experimental and theoretical investigations of selenium...

Experimental and Theoretical Investigations of Selenium

Chemical Shielding Tensors in Planar Heterocycles

By

Andre Sutrisno

An Undergraduate Thesis

Submitted to the Department of Chemistry & Biochemistry

in Partial Fulfillment of the Requirements for 59-410

University of Windsor

Windsor, Ontario, Canada

April 2007

© 2007 Andre Sutrisno

Experimental and Theoretical Investigations of Selenium Chemical Shielding

Tensors in Planar Heterocycles

by

Andre Sutrisno

Approved By:

_____________________________________

Charles L.B Macdonald, Departmental Reader

Chemistry & Biochemistry

_____________________________________

R. Schurko, Advisor

Chemistry & Biochemistry

April 23, 2007

-iii-

Abstract

Solid-state NMR (SSNMR) has proven to be a powerful tool for the analysis of

structure and dynamics in solids at the molecular level. It is used in tandem with techniques

like others like X-ray crystallography, powder X-ray diffraction and ab initio calculations, in

order to correlate NMR parameters with bonding and structure. SSNMR experiments are

very sensitive to the changes in the local environment of nuclei, so slight changes in bond

lengths, angles and molecular symmetry will have significant effects on NMR spectra.

Heterocyclic selenium compounds are used in different kinds of applications, such as

semi-conduction materials, photo-conducting agents and as precursors or intermediates to a

variety of synthetic and self assembly applications. 77Se CP/MAS NMR spectra of selenium-

containing heterocyclic compounds reveal expansive selenium chemical shift tensors, which

are extremely sensitive to molecular geometry, symmetry and ligand substitution. Gaussian

03 and ADF calculations are capable of reproducing experimental CS tensors, as well as

predicting their orientations with respect to the molecular frames. A large series of selenium

heterocyclic compounds are studied by NMR spectroscopy and theoretical calculations.

Selenium CS tensor analysis will be applied to deduce molecular structure in numerous

organic semi-conductors and heterocycles for which crystal structures are not available. Of

particular interest are paramagnetic selenium heterocycles.

-iv-

Acknowledgements

First of all, I would like to thank Prof. Robert W. Schurko for giving me the

opportunity to work in his lab for the last two years. I have to admit that it is even harder for

me, especially as an international student, to get such a wonderful opportunity. He has always

been there for any help and assistance throughout this entire research process. I would also

like to thank to all members of the solid-state NMR group at the University of Windsor,

namely Andy Lo, Joel Tang, Aaron Rossini, and Hiyam Hamaed for their help. I would like

to extend an additional thank you to Andy Lo who helped me set-up my first NMR

experiments and to both Prof. Schurko and Joel Tang for proofreading this thesis. Aaron

Rossini and Cory Widdifield are also thanked for the assistance with ADF calculations.

I would also like to thank Prof. Charles L. B. Macdonald for reading this thesis and

the extensive use of his laboratory and glove boxes to pack air sensitive samples. Prof.

Schurko and I would like to thank Prof. Paul J. Ragogna at University of Western Ontario

and Prof. Richard T. Oakley at the University of Waterloo for providing the selenium

samples.

In addition, I would like to thank the Centre for Catalysis and Materials Research

(CCMR) for the undergraduate research scholarships. Prof. Schurko and I would like to

thank the Canadian Foundation for Innovation (CFI), the Ontario Innovation Trust (OIT) and

the University of Windsor for funding the solid-state NMR facility at the University of

Windsor.

-v-

Table of Contents

Abstract.......................................................................................................................... iii

Acknowledgements......................................................................................................... iv

Table of Contents............................................................................................................. v

List of Tables..................................................................................................................vii

List of Figures............................................................................................................... viii

List of Abbreviations....................................................................................................... xi

Chapter 1 Introduction to Solid State NMR

1.1 Introduction....................................................................................................1

1.2 NMR Interactions...........................................................................................2

1.2.1 Zeeman Interaction........................................................................3

1.2.2 Radiofrequency Interaction............................................................5

1.2.3 Chemical Shielding Interaction.......................................................5

1.2.4 Direct Dipolar Interaction............................................................10

1.2.5 Indirect Spin-Spin Interaction or J-coupling.................................12

1.3 Solid State NMR Techniques........................................................................13

1.3.1 Magic Angle Spinning (MAS)......................................................13

1.3.2 Cross Polarization MAS (CP/MAS).............................................14

References..........................................................................................................18

-vi-

Chapter 2 Solid State Se-77 NMR studies on heterocyclic selenium compounds

2.1 Introduction..................................................................................................21

2.2 Experimental................................................................................................ 25

2.3 Results and Discussion..................................................................................29

2.3.1 Solid-state 77Se VACP/MAS NMR of N-heterocyclic carbenoid compounds..................................................................................30

2.3.2 Solid-state 77Se VACP/MAS NMR of 10 and 11..........................41

2.3.3 Ab Initio Calculations of Selenium Chemical Shielding Tensors....45

2.3.4 DFT Calculations of Selenium Chemical Shielding Tensors..........49

2.4 Conclusions...................................................................................................68

2.5 Future Work.................................................................................................69

References..........................................................................................................70

Appendices

Appendix A Experimental details for the synthesis of un-published 77Seheterocylic carbenoid compounds.................................................79

Appendix B 77Se VACP/MAS NMR experimental parameters.........................81

Appendix C 13C CP/MAS NMR experimental parameters................................82

Appendix D 13C CP/MAS spectra of 1-11........................................................83

Appendix E Powder X-ray diffraction pattern for 3, 6 and 7............................89

Appendix F All calculated 77Se NMR parameters for 2-7 and 9-10..................91

Vita Auctoris..................................................................................................................95

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List of Tables

Table 2.1. Summary of observed 77Se CS tensor parameters for compounds 1-11.....30

Table 2.2. Calculated 77Se NMR parameters using Gaussian 03 showing bestagreement with experimental results........................................................ 46

Table 2.3. Non-relativistic (NR) ADF calculations of 77Se CS tensor parameters......50

Table 2.4. Contributions to paramagnetic shielding from mixing of occupied andvirtual MOs in A-F..................................................................................52

Table 2.5. Composition of the MOs in A-F from NR calculations............................ 55

Table 2.6. Mulliken charge distribution of each individual atom on A-E generatedfrom ADF calculations using BLYP method and TZ2P basis set.............. 66

Table F.1. Calculated 77Se NMR parameters for (2)................................................. 91







Table F.8. Calculated 77Se NMR parameters for (10)............................................... 94

-viii-

List of Figures

Figure 1.1. Zeeman interaction for a spin-1/2 nucleus with γ > 0 under the presence ofmagnetic field............................................................................................3

Figure 1.2. Four different possible scenarios of CSA powder patterns in solid samplethat might be observed.............................................................................. 9

Figure 1.3. The chemical shielding and shift(deshielding) scale...................................10

Figure 1.4. Schematic diagram of a rotor that is aligned at the magic angle w.r.t. theapplied magnetic field B0 in an MAS experiment......................................13

Figure 1.5. Transfer of magnetization during the CP experiment................................15

Figure 1.6. Cross-Polarization (CP) pulse sequence...................................................16

Figure 2.1. Schematic drawings of selenium heterocylic compounds..........................29

Figure 2.2. 77Se VACP/MAS NMR spectra of (1) and (2) at two different spinningspeeds.....................................................................................................31



Figure 2.5. 77Se VACP/MAS NMR spectra of (7) at two different spinning speeds....38



Figure 2.8. 77Se VACP/MAS NMR spectra of (10) at two different spinning speeds..41

Figure 2.9. 77Se VACP/MAS NMR spectra of (11) at two different spinning speeds..43

Figure 2.10. CS tensor orientations generated by Gaussian 03.....................................47

Figure 2.11. Schematic diagram of model compounds used in ADF calculations..........49

-ix-

Figure 2.12. CS tensor orientations for A-F generated from ADF calculations.............51

Figure 2.13. The occupied and virtual MOs of A that make significant contributions tothe paramagnetic shielding term.............................................................. 58

Figure 2.14. A representation of 34-37 MO pair, in which their mixing contributes toparamagnetic deshielding along σ33..........................................................60



Figure 2.17. The occupied and virtual MOs of B that make significant contributions tothe paramagnetic shielding term...............................................................62

Figure 2.18. The occupied and virtual MOs of C that make significant contributions tothe paramagnetic shielding term...............................................................63

Figure 2.19. The occupied and virtual MOs of D that make significant contributions tothe paramagnetic shielding term...............................................................64

Figure 2.20. The occupied and virtual MOs of E that make significant contributions tothe paramagnetic shielding term...............................................................65

Figure 2.21. Labeling of the atoms for A-F................................................................. 66

Figure 2.22. The occupied and virtual MOs of F that make significant contributions tothe paramagnetic shielding term...............................................................67

Figure D.1 13C CP/MAS NMR spectra of (1) at two different spinning speeds...........83


Figure D.3 13C CP/MAS NMR spectra of (3) at 7000 Hz..........................................84

Figure D.4 13C CP/MAS NMR spectra of (4) at two different spinning speeds.......... 84


-x-




Figure D.9 13C CP/MAS NMR spectra of (9) at two different spinning speeds..........87

Figure D.10 13C CP/MAS NMR spectra of (10) at two different spinning speeds........87

Figure D.11 13C CP/MAS NMR spectra of (11) at two different spinning speeds.........88

Figure E.1 Powder XRD pattern of 3....................................................................... 89



-xi-

List of Abbreviations

ADF Amsterdam Density Functional

AO atomic orbital

B3LYP Becke’s 3-parameter hybrid density functional theory with Lee, Yang, and Parr correlation functional

BLYP Becke's exchange functional with the correlation functional of Lee, Yang, and Parr

CP cross-polarization

CS chemical shielding

CSA chemical shielding anisotropy

DFT density functional theory

DZ double-zeta (ζ)

EPR electron paramagnetic resonance

FID free induction decay

FT Fourier transform

GIAO gauge-including atomic orbitals

HB Herzfeld-Berger

HOMO highest occupied molecular orbital

LUMO lowest unoccupied molecular orbital

MAS magic-angle spinning

MO molecular orbital

NMR nuclear magnetic resonance

-xii-

occ. occupied molecular orbital

PAS principal axis system

ppm parts per million

pXRD powder x-ray diffraction

rf radio-frequency

RHF restricted Hartree-Fock

SCF self-consistent field

S/N signal-to-noise

SSNMR solid-state nuclear magnetic resonance

TMS tetramethylsilane, SiMe4

TZ2P triple-zeta (ζ) doubly polarized

vir. unoccupied (virtual) molecular orbital

VWN-BP Vosko-Wilk-Nusair local density approximation with Becke-Perdewgeneralized gradient approximation

XRD x-ray diffraction

VACP variable amplitude cross-polarization

-1-

Chapter 1

Introduction to Solid-State NMR

1.1 Introduction

Solid-state Nuclear Magnetic Resonance (SSNMR) has proven to be a powerful tool

for the analysis of structure and dynamics in solids at the molecular level. It is commonly

used in tandem with other techniques like X-ray crystallography, powder X-ray diffraction

and ab initio calculations, in order to correlate NMR parameters with bonding and structure.

SSNMR experiments are very sensitive to the changes in the local environment of nuclei, so

any slight changes in bond lengths, angles and molecular symmetry will have significant

effects on NMR spectra.1-4 SSNMR surpasses crystallographic techniques in structural

characterization, since it can be applied to disordered solids (e.g., amorphous solids, glasses,

aggregates, biological samples, etc.).

In solution NMR, sharp, well-resolved peaks are normally observed due to the fast,

isotropic molecular tumbling that serves to average the anisotropic (orientation-dependant)

NMR interactions. In solids, anisotropic NMR interactions generally lead to very broad

powder patterns that reduce both resolution and sensitivity. However, these broad spectra

often contain much more valuable information about the chemistry of the sample, such as

structural and dynamic properties, which can not be obtained from the solution NMR data.

Although solid-state NMR experiments are more difficult to perform and the spectra

are harder to analyze than solution NMR, techniques have been developed that make the

acquisition and interpretation of solid-state NMR spectra easier. A number of techniques,

-2-

,NMR ' ,Z % ,RF % ,CS % ,DD % ,J % ,Q (1)

such as magic angle spinning,5-8 cross-polarization9,10 and multiple-pulse sequences,5,6,11-20 have

been developed in order to reduce the spectral influence of large anisotropic NMR

interactions and to increase signal-to-noise ratio (S/N) in dilute/rare spins (e.g., 13C, 15N).1,21-23

1.2 NMR Interactions

The origin of NMR lies in the interactions between nuclear spins (I) and an applied

external magnetic field (B0). However, there are numerous nuclear spin interactions which

can be classified as external and internal interactions. The external interactions refer to the

interaction of nuclear spins with external magnetic fields (i.e., large static magnetic field and

smaller oscillating field). Internal interactions refer to interactions between one spin and other

spins, as well as magnetic and electric fields with their origins in the sample. A general

Hamiltonian describing the NMR interaction is given by:22

where ,Z, ,RF, ,CS, ,DD, ,J, and ,Q describe the Zeeman, radio-frequency (rf), chemical

shielding, direct-dipolar coupling, indirect (scalar, J) spin-spin coupling and quadrupolar

interactions, respectively. In most cases, the high-field approximation is generally utilized to

simulate and explain NMR phenomena, and assumes that the Zeeman interaction is much

larger than all other external and internal interactions, such that the latter can be treated as

perturbations on the former.

-3-

,Z ' &γ£B0ÎZ (2)

B0 = 0

)E = £(B0

mI = +1/2

mI = -1/2ENERGY

B0 > 0

Figure 1.1 Zeeman interaction for a spin-1/2 nucleus with γ > 0 under thepresence of magnetic field.

1.2.1 Zeeman Interaction

The Zeeman interaction is the interaction between nuclear spins and the applied

external magnetic field, and is expressed by:1

where γ is the gyromagnetic ratio, £ is Planck's constant and ÎZ is the projection of the nuclear

spin angular momentum along the z-axis (direction of B0).

In the absence of the magnetic field, nuclei, on average, have the same energy (i.e.,

degenerate). But in the presence of the magnetic field, the nuclear spin energies become non-

degenerate (Figure 1.1), since nuclei with different nuclear spin states start to precess about

the field axis. For a nucleus with nuclear spin I, there are 2I + 1 possible energy levels, each

associated with a magnetic quantum number, mI, where mI = +I, +I – 1, +I – 2, …, -I.

-4-

ΔE ' hυ0 ' £ω0 ' γ£B0 (3)

Nβ

Nα

' e &ΔE /kT(4)

The frequency of precession about the applied field axis is also known as the Larmor

frequency, ω0 = 2πυ0. It is proportional to the product of gyromagnetic ratio and the

magnitude of the applied external magnetic field. Therefore, the energy level difference

between adjacent spin states is given by the following equation:1

As the energy difference between nuclear spin states increases, the population

differences between energy levels also increase. The ratio of population difference between

energy states is determined by the Boltzmann distribution:3

where Nβ and Nα are the populations of the higher (mI = -1/2) and lower energy (mI = +1/2)

levels, respectively, k is the Boltzmann constant and T is the temperature in K. From this

equation, it is obvious that both temperature and magnetic field strength affect the population

distribution. These population differences between spin states is relatively small compared

to other spectroscopic methods, making NMR one of the less sensitive spectroscopic

techniques in terms of sample quantities.

-5-

, RF ' &B1(t) cos[ωrf(t) % φ] ji

γinI

ix (5)

1.2.2 Radio-frequency Interaction

The radio-frequency interaction is the interaction between the nuclear spin and the

applied rf field (B1), which is perpendicular to the direction of the static external magnetic

field. The Hamiltonian is expressed by:3

where ωrf is the radio wave frequency and φ is its phase. The applied rf field is often set close

to ω0, such that their difference, Ω = ωrf - ω0, is in the audio frequency regime. The above

Hamiltonian can be reexpressed in the so-called “rotating frame”, which provides a simple

means of visualizing classical rotation of magnetization vectors. Appropriate application of

the rf field in the form of a short pulse causes transitions between the different nuclear spin

energy levels, that ultimately lead to the observation of the NMR signal, which is detected in

the transverse plane, perpendicular to the applied field, B0. These transitions between these

energy levels have a selection rule of ΔmI = ±1.

1.2.3 Chemical Shielding Interaction

Chemical shielding results from the interaction between nuclear spins and a local

secondary magnetic field induced by the circulating motion of electrons within a molecule (or

an atom).

When a molecule/atom is placed within a strong magnetic field, electrons are induced

to circulate within their orbitals, and certain pairs of orbitals are induced to mix with one

another. The magnetically induced circulation of electrons and mixing of orbitals result in the

-6-

σtot ' σdiamagnetic % σparamagnetic (6)

,CS ' γ£Î ZPσB0 (7)

production of small local magnetic fields within the molecule, which is so much smaller

compared to B0, usually in the range of parts per million, or ppm. This means that the total

effective field experienced at the nucleus (Beff) is not equal to B0.

Hence, the phenomenon of chemical shielding (or chemical shifts), arises from the

interaction of the nuclear spin with these small local fields, which serve to increase or

decrease the Larmor precession frequency, depending upon whether the nucleus is deshielded

or shielded, respectively.

Chemical shielding is arbitrarily decomposed into two components according to the

formalism of Ramsey, which are known as the diamagnetic and paramagnetic contributions

to chemical shielding:24-26

The diamagnetic shielding arises from the field-induced electron circulations in the ground

electronic state and creates a local field which is anti-parallel to the applied field; hence, it is

responsible for shielding the nucleus and gives rise to a negative frequency shift. The

paramagnetic shielding arises from symmetry-allowed mixing of ground and excited states.

In most cases it gives rise to a local field that is parallel to the applied magnetic field, thereby

deshielding the nucleus and causing a positive frequency shift.

The chemical shielding Hamiltonian can be written as:22

-7-

Pσ '

σxx σxy σxz

σyx σyy σyz

σzx σzy σzz

(8)

PσPAS '

σ11 0 0

0 σ22 0

0 0 σ33

(9)

where is the chemical shielding tensor, a 3 x 3 second-rank matrix used to describePσ

chemical shielding anisotropy (CSA).

This CS tensor can be diagonalized to yield a tensor with three principle components in its

own principle axis system (PAS), such that:

The tensor components are ordered such that σ11 corresponds to the least shielded component

and σ33 to the most shielded component, i.e., σ11 # σ22 # σ33.

CS tensor describes the orientation dependence of chemical shielding, meaning that

the shielding of the nucleus depends on the orientation of the molecule with respect to the

magnetic field. The CS tensor is non-symmetric and also non-traceless (i.e., the sum of the

diagonal components is not equal to 0); however, the anti-symmetric portion can be

disregarded, as it has no contribution to the secular frequency shifts observed in NMR

spectra.27

In solution, the molecules are rapidly tumbling, which averages the anisotropic effects

of the CS tensor, leading to the observation of sharp peaks. However, in the solid state (e.g.,

-8-

σiso '(σ11 % σ22 % σ33)

3(10)

Ω ' σ33 & σ11 (11)

κ '3(σiso & σ22 )

Ω(12)

a powdered microcrystalline sample, for instance), molecules are oriented in an infinite

number of possible orientations with respect to the magnetic field, and all of the orientation-

dependant chemical shifts can be seen. The shape of the chemical shielding anisotropy (CSA)

powder pattern depends on the principal components of the shielding tensor, which vary with

the symmetry and structure of the molecule. There are three parameters used to describe the

CSA which are derived from σ11, σ22 and σ33: the isotropic chemical shielding (σiso), the span

(Ω) and the skew (κ).4,14,22

The isotropic chemical shielding is the average of the three chemical shielding

components, and corresponds to what is observed in solution NMR spectra. The span is the

difference between the most and the least shielded components, and describes the breadth of

the powder pattern. The skew is the degree of axial symmetry, and describes the shape of the

powder pattern and the axial symmetry of the CS tensor. The value of κ varies between -1.0

and 1.0 (see Figure 1.2).

-9-

δ (ppm) 'νsample & νref

νref

× 106(13)

F11

F22 = F33

6 = -1.0, axial symmetry

F11

6 = +0.3 non-axial symmetry

F33

F11 = F22F iso

6 = +1.0, axial symmetry

F iso = F11 = F22 =F33

S = 0, spherical symmetry

F iso

F iso

F33

F22

A B

C D

Figure 1.2 Four different possible scenarios of CSA powder patterns in solidsample that might be observed. Note that σ11 # σ22 # σ33.

The chemical shielding scale describes the chemical shielding of a given nucleus

relative to a the bare nucleus (e.g., for 31P, P15+ is the bare nucleus); of course, a bare nucleus

of this sort is not a practical experimental standard. For that reason, the chemical shift values

are often cited instead of chemical shielding values.

where νsample is the frequency of the signal for the nucleus in the sample of interest and νref is

the frequency of the same nucleus in the reference compound.

Chemical shift scales are constructed by assigning an arbitrary shift (usually 0.0 ppm) to a

-10-

δsample 'σref & σsample

1 & σref

. σref & σsample (14)

,DD ' RDD [I @ S & 3 (I @r)(S @r)

r 2] (15)

higher frequency

increase in shift / deshielding

downfield

increase in shielding

upfield

lower frequency

Figure 1.3 The chemical shielding and shift(deshielding) scale.

stable reference compound. All chemical shifts are reported relative to this reference, making

the relationship between chemical shift and chemical shielding scales as follows:

If δ > 0, then the nucleus is said to be deshielded relative to the reference, and if δ < 0, the

nucleus is said to be shielded (Figure 1.3). The tensor components of chemical shifts are

ordered such that δ11 $ δ22 $ δ33.

1.2.4 Direct Dipolar Interaction

The direct dipolar interaction, also known as direct dipole-dipole coupling, is a

through-space interaction between the magnetic moments of the two spins, I and S. Its

Hamiltonian can be written as:1

where RDD is the dipolar coupling constant in Hz, expressed by:

-11-

RDD ' (µ0

4π)γIγS£

r 3IS

(16)

where γI and γS are the gyromagnetic ratios between the two spins, rIS is the average

internuclear distance between spin I and S, and µ0 is the magnetic or permeability constant,

which is equal to 4π × 107 N A-2. The dipolar interaction increases proportionally to the

gyromagnetic ratios of the nuclei involved, and is inversely proportional to the cube of the

internuclear distance between two nuclei.

In solution, the dipolar interaction is averaged to zero due to rapid molecular tumbling

and only sharp peaks are observed. No shifts are observed because the dipolar tensor is

traceless; however, the dipolar interaction is of paramount importance for non-secular effects

in solution NMR (i.e., dipolar relaxation). On the other hand, in solid-state, frequency shifts

arising from all of the different orientations of the internuclear dipolar vectors are observed

at the same time, giving rise to a powder pattern. For a single crystal, a dipolar interaction

of a different magnitude is observed for each unique crystallite orientation with respect to B0.

It varies with an angular orientation dependance of (3cos2θ - 1), where θ is the angle between

B0 and the internuclear distance vector, r. Note that when θ = 54.74E(i.e., magic angle), the

“3cos2θ - 1” term is equal to 0. When all orientations are present in a microcrystalline sample

or amorphous sample, the classic Pake doublet is observed.28

-12-

, J ' I @PJ @ S (17)

1.2.5 Indirect Spin-Spin Interaction or J-coupling

The indirect spin-spin interaction, better known as J-coupling, is the interaction

between two magnetic dipole moments, I and S, mediated by electrons in molecular orbitals

that are involved in chemical bonding.1 The J-coupling hamiltonian is written as:

where is the J-coupling tensor, which is not symmetric and does have a trace (leading toPJ

the observation of J-splittings in solution NMR spectra, despite isotropic tumbling). In most

cases, only the isotropic J-splittings are observed, with the exception of coupling between

spin pairs involving heavy nuclei, where the J-anisotropy can be detected.29-39 J-coupling

provides a direct information on the nature of the chemical bonds at a given atom, and its

magnitude is field independent, with values expressed in Hz. It is very sensitive to changes

in molecular structure, making it an excellent complimentary structural probe with the

chemical shift.

-13-

54.74E

B0

Figure 1.4 Schematic diagram of a rotor that is alignedat the magic angle w.r.t. the applied magnetic field B0 inan MAS experiment.

1.3 Solid State NMR Techniques

1.3.1 Magic-Angle Spinning (MAS)

In solution, rapid molecular tumbling leads to an averaging of chemical shielding

resulting in the observation of isotropic chemical shifts as sharp resonances. However, in

solids, molecules do not isotropically reorient. Solid-state spectra are normally acquired from

powdered samples, which have many crystallites in all possible orientations with respect to

the external magnetic field, resulting in broad powder patterns whose lineshapes are

dependant upon the anisotropic CS an dipolar interactions.

In order to narrow these broad powder patterns, the magic-angle spinning (MAS)

technique is used.4,14 This involves a fast spinning of the sample, with frequencies on the

order of kHz, with the sample axis aligned at an angle of 54.74E with respect to the external

magnetic field (the so-called “magic angle”, Figure 1.4). All of the NMR interactions

-14-

discussed above, have a spatial dependance of (3cos2θ - 1), where θ is the angle between the

rotational axis of the sample rotor and the magnetic field. So when θ = 54.74E, the term

(3cos2θ - 1) = 0 and the spatial dependencies of the CSA and dipolar interactions are average

to 0.5-8,40,41

In order to completely average the anisotropic interactions, the spinning speed of the

sample has to be greater than/on the order of the magnitude of the anisotropy in Hz. The

spinning of the sample to average such interactions often creates spin echoes in the FID,

which give rise to spinning sidebands in the corresponding Fourier-transformed frequency

spectrum. These spinning sidebands flank the isotropic peak, which remains invariant in

position regardless of spinning speed, and are separated by a distance equal to the spinning

speed in Hz.

1.3.2 Cross-Polarization Magic-Angle Spinning (CP/MAS)

Cross-polarization (CP) is a common technique used to enhance S/N of dilute spins

via polarization transfer from abundant spins. It was first introduced by Pines, Gibby and

Waugh in 1972.9,10 CP enhances the signal by a factor of γI/γS (under ideal conditions), where

I is the abundant spin and S is the dilute spin. This happens as a result of transfer of

magnetization from the abundant spins (i.e., 1H) to dilute spins (i.e., 77Se, 13C), provided that

there is a substantial dipolar interaction. The transfer of magnetization (Figure 1.5) takes

place during a short time (on the order of ms), which is known as the contact time (since both

channels of the NMR probe are simultaneously irradiating the sample during this time period).

-15-

ω1I ' γIB1I ' γSB1S ' ω1S (18)

1H 13CT0H

T1H1H 13C

T0C

T1CPolarization

A. Lab frame: T0H > T0C

B. Rf rotating frame: T1H • T1C

Figure 1.5 Transfer of magnetization during the CP experiment

In order for CP to occur, the Hartmann-Hahn matching condition must be satisfied,

where the frequencies of precession of the both spins in the rotating frame are equal:

where B1I and B1S are the strengths of the applied rf fields on the I and S spins, respectively,

during the contact period.

The most common cross-polarization experiment is 1H-13C CP/MAS, which can

theoretically provide a signal-to-noise increase of about 4 times in the observed 13C NMR

spectrum. For 1H and 13C spin pairs, CP/MAS works best at relatively low spinning speeds

(υrot ca. 3 to 5 kHz). In addition, the spin lattice relaxation T1 that determines the delays

between acquisitions (i.e., recycle delay .5T1), only depends on the abundant spins; the T1

constants of protons are typically much shorter than those of 13C nuclei, due to the larger γ

-16-

I (1H) spinlocking decoupling relaxation

delay

S (13C) mixing acquisition relaxationdelay

(B/2)x

JCT JAQ JR

contact time

Figure 1.6 Cross-Polarization (CP) pulse sequence.

and higher mobility of the former. The combination of CP enhancement and shorter recycle

delays greatly reduces overall experimental times while achieving much higher S/N compared

to corresponding Bloch decay MAS NMR experiments.

The CP pulse sequence1,22 (Figure 1.6) starts with a (π/2)x pulse on the abundant (I)

spins, which rotates the spin magnetization along -y axis in the transverse plane of the rotating

frame. After this pulse, rf fields which satisfy Hartmann-Hahn matching conditions are

applied simultaneously to I and S spins. After the contact time, the FID of the dilute (S) spin

is detected, while high-power decoupling is applied on the abundant (I) spin channel.

When dealing with larger powder patterns arising from anisotropic NMR interactions

of large magnitude, higher spinning speeds are required to average the anisotropic NMR

interactions. CP becomes less efficient at higher spinning speeds, since the dipolar interaction,

-17-

which is necessary for efficient CP, is averaged at these higher spinning speeds. Therefore,

a modified version of CP pulse sequence, known as variable-amplitude CP (VACP),42,43 is

utilized. It uses a spin-lock pulse that is divided into train of shorter pulses with gradually

increasing amplitude and constant phase during the contact time, which reduces the amplitude

mismatch of the two spin-locking fields. This creates a series of different Hartmann-Hahn

conditions that result in much smaller loss of CP efficiency and hence signal intensity

compared to conventional CP/MAS.

-18-

References:

(1) Duer, M. J. Solid-State NMR Spectroscopy 2002, 73-110.

(2) Duer, M. J. Introduction to Solid-State NMR Spectroscopy, 2004.

(3) Harris, R. K. Nuclear Magnetic Resonance Spectroscopy, 1986.

(4) Hore, P. J. J., J.A.; Wimperis,S. NMR: The toolkit; Oxford University Press, 2000.

(5) Goldbourt, A.; Madhu, P. K. Monatshefte fuer Chemie 2002, 133, 1497-1534.

(6) Goldbourt, A.; Madhu, P. K. Annual Reports on NMR Spectroscopy 2005, 5, 81-153.

(7) Hediger, S.; Meier, B. H.; Ernst, R. R. Chemical Physics Letters 1993, 213, 627-635.

(8) Hafner, S.; Demco, D. E. Solid State Nuclear Magnetic Resonance 2002, 22, 247-

274.

(9) Pines, A.; Gibby, M. G.; Waugh, J. S. Chemical Physics Letters 1972, 15, 373-376.

(10) Pines, A.; Gibby, M. G.; Waugh, J. S. Journal of Chemical Physics 1973, 59, 569-

590.

(11) Aliev, A. E.; Law, R. V. Nuclear Magnetic Resonance 2006, 35, 234-312.

(12) Aliev, A. E.; Law, R. V. Nuclear Magnetic Resonance 2003, 32, 238-291.

(13) Amoureux, J. P.; Pruski, M. Molecular Physics 2002, 100, 1595-1613.

(14) Claridge, T.; Editor High-Resolution NMR Techniques in Organic Chemistry, 2000.

(15) Dybowski, C.; Bai, S. Analytical Chemistry 2000, 72, 1-7.

(16) Eckert, H. Current Opinion in Solid State & Materials Science 1996, 1, 465-476.

(17) Fitzgerald, J. J.; DePaul, S. M. ACS Symposium Series 1999, 717, 2-133.

(18) Fyfe, C. A. In Solid State NMR for Chemists; C.F.C Press: Guelph, 1983, pp 268-526.

(19) Klinowski, J.; Kolodziejski, W.; Editors Solid-State NMR Techniques, 1998.

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(20) MacKenzie, K. J. D. Multinuclear Solid-State Nuclear Magnetic Resonance of

Inorganic Materials, 2002.

(21) Duer, M. J. In Introduction to Solid-State NMR Spectroscopy, 2004, pp 96-101.

(22) Levitt, M. H. Spin Dynamics: Basics of Nuclear Magnetic Resonance; John Wiley

& Sons Ltd., 2001.

(23) Wind, R. A. Practical Spectroscopy 1991, 11, 125-215.

(24) Ramsey, N. F. Physical Review 1951, 83, 540-541.



(27) Mehring, M. Principles of High Resolution NMR in Solids. 2nd Ed, 1983.

(28) Verber, C. M.; Lecander, R. G. Physics Letters A 1967, 25, 179-180.

(29) Kroeker, S.; Hanna, J. V.; Wasylishen, R. E.; Ainscough, E. W.; Brodie, A. M.

Journal of Magnetic Resonance 1998, 135, 208-215.

(30) Wu, G.; Kroeker, S.; Wasylishen, R. E.; Griffin, R. G. Journal of Magnetic

Resonance 1997, 124, 237-239.

(31) Wu, G.; Wasylishen, R. E. Inorganic Chemistry 1996, 35, 3113-3116.

(32) Lumsden, M. D.; Wasylishen, R. E.; Britten, J. F. Journal of Physical Chemistry

1995, 99, 16602-16608.

(33) Lumsden, M. D.; Eichele, K.; Wasylishen, R. E.; Cameron, T. S.; Britten, J. F.

Journal of the American Chemical Society 1994, 116, 11129-11136.

(34) Power, W. P.; Wasylishen, R. E. Inorganic Chemistry 1992, 31, 2176-2183.

(35) Power, W. P.; Lumsden, M. D.; Wasylishen, R. E. Journal of the American Chemical

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Society 1991, 113, 8257-8262.

(36) Johannsen, I.; Eggert, H. Journal of the American Chemical Society 1984, 106, 1240-

1243.

(37) Johannsen, I.; Henriksen, L.; Eggert, H. Journal of Organic Chemistry 1986, 51,

1657-1663.

(38) Grossmann, G.; Potrzebowski, M. J.; Fleischer, U.; Kruger, K.; Malkina, O. L.;

Ciesielski, W. Solid State Nuclear Magnetic Resonance 1998, 13, 71-85.

(39) Eggert, H.; Nielsen, O.; Henriksen, L. Journal of the American Chemical Society

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(40) Hediger, S.; Meier, B. H.; Ernst, R. R. Chemical Physics Letters 1995, 240, 449-456.

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4011.

(42) Peersen, O. B.; Wu, X. L.; Kustanovich, I.; Smith, S. O. Journal of Magnetic

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106, 127-131.

-21-

Chapter 2

Solid-State 77Se NMR studies on heterocyclic selenium compounds

2.1 Introduction

The scope of selenium chemistry is enormous, covering large areas in inorganic,

organic, organometallic and materials chemistry, biochemistry, and environmental sciences.1-13

Selenium-containing compounds have been used in various chemical applications, including

catalytic agents, magnetic salts, semi-conducting materials, and also photo-conducting agent

in photocopying machines and printers.14-19 Although many of these selenium compounds are

known for being toxic and having unpleasant odours, many organoselenium compounds are

found to be useful intermediates in a great variety of reactions since they often react regio-

and/or stereo-selectively.3,20-22 They are generally more nucleophilic and more acidic, hence

more reactive than the corresponding sulfur and oxygen analogues. Many of these

organometallic selenium complexes are of the heterocyclic variety, where the Se atom is the

often the crucial substituent in a five- or six-membered ring.23 The complexes of pertinence

to this thesis are (i) N-heterocyclic main group carbenoids, (ii) low-temperature semi-

conducting Bechgaard’s salts and (iii) N-Se-S containing heterocyles.

Carbenes are known to be important reaction intermediates, an there is an enormous

amount of literature surrounding their synthesis and reactivity.24-27 Since the first report of

the synthesis of a stable crystalline N-heterocyclic carbenes by Arduengo,28 there have been

a substantial amount of efforts in synthesizing, characterizing and examining the reactivity of

the isovalent p-block analogues, known as the main group carbenoids.29-33 In all of these N-

-22-

heterocyclic carbenes, the main group element bears a lone pair of electrons and is bi-

coordinate. These p-block heterocyclic carbene analogues have been successfully synthesized

and characterized for group 13 (Al-, Ga-, In-, Tl-), group 14 (Si, Ge, Sn) and group 15 (P+,

As+, Sb+), with its main group element being formally drawn as anionic, neutral or cationic,

respectively.34-39 However, the recent synthesis of dicationic N-heterocyclic selenium

carbenes have extended the series into group 16, and the change in the formal charge at the

Se atom (+2) has a significant impact on the reactivity of these carbenoids.40,41

Some organic selenium compounds, (TMTSF)2X, which belong to the Bechgaard’s

salts family, are known as good low temperature organic conductors and superconductors,

due to their properties stemming from extensive π-π conjugation.42-49 TMTSF stands for

tetramethyltetraselenafulvalene, and X- are inorganic anions with various possible symmetry

such as spherical (PF6-, AsF6

-), tetrahedral (BF4-, ClO4

-, ReO4-), and trigonal planar (NO3

-).

The solid state structure of these salts reveal two different TMTSF molecules, one being

cationic and the other being neutral.

Neutral π-electrons systems have been used extensively as the building blocks for the

development of single component molecular conductors.50-53 However, most molecular

radicals dimerize in the solid state, resulting in low bandwidth and conductivity.51,52 However,

a resonance stabilized bis-1,2,3-thiaselenazolyl radical has recently been synthesized and

characterized, and was the first example of an undimerized π-stacked radical.54 The effect of

replacing sulfur with selenium into this neutral radical conductors has been shown to increase

the bandwidth and conductivity of these molecular conductors by reducing the thermal

activation energy for electronic conduction.55-58

-23-

Selenium also has many biochemical applications, as it is an essential trace element in

mammalian systems, and also some selenoenzymes have been discovered, e.g.,

selenadigluthathione,8,13,59 iodothyronine deiodinase, thioredoxin reductase, and most

importantly selenocysteine,7,10,60,61 which is considered as the 21st amino acid. It has also been

recently believed that some selenium compounds have protective anti-carcinogenic

properties.1,4,11,62

Out of six natural selenium isotopes, 74Se (0.87%), 76Se (9.02%), 77Se (7.58%),

78Se(23.52%), 80Se (49.82%), and 82Se (9.19%), only one of them, which is 77Se, is NMR-

active. 77Se has a spin quantum number I = ½ and no quadrupolar moment, with a

gyromagnetic ratio of 5.1214 × 107 rad T-1 s-1. These factors result in a relative receptivity

of 2.9 in comparison to 13C. The 77Se chemical shift range is about 3800 ppm with the

extremes being marked by selenoaldehydes, molybdenum selenides, and cationic heterocycles

(δ = 2434 ppm) at the high-frequency end, and [(CH3)4N]SeO2Cl (δ = -1371 ppm) at the

other end.22,63-78 77Se chemical shift is very sensitive to the local electronic environment of the

selenium atom itself. Due to high electronegativity of nitrogen, the resonances of selenium

atom attached to N in the N-heterocylic selenium compounds have generally high frequencies;

δ-values between 1200-1800 ppm may appear depending on the molecular form of the

molecule, its atomic charge and its bonding properties.64,65

Herein, a preliminary study of selenium heterocyclic compounds is reported. The aims

of this study are to experimentally measure the large selenium chemical shift tensors for two

classes of selenium heterocycles using solid-state 77Se NMR, and to investigate the

relationships between the selenium CS tensor parameters and the molecular structure, bonding

-24-

and symmetry in these systems using high-level first-principles calculations. The first class

of complexes is comprised of a variety of N-heterocyclic selenium carbenoids (featuring Se

in N-Se-N environments) with variation in both organic substituents and counteranions,

including t-butyl and dipp groups, and OTf -, GaCl4-, SeCl6

2-, Cl- and SnCl62- counteranions.

The second class of complexes feature Se in two distinct heterocyclic environments: C-Se-C

and N-Se-S heterocylic compounds. Density-functional theory (DFT) calculations using the

Amsterdam density functional (ADF) software package79 are applied to a series of model

systems in order to examine the origin of the selenium CS tensors from the perspective of

contributions of single and pairs of molecular orbitals (MOs).

-25-

2.2 Experimental

Sample Preparation. Samples of 1,2,5-selenadiazolium trifluoromethane sulfonate

([tBu-N2C2Se][OTf]) (1), 1,2,5-selenadiazolium gallium(III) tetrachloride ([tBu-

N2C2Se][GaCl4]) (2), bis(1,2,5-selenadiazolium) selenium(IV) hexachloride ([tBu-

N2C2Se]2[SeCl6]) (3), 1,2,5-selenadiazolium chlorine ([tBu-N2C2Se-Cl]) (4), selenium

"carbene" 2-chloro-1,2,5-selenadiazole trifluoromethane sulfonate ([SeN2C26H36Cl][OTf]) (5),

selenium "carbene" 2-chloro-1,2,5-selenadiazole t in(IV) hexachloride

([SeN2C26H36Cl]2[SnCl6]) (6), selenium "carbene" 1,2,5-selenadiazole tin(IV) hexachloride

([SeN2C26H36]2[SnCl6]) (7), selenium "carbene" SeCl2-dipp ([SeN2C26H36Cl2]) (8) and

selenium "carbene" SeCl2-BIAN (bis(imino)acenapthene) ([SeN2C36H40Cl2]) (9) were

synthesized in the research laboratories of Professor Paul J. Ragogna at the University of

Western Ontario using previously reported procedures,40,41 as well as some new modifications

(see Appendix A for details). Tetramethyltetraselenafulvalene (TMTSF) (10) was purchased

from Sigma-Aldrich Canada, Ltd. and used without further purification. The paramagnetic

version of 10, (TMTSF)2PF6 (10*) was synthesized by Professor Mark S. Workentin at the

University of Western Ontario according to previously reported procedures.80 Methyl

bis(pyridine thiaselenazolyl) ethyl trifluoromethane sulfonate ([MeBPTSEt][OTf]) (11)57 and

its paramagnetic version, MeBPTSEt radical, (11*)58 were provided by Professor Richard T.

Oakley from the University of Waterloo, Canada. All samples were finely ground and packed

into 4 mm outer diameter zirconia rotors. All samples, except 10 and 11, were sealed with

airtight caps under a nitrogen atmosphere.

-26-

Solid-State NMR Spectroscopy. Solid-state 77Se and 13C VACP/MAS NMR spectra

were acquired on a Varian Infinity Plus Spectrometer with an Oxford 9.4 T (ν0 (1H) = 400

MHz) wide bore magnet, operating at resonance frequencies of ν0 (77Se) = 76.28 MHz and

ν0 (13C) = 100.52 MHz. A Varian/Chemagnetics 4 mm double resonance (4mm HX) MAS

probe was used for all the experiments. 77Se chemical shifts were referenced to a solution of

Me2Se in CDCl3 (60% v/v) at 0.0 ppm, using solid (NH4)2SeO4 as a secondary standard (δiso

= 1040.2 ppm).67 13C chemical shifts were referenced to tetramethylsilane (TMS) at 0.0 ppm,

using solid adamantane as a secondary standard (δiso = 38.57 ppm).81 The magic angle

(54.74°) was accurately set by optimizing the number of rotary echoes of 79Br resonance in

a 50-50 mixture of KBr and adamantane.

77Se VACP/MAS NMR. Proton-decoupled 77Se VACP/MAS NMR spectra were

acquired with typical spinning speeds ranging from 4.5 kHz to 12.0 kHz. Calibrated recycle

delays from 1.0 to 30.0 seconds were used. The proton π/2 pulse width used in all the

experiments was between 1.6 and 4.0 µs (ν1 (1H) = 62.5 to 156 kHz). The 77Se rf

frequencies, ν1 (77Se), were calibrated at various power levels using a solution of 1.0 M

(NH4)2SeO4. The Hartmann-Hahn matching condition was set by fixing the proton rf field,

ν1(1H), and adjusting the 77Se channel rf field in order to obtain maximum S/N (and therefore

maximum cross polarization) on a sample of solid (NH4)2SeO4. Calibrated contact times of

15 to 22 ms were used in all the experiments. The spectral widths were set from 200 to 300

kHz, with 164 to 73500 transients collected during the experiments (see Appendix B for the

detailed experimental parameters).

-27-

13C VACP/MAS NMR. Proton-decoupled 13C VACP/MAS NMR spectra were

acquired with νrot from 1.95 kHz to 7.9 kHz. The 13C rf frequencies, ν1 (13C), were

determined at various power levels using a solid adamantane as a standard. Calibrated

contact times of 9 to 16 ms were used in all the experiments. The spectral widths were set

from 50 to 100 kHz, with 88 to 3368 transients collected during the experiments. Variation

in recycle delays is quite large, ranging from 3 to 60 seconds, and obviously very sample

dependent (see Appendix C for full listings of parameters).

Spectral Simulations. 77Se chemical shift tensor parameters, δiso, Ω and κ (see

introduction and tables for their definitions) were extracted using Herzfeld-Berger Analysis

(HBA)82 and analytical simulations of solid-state 77Se NMR spectra were performed using the

WSOLIDS simulation package.83

Theoretical Methods. Ab initio calculations were conducted using Gaussian 0384

running on a dual-733 MHz Pentium III Dell Precision 420, a dual-2.8 GHz Xenon Dell

Precision 650 workstation or a dual-3.6 GHz Xenon Dell Precision 670n workstation running

Red Hat Linux 9.0. Chemical shielding tensors were calculated using B3LYP85 (Becke’s

three-parameter hybrid density functional theory with Lee, Yang, and Parr correlation

functional) and RHF (Restricted Hartree-Fock) methods with basis sets of 3-21G**, 6-

31G**, 6-311G**, 6-311++G** as well as the all-electron Huzinaga basis sets86 (13s10p4d),

(14s10p5d), (17s13p6d) and (24s20p10d). Selenium chemical shifts were referenced to

Me2Se by comparing the calculated shielding values of all compounds to the absolute

shielding value of Me2Se.67,87 The Amsterdam Density Functional (ADF) software package88

was applied to several model compounds; individual MO contributions to the CS tensors were

-28-

obtained using the NMR-EPR module.89 The BLYP90,91 (Becke’s density functional theory)

and VWN-BP (Vosko-Wilk-Nusair92 local density approximation with Becke-Perdew85,90

generalized gradient approximation) exchange-correlation functional were applied to NR

(non-relativistic) calculations, and all-electron gauge including atomic orbitals (GIAO)

double-ζ (DZ) or triple-ζ doubly polarized (TZ2P) basis sets were used on all atoms. Crystal

structure coordinates with optimized proton positions were used for Gaussian 03 calculations,

while symmetry-constrained geometry optimized structures were used for ADF calculations.

Powder X-ray Diffraction. All samples were finely powdered, then carefully packed

into 0.7 mm glass capillary tubes under nitrogen and flame sealed. Powder X-ray diffraction

patterns were collected using a Bruker AXS HI-STAR system using a General Area Detector

Diffractions System (GADDS). The X-ray source employed was a Cu Kα radiation

(1.540598 Å) with an area detector using a 2θ range between 5° and 45°. Powder X-ray

diffraction patterns were then simulated using the Powder Cell software package.93,94 All the

unpublished crystallographic data on the selenium carbenoids was provided courtesy of Prof.

Paul Ragona, University of Western Ontario.

-29-

+

NSe

NtBu OTf N

SeN

tBu

+

GaCl4 NSe

NtBu

2

SeCl6

+

2-

NSe

NtBu

Cl

NSe

Ndipp dipp

Cl

OTf-

+

SnCl62-

2

NSe

Ndipp dipp

Cl+

2

NSe

Ndipp dipp

2

SnCl62-

+

NNdipp dippSe

ClClNNdipp dipp

SeClCl

Se

Se

Se

SeOTf

+

N

SSe

NNSe

S

Et

Me

1 42 3

10

65

78

9

11

Figure 2.1 Schematic drawings of selenium heterocylic compounds.

2.3 Results and Discussion

The following section is broken down into four parts. In the first two parts of this

section, the acquisition and interpretation of solid-state 77Se VACP/MAS NMR spectra of all

the N-heterocyclic selenium carbenoid compounds (1 to 9) are discussed, followed by

discussion of TMTSF (10) and [MeBPTSEt][OTf] (11). The carbenoid complexes are

grouped according to common structural features (Figure 2.1).

The third section features a discussion of Gaussian 03 RHF and B3LYP calculations of

selenium CS tensors for most of the systems discussed herein. Finally, the fourth section

discusses the application of ADF calculations on model compounds to examine which

molecular orbitals are key in determining the nature of the selenium CS tensor.

-30-

A summary of the observed 77Se NMR parameters from all the compounds studied are

shown in Table 2.1.

Table 2.1 Summary of observed 77Se CS tensor parameters for compounds 1-11.

Compound δ11 (ppm)a δ22 (ppm) δ33 (ppm) δiso (ppm)b Ω (ppm)c κd

1 (site 1) 2005 1520 1275 1600 (10)e 730 (10) -0.33 (3)

(site 2) 1997 1504 1277 1593 (5) 720 (10) -0.37 (2)

2 1959 1489 1247 1565 (15) 712 (10) -0.32 (2)

3 (site 1) 1861 1384 1322 1522 (10) 539 (20) -0.77 (4)

(site 2) 1849 1382 1290 1507(10) 559 (20) -0.67 (3)

4 1789 1316 1252 1452 (5) 537 (10) -0.76 (4)

5 1970 1818 754 1514 (5) 1216 (15) 0.75 (4)

6 2030 1789 761 1527 (5) 1269 (15) 0.62 (3)

7 2115 1692 772 1526 (50) 1343 (70) 0.37 (9)

8 1983 1896 831 1570 (15) 1152 (20) 0.85 (6)

9 2362 2008 976 1782 (15) 1386 (25) 0.49 (3)

10 (site 1) 979 837 194 670 (5) 785 (10) 0.64 (6)

(site 2) 976 807 192 658 (5) 784 (10) 0.57 (3)

11 (site 1) 2161 1025 1014 1400 (15) 1147 (15) -0.98 (9)

(site 2) 2099 1040 860 1333 (15) 1239 (20) -0.71 (9)

aThe chemical shift tensor is described by three principal components, ordered such that δ11 $δ22 $δ33. bδiso = (δ11 + δ22 + δ33)/3. δiso is given relative to Me2Se [δiso(77Se) = 0.0 ppm].cΩ = δ11 - δ33.dκ = 3(δ22 - δiso)/Ω.eThe uncertainty in the last digit of each value is denoted in brackets.

2.3.1 Solid-state 77Se VACP/MAS NMR of N-heterocyclic carbenoid compounds

[tBu-N2C2Se][OTf] (1) and [tBu-N2C2Se][GaCl4] (2). Complexes 1 and 2 are

structurally similar, in that they are 1,2,5-selenadiazolium heterocyclic cations which are

singly substituted by t-butyl groups at one nitrogen position. Furthermore, atoms in the

counteranions of these species are not expected to make any distant contacts or formal bonds

with the Se atoms. The crystal structure has been determined for 2,40 revealing a dimeric

-31-

200 180 160 140 120 100 80 60 40 kHz

<rot = 7300 HzCP/MAS experiment

<rot = 7300 Hz

2500 2000 1500 1000 500 ppm

< rot = 4500 HzCP/MAS experiment

WSOLIDS simulation

* iso

2500 2000 1500 1000 500 ppm

200 180 160 140 120 100 80 60 40 kHz

< rot = 5700 HzWSOLIDS simulation



* iso

A)

B)

Figure 2.2 A) 77Se VACP/MAS NMR spectra of (1) at two different spinningspeeds. Inset: the deconvolution of the isotropic peak. B) 77Se VACP/MASNMR spectra of (2) at two different spinning speeds.

structure with a Se@@@N interaction of 2.605 Å and an inversion centre at the heart of a 4-

membered Se2N2 ring.

77Se VACP/MAS NMR spectra of 1 and 2 acquired at two different spinning speeds

are shown in Figure 2.2A and B, respectively.

-32-

A minimum of two spinning speeds are required to identify the isotropic chemical shifts, δiso.

The spinning sidebands change positions and intensities with variation in spinning speed;

however, provided one does not spin too fast, the spectrum is still rich with information on

the CSA.82 Superficially, the spectra of 1 and 2 look almost identical in appearance, except

for the obvious appearance of two separate resonances in the spectrum of 1. Herzfeld-Berger

(HB) analyses of these spectra reveal similar NMR parameters. There are two isotropic

chemical shifts observed for 1 at 1600 and 1593 ppm, and the CS tensor parameters for these

sites are almost identical. This indicates that there are two magnetically and

crystallographically distinct Se sites, the environments of which must differ only slightly as

indicated by the similarity of their NMR parameters. For 2, there is only one isotropic

chemical shift observed at 1565 ppm, which is consistent with its crystal structure.

Though the crystal structure is unknown for 1, the similarity of the NMR parameters

suggest that the local Se environment in 1 is very similar to that of 2; however, the

crystallographic space group must be different, due to the appearance of two distinct peaks

in the spectrum of 1. Note that right peak is broader than the left one, with a full-width at

half-height (FWHH) = 550 and 400 Hz, respectively (see the inset in Figure 2.2A). The

broadening effect in the Se resonance might be caused by a dipolar or J interactions, likely

resulting from shorter Se-N bond distances. This is confirmed by the solid-state structure of

2,40 where the bond distance between Se-N1 is 1.786 Å, compared to 1.851 Å for Se-N2.

The large spans and non-axial skews are not surprising given the planar structure of these

molecules.66 In both cases, the skew is negative, indicating that δ11 is the most distinct

component of the CS tensor. Since these types of molecules are known to have significant

-33-

aromatic character, and a π-system delocalized over the five-membered ring, the δ33

component is likely directed perpendicular to the plane of the heterocycle. The tensor

orientations will be further discussed in the theoretical section (vide infra).

[tBu-N2C2Se]2[SeCl6] (3) and [tBu-N2C2Se-Cl] (4). Complexes 3 and 4 have the

same heterocyclic structures as 1 and 2; however, the counteranions are positioned such that

one chlorine atom makes a distant contact with the Se atom (we define a “distant contact” as

a weak bonding interaction with energy significantly less than a formal covalent bond). The

distances between Se and Cl (2.850 Å for 3 and 2.605 Å for 4)40 are significantly shorter than

that in 2 (3.510 Å). Solid-state structures of 3 and 4 also reveal different types of N-Se-N

bonding arrangements: 3 has a longer Se@@@N contact (3.714 Å) than 2, which fall outside of

the sum of the van der Waals radii (3.54 Å); while in the solid-state structure for 4, the

presence of the Cl atom on Se interferes with the potential alignment of a second ring.

77Se VACP/MAS NMR spectra of 3 and 4 acquired at two different spinning speeds

are presented in Figure 2.3A and B, respectively. HB analyses of spectra 3 and 4 also reveal

similar NMR parameters to each other. Three isotropic chemical shifts were observed in the

spectrum of 3, two of which are assigned to the two [tBu-N2C2Se] cations (1522 and 1507

ppm), and one to the SeCl62- anion (935 ppm). The two observed isotropic chemical shifts

for 3 correspond to the two different crystallographic cations, and there is clearly no centre

of inversion; nonetheless, the cations are structurally similar enough that CS tensor

parameters of the two Se sites are similar. For 4, there is only one isotropic chemical shift

observed at 1452 ppm.

-34-

2000 1800 1600 1400 1200 1000 800 600 ppm

<rot = 7300 Hz

WSOLIDS simulation



140 120 100 80 60 kHz

*iso

*

#

2500 2000 1500 1000 500 ppm

200 180 160 140 120 100 80 60 40 kHz

<rot = 5700 HzWSOLIDS simulation



*iso

A)

B)

Figure 2.3 A) 77Se VACP/MAS NMR spectra of (3) at two different spinning speeds.Note:# denotes the istotropic shift for the SeCl62- anion, while * denotes impurities inthe sample. B) 77Se VACP/MAS NMR spectra of (4) at two different spinning speeds.

-35-

Compared to 1 and 2, the span of 3 and 4 is reduced by about 200 ppm, and the skews

becomes more negative (δ11 becomes even more distinct). This suggests that the contacting

Cl atom plays a role in reducing the span, as well as causing the isotropic chemical shift to

shift to a lower frequency (shift becomes more negative). The selenium nucleus is more

shielded compared to 1 and 2, with δ11 and δ22 in the plane of the molecule (where the Cl

atom is), and δ33 perpendicular to the plane of the ring. Due to the reduced symmetry of this

system, no direct comment on the precise orientations of δ11 and δ22 can be made until the

theoretical section.

[SeN2C26H36Cl][OTf] (5) and [SeN2C26H36Cl]2[SnCl6] (6). Complexes 5 and 6 are

2-chloro-1,2,5-selenadiazolium heterocylic cations which are doubly substituted by dipp

groups at each of the nitrogen position. These two complexes also have similar Se-Cl bond

lengths, of 2.323 and 2.458 Å, respectively.41 Solid-state structure for 5 indicates a planar

arrangement of the heterocyles, with a presence of a disorder (80% : 20% occupancy of

chlorine on each side). The N-Se-Cl bond angles are 92.33E and 93.14E for 5, while it is

approximately 92.8E for 6.

HB analyses of the 77Se VACP/MAS NMR spectra of 5 and 6 (Figure 2.4A and B)

reveal isotropic shifts of 1514 and 1527 ppm, respectively, and have similar CS tensor

parameters to one another.

-36-

3500 3000 2500 2000 1500 1000 500 0 ppm

250 200 150 100 50 0 kHz




*iso

3500 3000 2500 2000 1500 1000 500 0 ppm

250 200 150 100 50 0 kHz




*iso

A)

B)

Figure 2.4 A) 77Se VACP/MAS NMR spectra of (5) at two different spinning speeds. B) 77Se VACP/MAS NMR spectra of (6) at two different spinning speeds.

-37-

There are some very notable changes in the CS tensor parameters compared to the

compounds discussed thus far. First, the spans of 5 and 6 are more than double those of 3

and 4, and second, the skew is positive, meaning that δ33 is now the distinct component. The

change in κ is especially noticeable in these spectra, since the pointed, discontinuous portion

of the pattern corresponding to δ22 is now centred to the left of the isotropic shift. Clearly,

the addition of another ligand to the second nitrogen position plays a role in increasing the

span of the CS tensor, although it does not necessarily mean that the selenium atom becomes

more shielded or deshielded. The clear difference between having one or two dipp ligands

is the δ33 component; that is, the second ligand results in a significant change in the electronic

configuration around the Se atom, and the magnetic shielding along the direction of δ33

increases substantially.

[SeN2C26H36]2[SnCl6] (7), [SeN2C26H36Cl2] (8) and [SeN2C36H40Cl2] (9). Complexes

7, 8, and 9 are 2,2-dichloro-1,2,5-selenadiazolium complexes with doubly substituted dipp

groups at each of the nitrogen position. The solid-state structure of 7 reveals short Se@@@Cl

contacts from the counteranion which are approximately in the same range as that in 3 (ca.

2.74 to 2.85 Å), while in 9, the bond distance between Se-Cl is similar to that in 5 and 6 (ca.

2.29 to 2.46 Å).

A comparison of 77Se VACP/MAS NMR spectra of 7, 8 and 9 obtained at two

different spinning speeds (Figure 2.5, 2.6 and 2.7, respectively) reveal the isotropic chemical

shifts varying from 1526 to 1782 ppm.

-38-

2500 2000 1500 1000 500 0 ppm

150 100 50 0 kHz

<rot = 12000 Hz

WSOLIDS simulation



*iso

Figure 2.5 77Se VACP/MAS NMR spectra of (7) at two different spinning speeds.

3500 3000 2500 2000 1500 1000 500 0 ppm

250 200 150 100 50 0 kHz




*iso


-39-

3500 3000 2500 2000 1500 1000 500 0 ppm

250 200 150 100 50 0 kHz




*iso


Analysis of the spinning sidebands of the slow-spinning speed spectra yields fairly large spans,

ranging from 1152 to 1386 ppm. The skews of these patterns are also positive (κ = 0.37,

0.85 and 0.49 for 7, 8 and 9, respectively). The VACP/MAS spectra for 7 has a very poor

S/N, making it more difficult to extract the NMR parameters; hence, the result may not be as

accurate as the others.

A comparison of the CS tensor parameters of 8 with 5 and 6 suggests that the

additional chlorine atom reduces the span, the same trend that was observed in the case of the

singly-substituted 1,2,5-selenadiazolium ring (comparison of 1 and 2 with 3 and 4). However,

the skew of 8 is more positive than that of 5 and 6, indicating that δ11 and δ22 are oriented in

almost identical electronic environments. Though the crystal structure is unknown for 8, the

-40-

similarity of the NMR parameters between 7, 8 and 9 suggest that the local Se environment

in 8 is very similar to that of 7 and 9.

Summary of the Observed 77Se NMR Parameters. A number of trends in the

selenium CS tensors for this series of N-heterocylic compounds (1 to 9) have been observed.

First, an extra substituent at the second N atom in the ring (i.e., 5-9 vs. 1-4), increases the

span of the CS tensor significantly, and changes the sign of the skew from negative to

positive. Second, additional chlorine atoms which coordinate with selenium (i.e., 1 and 2 vs.

3 and 4, and 5 vs. 6 to 8), result in a reduction of the CSA. Formal Se&Cl bonds are best

described in the range of 2.23 to 2.46 Å, while Se@@@Cl contact falls under 2.74 to 2.85 Å

range. Also, close Se@@@Cl contacts result in a distinctive δ33 component, and similar δ11 and

δ22 components. In all cases, δ33 must be oriented perpendicular to the planar ring, and it

should be a good indicator of the direction of the lone pair (vide infra).

Finally, intra-molecular interactions between the cation and anion (e.g., 3 vs. 7) and

the Se&Cl bond distances have a great influence on the NMR parameters. This confirms that

77Se SSNMR is very sensitive to the local bonding environment of an atom, and is an excellent

probe of intra-molecular interactions between charged species; this has been demonstrated

in a large number of cases (for example Se@@@H, Se@@@N, Se@@@F, Se@@@Se, Se@@@Te

interactions).64,65

-41-

2000 1500 1000 500 0 -500 ppm

150 100 50 0 -50 kHz




*iso


2.3.2 Solid-state 77Se VACP/MAS NMR of 10 and 11

TMTSF (10). TMTSF is a precursor to the synthesis of (TMTSF)2X. 77Se

VACP/MAS NMR spectra of 10 acquired at two different spinning speeds are shown in

Figure 2.8.

The comparison of the two spinning speeds indicates that there are two similar selenium sites

at δiso = 658 and 670 ppm, both with Ω = 784 ppm, and κ = 0.64 and 0.57. These similar

NMR parameters indicates that the electronic environments of the magnetically inequivalent

selenium atoms are very similar, and they are in very good agreement with the previously

reported 77Se SSNMR study on this compound.66,67 There is a notable change in the δiso

compared to the compounds discussed so far. This unique δiso indicates an extremely different

local Se environment (C-Se-C), where the two C’s are extremely different from one another.

-42-

Compound 10 is very symmetric compared to the previous systems, and it has very different

CS tensors parameters. Also, despite the fact that this is a 5-membered ring system, there is

a double bond in between the two 5-membered rings, which has an enormous impact on the

selenium CS tensor.

(TMTSF)2PF6 (10*). An attempt has been made to acquire the solid-state 77Se

CP/MAS NMR spectrum, but was not successful mostly due to sample quality and limited

sample quantity. In addition to that, under the conventional CP/MAS, the 1H rf decoupling

is often not effective for paramagnetic systems. Very fast magic-angle spinning (VFMAS)95-97

experiment with νrot $20 kHz should have been done on this paramagnetic system to decouple

some of the strong dipolar interactions between the electrons which are delocalized in the π-

system, and the nuclei we are observing (more on this in the “future work” section).

[MeBPTSEt][OTf] (11). Complex 11 is a bis-1,2,3-thiaselenazolium cation with a

triflate anion. 77Se VACP/MAS NMR spectra of 11 acquired at two different spinning speeds

are shown in Figure 2.9. Two selenium sites were observed with two similar CS tensor

parameters, corresponding to two different local Se environments (Se and Se+). Note that

the skew of the powder pattern of one of the selenium site is close to -1, indicating that δ22

= δ33 with δ11 being the most distinct component out of the three CS tensor components. The

negative skew is like that of 1-4, likely because there are no bulky ligands at the N and S

positions.

-43-

3000 2500 2000 1500 1000 500 0 ppm

200 150 100 50 0 kHz




*iso


MeBPTSEt* radical (11*). The low sensitivity or resolution and difficulties in 1H

rf decoupling associated with large anisotropic paramagnetic shifts limit the amount of

paramagnetic systems being studied by SSNMR, and has been a long-time problem. 77Se

MAS experiment at 14 kHz with a long recycle delay (180 s) has been performed on 11*

although no signals were able to be observed. And as the case for 10*, this experiment

should have been done under very fast magic-angle spinning (VFMAS) to improve the

sensitivity by suppressing the spinning sidebands.

-44-

Solid-State 13C NMR. The 13C CP/MAS spectra of all the compounds are presented

in Appendix D. These spectra were acquired at two different spinning speeds to distinguish

the isotropic peaks from the spinning sidebands. The spectra confirm the identities and purity

of all the samples. All peaks can be assigned, and are in good agreement with solution 13C

NMR shifts.40,41

Powder X-ray Diffraction. Powder XRD of some of the compounds that have solid-

state structures are presented in Appendix E. This technique has been commonly used for a

qualitative analysis for sample impurities. For 3, some additional peaks that are observed

confirmed the sample impurities indicated by both 77Se and 13C SSNMR. For 6, the powder

XRD pattern indicates that the sample was pure. And finally, for 7, the crystal structure used

for the pattern simulation is derived from crystals grown in CH2Cl2 (where there is a CH2Cl2

solvate present), while the bulk powder is made from a THF solution, where THF remains

in the sample after the synthesis.

-45-

2.3.3 Ab Initio Calculations of Selenium Chemical Shielding Tensors

B3LYP and RHF calculations. Ab initio calculations were performed to calculate

the selenium CS tensor parameters and the orientations of the CS tensors in the molecular

frames, with the aim of correlating molecular structure, symmetry and the experimental NMR

parameters. Calculated CS tensor parameters having the best agreement with the

experimental results are presented in Table 2.2. All other calculations are summarized in

Appendix F. There were no solid-state structures for 1, 8 and 11, so no results are available

for these molecules at this time.

In general, for the singly-substituted compounds (2-4), all the calculations were in

good agreement with experimental results. For 3, intramolecular interactions between the

cation and the anion have a large influence on the magnitudes of the NMR parameters (refer

to Table F.2 in Appendix F). Calculations that were performed without considering the

SeCl62- anion seem to overestimate the magnitudes of the spans, while those performed with

the dianion yielded a better agreement with the experimental results. Calculations for 10 were

also in good agreement with the experimentally observed parameters, as shown in Table 2.2.

However, for the doubly-substituted compounds (5-7 and 9), there are significant

discrepancies between experimental and theoretical CS tensors. While the calculations do

predict larger spans in 5, 6 and 7, the magnitudes of the spans are largely overestimated. The

negative values of skews predicted by the calculations are also distinct from the

experimentally positive values. One main reason that could have caused this is insufficiently

large basis set applied on the chlorine atoms, which are very important atoms in determining

these shielding parameters due to the fact that the chlorine atoms are in the first coordination

-46-

sphere of the selenium. In general, all these calculations took a much longer time because

they have bulky ligands (dipp = 2,6(diisopropyl phenyl).

Table 2.2 Calculated 77Se NMR parameters using Gaussian 03 showing best agreement with

experimental results.

CompoundaMethod Basis Set σ11 σ22 σ33 σiso δiso (ppm) Ω (ppm) κ2 exp. 1565 (15) 712 (10) -0.32 (2)

B3LYP 14s10p5d 121.6 475.2 739.7 445.5 1327.9 618 -0.14

17s13p6db 30.7 406.3 627.2 354.7 1352.6 596.4 -0.26

3 exp. site 1 1522 (10) 539 (20) -0.77 (4)

site 2 1507(10) 559 (20) -0.67 (3)

B3LYP 13s10p4d 276.4 716.3 867.2 620.0 1371.4 590.9 -0.49

14s10p5db -71.9 398.9 552.4 293.2 1480.2 624.3 -0.51

4 exp. 1452 (5) 537 (10) -0.76 (4)

B3LYP 6-311++G**b -319.3 317.2 439.8 145.9 1490.4 759.1 -0.685 exp. 1514 (5) 1216 (15) 0.75 (4)

B3LYP 6-311++G**b 1506.4 191.2 691 -208.1 1844.3 2197.4 -0.556 exp. 1527 (5) 1269 (15) 0.62 (3)

B3LYP 14s10p5db -353.9 1194.9 1603.4 814.8 1102.5 1957.4 -0.58

RHF 14s10p5d -631.1 779.8 1112.3 420.4 1215.9 1743.4 -0.62

7 exp. 1526 (10) 1343 (25) 0.37 (2)

B3LYP 14s10p5db -2173.5 -2.2 1536.6 -213.0 1986.4 3710.1 -0.17

17s13p6d -2302.5 -105.3 1529.6 -292.8 2000.1 3832.1 -0.15

9 exp. 1782 (15) 1386 (25) 0.49 (3)

B3LYP 14s10p5d -227.2 191.5 455 139.8 1633.6 682.2 -0.23

17s13p6db -320.7 112.2 400.4 64 1643.3 721.2 -0.2

10 exp. site 1 670 (5) 785 (10) 0.64 (6)

site 2 658 (5) 784 (10) 0.57 (3)

B3LYP 3-21G**b 1050.2 1164.3 1906.1 1373.5 521.9 855.8 0.73

RHF 3-21G** 1233.4 1452.2 2190.1 1625.2 504.4 956.8 0.54

aNo calculations are done to compounds 1, 8, and 11 due to the unavailability of the crystal structure.bThese particular methods and basis sets were used to generate the CS tensor orientations in Figure 2.10.

-47-

*33

*11

*22 z to the page

2

*11

*22

*33 z to the page

3,4

*22

*33 z to the page*11

dippdipp

5,6

dippdipp

9

*11

*22

*33 z to the page

dippdipp

*11

*22

*33 z to the page

710

*11

*33

*22 z to the page

Figure 2.10 CS tensor orientations generated by Gaussian 03.Note: refer to Table 2.2 for method and basis set used for each compound.

The tensor orientations of the CS tensors in 2-7 and 9-10 are shown in Figure 2.10.

It is well known that the CS tensor components are oriented along/near the symmetry

elements, thus the most distinct component is likely oriented along the direction of the

presumed lone pair of electrons. Calculations performed upon 2 orient δ11, the most distinct

component of the tensor orientation, along the direction of the centroid of the 5-membered

ring. In 3 and 4, the Cl contact on the selenium slightly tilts δ11, the least shielded component,

from the pseudo-C2 axis to the direction of the Se-Cl bond. For 5-9, no correlation between

the molecular structure and the experimental NMR parameters can be drawn because the

theoretical calculations do not correspond to the experimental results. Finally, in 10, since

-48-

the skew is positive, the most distinct component of the tensor orientation is δ33 which has an

orientation towards the centroid of the neighbouring ring.

Summary of ab initio calculations. Inspection of all the calculations reveals that the

B3LYP method is generally superior to the RHF method. The former typically results in CS

tensor parameters that are reasonably close to the experimentally observed values (about 5

to 15% error), while the latter do not correctly predict CS tensor parameters in most cases.

In general, Huzinaga’s basis set (14s10p5d) seems to give the best agreement in predicting

the CS tensor parameters.

-49-

NSe

NMe

A

TMTSF

Se

Se

Se

Se

test molecule A

NSe

NMe Me

2

test molecule B

B

NSe

NMe

Cl

C

test molecule C

D

test molecule D

NSe

NMe Me

Cl

test molecule E

E

NNMe MeSe

ClCl

F

Figure 2.11 Schematic diagram of all the model compounds used in ADFcalculations.

2.3.4 DFT Calculations of Selenium Chemical Shielding Tensors

Amsterdam Density Functional (ADF) calculations. Pure DFT computational

methods can also be used to calculate the CS tensor parameters and their orientations within

the molecular frames. The ADF software suite has a unique program which permits a detailed

analysis of contributions from individual and mixed pairs of molecular orbitals (MOs) to the

total CS tensor. Such calculations are applied to simple model compounds with symmetry-

constrained geometry optimized structures (gas phase structures) in order to understand the

origin of the selenium chemical shielding tensors in these heterocylic systems. All of the

model compounds used for the ADF calculations are presented in Figure 2.11. Note that A

is the analogue of 1 and 2, C for 3 and 4, D for 5 and 6, E for 7, 8 and 9, and F for 10.

-50-

Selenium CS parameters from calculations performed with ADF are summarized in

Table 2.3.

Table 2.3 Non-relativistic (NR) ADF calculations of 77Se CS tensor parameters.

Compound Method Basis Set σ11a σ22 σ33 σiso

b δisoc Ωd κe

A BLYP DZf -1139.8 -517.9 -489.6 -715.7 2108.4 650.2 -0.91

TZ2Pf -706.3 32.3 183.3 -163.5 1646.8 889.6 -0.66

VWN-BP DZ -1353.3 -478.6 -197.2 -676.4 2204.2 1156.1 -0.51

TZ2P -486.9 5.1 158.6 -107.7 1718.9 645.5 -0.52

B BLYP DZ -2015.4 -632.5 441.9 -735.3 2128.0 2457.2 -0.13

TZ2P -1338.7 -111.6 530.4 -306.6 1789.9 1869.2 -0.31

VWN-BP DZ -1935.7 -594.3 462.8 -689.1 2216.9 2398.6 -0.12

TZ2P -1240.2 -57.6 555.4 -247.5 1858.6 1795.7 -0.32

C BLYP DZ -380.2 328.2 397.9 115.3 1277.4 778.1 -0.82

TZ2P -293.4 270.9 410.6 129.4 1353.9 704.1 -0.6

VWN-BP DZ -332.3 375.6 423.6 155.7 1372.2 755.8 -0.87

TZ2P -231.8 331.3 438 179.1 1432.0 669.8 -0.68

D BLYP DZ -1513.5 -542.4 -199.4 -751.7 2144.4 1314.1 -0.48

TZ2P -915.4 -13.6 296.4 -210.9 1694.1 1211.9 -0.49

VWN-BP DZ -1437.3 -480.1 -139.4 -685.6 2213.4 1297.9 -0.48

TZ2P -825.3 58.5 346.2 -140.2 1751.4 1171.6 -0.51

E BLYP DZ -1291.8 -777.7 -339.6 -803 2195.7 952.1 -0.08

TZ2P -873.6 -307.1 123.6 -352.4 1835.6 997.2 -0.14

VWN-BP DZ -1212.3 -693.4 -259.6 -721.8 2249.6 952.8 -0.09

TZ2P -780.5 -214 191.8 -267.6 1878.8 972.3 -0.17

F BLYP DZ 289.7 419.4 1405.2 704.8 687.9 1115.6 0.77

TZ2P 447.7 487.2 1477.2 804.0 679.2 1029.5 0.92

aThe chemical shielding tensor is described by three principal components, ordered such that σ11 # σ22 # σ33.bσiso = (σ11 + σ22 + σ33)/3.cThe chemical shift values were referenced to Me2Se by comparing the calculated shielding values of all compoundsto the absolute shielding value of Me2Se.dΩ = σ33 - σ11.eκ = 3(σiso - σ22)/Ω.f DZ is double-ζ and TZ2P is triple-ζ doubly polarized.

-51-

F11

F22

E

C

F22

F11

F22

B

F11

F22

A

F11

F22

D

F11

F33

F22 z to the page

F11

F33 z to the page for all molecules, except for F

Figure 2.12 CS tensor orientations for A-F generated from ADF calculations

In A and C, all the ADF calculations were in good agreement with the experimental results.

In D and E, the larger span values correspond to those observed experimentally (see Table

2.1 on 5-9), but the all skews have an opposite sign. For F, although the span was slightly

overestimated, the ADF calculations yield positive skews.

In general, the same trends in experimental results were also observed using the model

compounds: additional chlorine atom coordinated to the selenium greatly reduces the span

(B vs. D vs. E), and an extra substituent on the nitrogen position on the ring results in an

increase in the span of the CS tensor (A vs. B and C vs. D).

The selenium CS tensor orientations generated from NR calculations are presented

in Figure 2.12. These NR calculations yield almost identical tensor orientations to those

calculated with Gaussian 03.

-52-

According to DFT-GIAO formalism developed by Ziegler and co-workers,89,98 the

paramagnetic shielding contribution of the total shielding can be partitioned into

contributions arising from the mixing of occupied MOs (σpocc-occ) and mixing of the occupied

and virtual MOs (σpocc-vir). The NMR/EPR module of the ADF program outputs

paramagnetic shielding contributions associated with each MO pair (Table 2.4).

Table 2.4 Contributions to paramagnetic shielding from mixing of occ. and vir. MOs in A-F.

Compound Occupied MO Virtual MO σiso of MOpairs

σ11 σ22 σ33

MO symm.label

MO symm. label

(ppm) (ppm) (ppm)

A (Cs)a 30 25A' 37 28A' -201 -603.1

32 26A' 36 9A''(LUMO)

-324 -240.7 -731.5

32 26A' 37 28A' -216.7 -650.0

34 27A' 37 28A' -437.8 -1313.4b

35 8A''(HOMO)

37 28A' -1068.6 -3193.3 -12.6

35 8A'' 38 29A' -464.4 -0.8 -1392.5

B (C2v) 32 2A2 40 7B1

(LUMO)-207.8 -623.5

34 17A1 40 7B1 -245.4 -736.3

34 17A1 41 13B2 -636.8 -1910.4

36 18A1 42 19A1 -159.2 -477.5

39 6B1

(HOMO)41 13B2 -1245.3 -3735.8

39 6B1 42 19A1 -710.1 -2130.4

C (Cs) 36 30A' 45 11A''(LUMO)

-131.5 -400.6 6.1

38 31A' 45 11A'' -416.3 -26.4 -1222.4

38 31A' 46 35A' -184.9 -554.8

40 32A' 45 11A'' -121.6 -476.7 112.0

40 32A' 46 35A' -135.2 -405.7

41 9A'' 46 35A' -332.9 -801.4 -197.3

41 9A'' 49 37A' -268.1 -10.9 -793.4

-53-

42 33A'(HOMO -2)

45 11A'' -198.8 -565.7 -30.6

D (Cs) 45 10A'' 50 12A'' -260.3 -534.6 -246.2

45 10A'' 51 38A' -400.3 -136.1 -1064.8

46 36 A' 51 38A' -95.6 -286.7

48 11A''(HOMO)

50 12A'' -484.2 -1462.4 9.8

48 11A'' 51 38A' -197.7 -226.7 -366.2

E (C2v) 47 23A1 58 9B1

(LUMO)-311.1 -933.3

47 23A1 59 20B2 -353.6 -1060.7

49 17B2 58 9B1 -226.4 -679.1

49 17B2 60 26A1 -282.7 -848.0

51 18B2 60 26A1 -109.6 -328.7

52 7B1 59 20B2 -436.1 -1308.3

52 7B1 60 26A1 -397.9 -1193.6

57 8B1

(HOMO)59 20B2 -322.7 -968.1

57 8B1 60 26A1 -510.8 -1532.4F (D2h) 97 18B2u 106 21B3u -176.9 -530.7

99 18B1g 105 22A1g

(LUMO)-347.4 -1042.3

99 18B1g 111 8B2g -118.1 -43 -311.6103 7B2g 105 22A1g -121.7 -357 -8.4104 8B1u

(HOMO)106 21B3u -144.1 -460.3 -27.5

aPoint group of the molecule is given in the brackets.bAll shielding values in bold correspond to the highest contribution for each tensor component.

Model complex A has been chosen as the focus of discussion for this section, due

to the generally good correspondence between experimental and theoretical parameters, and

the simplicity of the molecule. The paramagnetic contributions account for approximately

90% of the total isotropic shielding and are the dominant contributions to the anisotropy

of the CS tensor; therefore, occ-occ mixing will not be considered. Due to the large

number of MO pairs, only those that contribute to greater than 6 % (ca. 200 ppm) of the

total isotropic shielding arising from occ-vir MO mixing (-3309.8 ppm) are listed. This

-54-

focuses the analysis of the MO contributions on a few MOs that are near the HOMO and/or

involved in the bonding.

The mixing of the occupied MO 34 (HOMO -1) with virtual MO 37 (LUMO +1)

contributes -437.8 ppm to the total paramagnetic shielding (σp) of -3703.6 ppm, resulting

in deshielding mainly along the direction of σ33 (Table 2.4). On the other hand, the mixing

of the occupied MO 35 (HOMO) with the virtual MO 37 and 38 accounts for -1068.6 and

-464.4 ppm of the total isotropic paramagnetic shielding, causing deshielding along the

direction of σ11 and σ22 respectively.

The analysis of contributions of individual MOs to paramagnetic shielding combined

with the calculation of gross AO populations of each MO (Table 2.5) can provide some

insights into the origin of chemical shielding at a nucleus.

Occupied MO 34 has mainly N1 2py and 2px character (35% and 21%) and a small

contribution from Se 4px and 4py atomic orbitals (AOs), while occupied MO 35 (HOMO)

has a relatively high Se pz-orbital character (68%) and slight contributions from the C1 and

C2 2pz orbitals (16 % and 12.5%) on the aromatic rings. Each of the virtual MOs 36, 37

and 38 are largely delocalized in the heterocyclic ring, but have varying amounts of Se, N1,

N2, and C2 AO character. The stereochemically active lone pair of selenium is described

by MO 35 (the HOMO), and is oriented along the direction of the σ33 component

(perpendicular to the plane of the ring).

-55-

Table 2.5 Composition of the MOs in A-F from NR calculations.

Compound MO Energy(eV)

Occ. ofMOa

Composition ofMO

SFOb (firstmember)

Occ. ofSFO

Fragment

A 34 -12.361 2 34.56% 2py 1 N120.55% 2px 1 N19.70% 4px 1.33 Se1.72% 4py 1.33 Se

35 -12.114 2 67.91% 4pz 1.33 Se15.91% 2pz 0.67 C112.49% 2pz 0.67 C2

36 -8.916 0 26.99% 2pz 1 N125.48% 2pz 0.67 C223.05% 2pz 1 N218.74% 4pz 1.33 Se

37 -8.413 0 57.70% 4py 1.33 Se16.50% 4px 1.33 Se14.88% 2py 1 N2

38 -6.634 0 42.57% 4px 1.33 Se32.34% 2px 1 N110.02% 4py 1.33 Se

B 38 -17.943 2 20.09% 2px 1 N120.09% 2px 1 N2

39 -17.094 2 71.04% 4px 1.33 Se12.90% 2px 0.67 C212.90% 2px 0.67 C1

40 -14.093 0 21.95% 2px 1 N121.95% 2px 1 N219.65% 4px 1.33 Se16.81% 2px 0.67 C216.81% 2px 0.67 C1

41 -13.699 0 75.01% 4py 1.33 Se5.92% 2pz 1 N25.92% 2pz 1 N14.17% 2py 1 N24.17% 2py 1 N1

42 -12.071 0 58.95% 4pz 1.33 Se8.05% 2pz 1 N18.05% 2pz 1 N2

C 40 -7.283 2 33.90% 2py 1 N121.51% 2px 1 N212.45% 3s 0.67 C15.23% 3s 0.67 C2

41 -6.912 2 59.66% 4pz 1.33 Se16.01% 2pz 0.67 C1

-56-

13.10% 2pz 0.67 C242 -5.501 2 71.50% 3py 1.67 Cl1

6.08% 4px 1.33 Se43 -5.039 2 91.29% 3pz 1.67 Cl144 -5.019 2 96.72% 3px 1.67 Cl145 -3.364 0 25.64% 4pz 1.33 Se

23.32% 2pz 1 N121.13% 2pz 1 N220.54% 2pz 0.67 C2

D 45 -12.098 2 43.14% 3pz 1.67 Cl139.97% 4pz 1.33 Se

46 -11.633 2 36.18% 3px 1.67 Cl122.82% 3py 1.67 Cl113.95% 2py 1 N2

47 -11.157 2 55.02% 3px 1.67 Cl117.17% 3py 1.67 Cl19.53% 2py 1 N2

48 -10.552 2 49.66% 3pz 1.67 Cl128.34% 4pz 1.33 Se

49 -8.511 0 21.50% 2pz 1 N220.55% 2pz 1 N119.05% 4pz 1.33 Se18.41% 2pz 0.67 C110.74% 2pz 0.67 C2

50 -7.127 0 51.53% 4px 1.33 Se13.94% 2px 1 N1

51 -6.02 0 50.29% 4py 1.33 Se18.92% 3py 1.67 Cl1

E 52 -7.549 2 45.83% 4px 1.33 Se49.12% 3px 1.67 Cl1, Cl2c

53 -6.825 2 54.02% 3pz 1.67 Cl1, Cl236.18% 3py 1.67 Cl1, Cl2

54 -6.569 2 98.02% 3px 1.67 Cl1, Cl255 -6.405 2 52.89% 3py 1.67 Cl1, Cl2

44.65% 3pz 1.67 Cl1, Cl256 -6.166 2 35.09% 3pz 1.67 Cl1, Cl2

25.79% 3py 1.67 Cl1, Cl257 -5.646 2 45.31% 3px 1.67 Cl1, Cl2

34.34% 4px 1.33 Se58 -4.069 0 44.84% 2px 1 N1

27.94% 2px 0.67 C116.79% 4px 1.33 Se

59 -2.1 0 60.92% 4py 1.33 Se19.08% 3pz 1.67 Cl1, Cl211.18% 3py 1.67 Cl1, Cl2

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60 -2.065 0 59.51% 4pz 1.33 Se19.24% 3py 1.67 Cl1, Cl2

F 99 -7.166 2 69.35% 4py 1.33 Se1,Se2,Se3,Se411.14% 2py 0.67 C1,C2

100 -6.71 2 60.55% 2pz 0.67 C3,C4,C5,C624.94% 2pz 0.67 C1,C2

101 -6.029 2 91.59% 4pz 1.33 Se1,Se2,Se3,Se46.96% 2pz 0.67 C3,C4,C5,C6

102 -5.775 2 89.80% 4pz 1.33 Se1,Se2,Se3,Se48.50% 2pz 0.67 C1,C2

103 -5.167 2 44.98% 4pz 1.33 Se1,Se2,Se3,Se437.82% 2pz 0.67 C3,C4,C5,C612.38% 2pz 0.67 C1,C2

104 -4.001 2 67.97% 4pz 1.33 Se1,Se2,Se3,Se419.04% 2pz 0.67 C1,C213.48% 2pz 0.67 C3,C4,C5,C6

105 -2.431 0 60.23% 4px 1.33 Se1,Se2,Se3,Se415.87% 2px 0.67 C3,C4,C5,C6

106 -1.511 0 52.20% 4px 1.33 Se1,Se2,Se3,Se419.52% 4py 1.33 Se1,Se2,Se3,Se419.13% 2s 2 C1,C28.40% 2px 0.67 C1,C2

aOcc stands for occupancy.bSFO stands for symmetrized fragment orbitals.cThere is an equal contribution from each atom, so the total contribution is shown.

-58-

+ψn | Ri | ψm , g A1

30 32 34 35(HOMO)

37 3836(LUMO)

x (F11)

y (F22)

z (F33) z into the page

Figure 2.13 The occupied and virtual MOs of A that make significantcontributions to the paramagnetic shielding term. The MOs are visualized at the96% electron density level.

All the occupied and virtual MOs that have a significant paramagnetic contribution

for A are shown in Figure 2.13.

An allowed transition (hence a contribution to chemical shielding) in spectroscopy

is based upon the symmetry considerations. With electronic spectroscopy, the translational

operators make transitions between different energy levels (hence called electronic

transitions). These electronic transitions are magnetic-dipole allowed transitions, and are

described by the three member integrands below:

where ψm is the wavefunction of the occupied MO, ψn is the wavefunction of the virtual

MO, and Ri (i = x,y,z) is the rotational operator along the direction of the magnetic

shielding; and A1 is the totally symmetric representation for the molecule.

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Γ(ψn ) q Γ( Rz ) q Γ(ψm ) ' A ) q A ) q A ) ' A )

Γ(ψn ) q Γ( Rx,y ) q Γ(ψm ) ' A ) q A )) q A )) ' A )

Model A has a point group of Cs, thus, the symmetry representations of the rotational

operators Rx, Ry and Rz are A'', A'' and A', respectively. Now, consider the 34-37 MO

mixing (see Table 2.5), that results in deshielding along the direction of σ33 (which is

coincident with the z-axis). The direct product of the symmetry representations of the three

member integrands above is evaluated:

In the case of 35-37 and 35-38 mixing, the direct product of the symmetry representations

evaluates as follows:

Simple visualization, often referred to as “charge rotation” 89,99,100 can also be used

to understand why certain MO pairs make large paramagnetic (de)shielding contributions.

MOs which contribute to the shielding along the direction of a particular principal

component must be relatively close in energy and have the correct symmetry to overlap with

one another when rotated about the direction of the tensor component. An arbitrary

convention of a right-handed ninety degree rotation of the virtual MOs about their nuclear

origins (i.e., rotating the contributing AOs about their own origins) is utilized. Three key

MO pairs which make the largest contributions to the three different CS tensor components

will be discussed (Figure 2.14 - Figure 2.16): the 34-37 MO pair for σ33, the 35-37 MO pair

for σ11, and the 35-38 MO pair for σ22.

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F33 z into the page

MO 3427A’ (HOMO -1)

top view

MO 3728A’ (LUMO +1)

top view

MO 3427A’ (HOMO -1)

side view

MO 3728A’ (LUMO +1)

side view

x (F11)

y (F22)

z (F33)

y (F22)

Figure 2.14 A representation of 34-37 MO pair, in which their mixing contributes toparamagnetic deshielding along σ33.

A right-handed 90Erotation of MO 37 about σ33 (into the page), which is coincident

to the z-axis, gives a constructive overlap (i.e., lobes of the same phase overlap) with MO

34 (see Figure 2.14). Similarly, a right-handed rotation of MO 37 about σ11 (pointing to

the direction near to the centroid of the 5-membered ring) results in a huge constructive

orbital overlap with MO 35, hence the largest deshielding contribution (Figure 2.15).

Finally, a right-handed rotation of MO 38 about the σ22 (pointing to the direction of the

methyl group) leads to a constructive overlap with MO 35 as well, but to a lesser extent

(Figure 2.16). All of the other paramagnetic contributions can be visualized in this manner

as well (see Table 2.4 and Figure 2.13 for the MO pairs).

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MO 358A’’ (HOMO)

top view

MO 358A’’ (HOMO)

side view

MO 3728A’ (LUMO +1)

top view

MO 3728A’ (LUMO +1)

side view

F11

x (F11)

y (F22)

z (F33)

y (F22)

F11 z into the page

Figure 2.15 A representation of 35-37 MO pair, in which their mixingcontributes to paramagnetic deshielding along σ11.

MO 3829A’ (LUMO +2)

top view

F 22

MO 3829A’ (LUMO +2)

side view

MO 358A’’ (H OM O )

top view

MO 358A’’ (H OMO )

side view

x (F 11)

y (F 22)

z (F 33)

y (F 22)

Figure 2.16 A representation of 35-38 MO pair, in which their mixingcontributes to paramagnetic deshielding along σ22.

-62-

39(HOMO)

34 3632

40(LUMO)

41 42x (F33) z into the page

z (F11)

y (F22)

Figure 2.17 The occupied and virtual MOs of B that make significant contributionsto the paramagnetic shielding term. The MOs are visualized at the 96% electrondensity level.

This analysis can also be applied to models B through F; however, due to the detail

and complexity of this discussion, only the main points are discussed. In the case of B, the

lone pair of electrons is also oriented perpendicular to the plane of the ring (along the

direction of σ33) and localized in the 4px orbital of Se on the MO 39 (HOMO) (see Figure

2.17).

B has a C2v symmetry with four symmetry representations (A1, A2, B1, B2), which means that

one of them does not correspond to an angular momentum operator; hence there are some

nice sets of MOs which can mix, and some which cannot. The symmetry representations

of the rotational operators Rx, Ry and Rz are B2, B1 and A2, respectively. The direct product

of the three key MO pairs which make the largest contributions to the three different CS

tensor components: the 34-41 MO pair for σ33, the 39-41 MO pair for σ11, and the 39-42

MO pair for σ22, respectively, are as follows:

-63-

Γ(ψn ) q Γ( Rx ) q Γ(ψm ) ' B2 q B2 q A1 ' A1

Γ(ψn ) q Γ( Rz ) q Γ(ψm ) ' B2 q A2 q B1 ' A1

Γ(ψn ) q Γ( Ry ) q Γ(ψm ) ' A1 q B1 q B1 ' A1

36 38 40 41

42 45(LUMO)

46 49

z (F33) z towards the page

Figure 2.18 The occupied and virtual MOs of C that make significantcontributions to the paramagnetic shielding term. The MOs are visualized atthe 96% electron density level.

In the presence of a chlorine atom attached to the selenium atom, as in the case of

C and D, the MO with large population contributions from the lone pair of electrons

localized on the selenium atom is no longer the HOMO. Population analysis for both C and

D shows that the HOMOs are now MOs which are largely comprised of p character

localized at the chlorine atoms. In C, for instance, the Se “lone pair” MO shifts to a lower

energy MO, which is MO 41 (HOMO -3). In MO 41, there is about 60% contribution from

Se 4pz atomic orbital, oriented perpendicular to the plane of the ring, as shown in Table 2.5.

The MOs that make significant contributions to the paramagnetic shielding term in C are

shown in Figure 2.18.

-64-

48(HOMO)

50 51

y (F11)

x (F22)

z (F33) z into the page

45 46

Figure 2.19 The occupied and virtual MOs of D that make significant contributionsto the paramagnetic shielding term. The MOs are visualized at the 96% electrondensity level.

If this MO containing lone pair (MO 41) is of the appropriate energy and symmetry to mix

with the virtual MOs (MO 46 and 49) which are perpendicular to it, this leads to a

significantly more deshielding in the plane (directions of σ11 and σ22), than along the

direction of the lone pair (Table 2.4). At the same time, there is less deshielding along the

direction of the lone pair (which is coincident with σ33), since there are not as many MOs

of appropriate symmetry and energy perpendicular to this direction which can mix and

produce large deshielding contributions.

The MOs that make significant contributions to the paramagnetic shielding term in

D are shown in Figure 2.19. Three key MO pairs which make the largest contributions to

the three different CS tensor components are: the 45-51 MO pair for σ22, the 46-51 MO pair

for σ33, and the 48-50 MO pair for σ11.

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47 5149 52

57(HOMO)

58(LUMO)

59 60

x (F33) z towards the pagez (F22)

y (F11)

Figure 2.20 The occupied and virtual MOs of E that make significant contributionto the paramagnetic shielding term. The MOs are visualized at the 96% electrondensity level.

In E, with the presence of the two chlorine atoms, the MO with Se “lone pair” shifts

even to a lower energy. MO 52 (HOMO -5) seems to be the best representation of the lone

pair of electrons on the selenium (its largest contribution is from the Se 4pz AO, 46%)

(Figure 2.20). Three key MO pairs which make the largest contributions to the three

different CS tensor components are: the 47-59 MO pair for σ33, the 52-59 MO pair for σ22,

and the 57-60 MO pair for σ11.

-66-

2

1

5

34

6

8

7

10 9

11

12

13 14

16

15

17

C1C2

C3

C5 C4

C6

b)a)

Figure 2.21 Labelling of the atoms for a) A-E and b) F

Mulliken charge distributions for each atom were also calculated for A-E (Table 2.6,

Figure 2.21 for labelling). The charges on the selenium atom vary from 0.68 to 1.18 in all

five different scenarios.

Table 2.6 Mulliken charge distribution of each individual atom on A-E generated from

ADF calculations using BLYP method and TZ2P basis set.

Compound A B C D E

Label Atom Charge

1 Se 0.946 1.180 0.874 0.827 0.681

2 N1 -0.451 -0.678 -0.501 -0.645 -0.557

3 C1 -0.067 0.132 -0.085 0.051 -0.010

4 C2 0.092 0.132 0.060 0.022 -0.010

5 N2 -0.676 -0.678 -0.771 -0.587 -0.557

6 H1 0.355 0.402 0.247 0.368 0.293

7 H2 0.349 0.402 0.263 0.350 0.293

8 C3 -0.415 -0.425 -0.4 -0.522 -0.517

9 H3 0.294 0.329 0.273 0.317 0.306

10 H4 0.279 0.321 0.263 0.291 0.244

11 H5 0.294 0.329 0.263 0.317 0.306

12 C4 -0.425 -0.514 -0.517

13 H6 0.329 0.337 0.306

14 H7 0.321 0.302 0.244

15 H8 0.329 0.337 0.306

16 Cl1 -0.485 -0.251 -0.407

17 Cl2 -0.407

Total Charge 1 2 0 1 0

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99

105 (LUMO)

104 (HOMO)

106 111

97 103

F33

F11

y

x

z (F22) z towards the page

Figure 2.22 The occupied and virtual MOs of F that make significant contribution tothe paramagnetic shielding term. The MOs are visualized at the 96% electron densitylevel.

And finally for F, the MO with two lone pairs of electrons seem to localize around

the HOMO of the molecule (Table 2.5). MO 99, 101, 102, 103 and 104 (HOMO) all show

a large contribution from the 4 Se 4pz orbitals, thus in a good position to easily mix with

virtual MOs perpendicular to it to create deshielding in directions perpendicular to the

mixing. For instance, MO 99 mixes with MO 111 to create the largest contribution to the

deshielding along σ33, and also the 104-106 MO pair mixing contributes the most to the

deshielding along σ11, which is perpendicular to the direction of the lone pairs (coincident

with σ22).

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2.4 Conclusions

77Se VACP/MAS NMR spectra of selenium-containing heterocycles reveal

expansive selenium chemical shift tensors, which are extremely sensitive to molecular

geometry, symmetry and ligand substitution. 77Se SSNMR is an excellent probe for

detecting intra-molecular interactions between charged species, as they have a great

influence on the NMR parameters. An extra substituent group at the second N atom in the

ring increases the span of the CS tensor significantly, and changes the sign of the skew from

negative from positive. Additional chlorine atoms coordinated with the selenium atom

result in a reduction of the CSA.

Gaussian 03 calculations are capable of reproducing experimental CS tensors in

cases not involving Cl contacts, as well as predicting their orientations with respect to the

molecular frames. In the cases involving Cl contacts, Gaussian 03 calculations overestimate

the spans and predict opposite signs of skew compared to the experimental values.

Applying larger basis sets on the chlorine atoms might have been done to improve these

calculations. If the chlorine is a part of an anion, perhaps the whole anion needs to be

included; and although this is doubtful, perhaps Gaussian 03 calculations are insufficient for

modeling these systems.

ADF calculations on model systems are being applied to explain the origin of the CS

tensors from the perspective of paramagnetic shielding arising from induced mixing of

occupied and virtual MOs. The analysis of contributions from individual and mixed pairs

of MOs generated by non-relativistic (NR) calculations provides insight into the molecular

origins of selenium chemical shielding in these heterocyclic systems, as well as the existence

-69-

and position of the stereochemically active lone pair of selenium. The stereochemically

active lone pair of selenium is oriented perpendicular to the plane of the heterocylic ring and

described by the MOs that are near the HOMO. This MO containing lone pair has to have

an appropriate energy and symmetry to mix with the virtual MOs which are perpendicular

to it, to be able to create deshielding along the direction of the lone pair.

2.5 Future Work

Preliminary results for both 10 and 11 have been obtained in order to get some

parameters that can help acquiring their corresponding paramagnetic species, 10* and 11*.

We will apply very fast magic-angle spinning (VFMAS) experiments95-97 in the hope that

effects of paramagnetic relaxation can be eliminated, thereby permitting the acquisition of

informative NMR spectra. These spectra, in turn, should provide information on the

delocalization of electron density throughout the π-systems of these molecules. Ab initio

calculations will be applied to model the distribution of unpaired electrons in these highly

conjugated systems. In addition, it is our strong belief that experimental and theoretical

investigations of selenium CS tensors can be applied to deduce molecular structures in

numerous organic semi-conductors and heterocycles for which crystal structures are not

available.

-70-

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(76) Barrie, P. J.; Clark, R. J. H.; Withnall, R.; Chung, D.-Y.; Kim, K.-W.; Kanatzidis,

M. G. Inorganic Chemistry 1994, 33, 1212-1216.

(77) Duddeck, H.; Wagner, P.; Mueller, D.; Jaszberenyi, J. C. Magnetic Resonance in

Chemistry 1990, 28, 549-552.

(78) Koch, W.; Lutz, O.; Nolle, A. Zeitschrift fuer Physik A: Atoms and Nuclei (1975)

1978, 289, 17-20.

(79) ADF2005.01; SCM, Theoretical Chemistry, Vrije Universiteit: Amsterdam.

(80) Bartlett, P. N.; Essex, J. W.; Koo, H. J.; Nandhakumar, I. S.; Robertson, N.;

Whangbo, M. H. Journal of Physical Chemistry B 2000, 104, 7394-7402.

(81) Harris, R. K. Nuclear Magnetic Resonance Spectroscopy, 1986.

(82) Herzfeld, J.; Berger, A. E. Journal of Chemical Physics 1980, 73, 6021-6030.

(83) Eichele, K.; Wasylishen, R. E. In WSolids1: Solid-State NMR Spectrum Simulation,

2001.

(84) Frisch, M. J. e. a.; Gaussian, Inc.: Pittsburgh, 2003.

(85) Becke, A. D. Physical Review A 1988, 38, 3098-3100.

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(86) Huzinaga, S.; Andzelm, J. Gaussian Basis Sets for Molecular Calculations;

Elsevier: New York, 1984; Vol. 16.

(87) Schreckenbach, G.; Ruiz-Morales, Y.; Ziegler, T. Journal of Chemical Physics

1996, 104, 8605-8612.

(88) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; Van

Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. Journal of Computational Chemistry 2001,

22, 931-967.

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1211.

(93) Kraus, W.; Nolze, G.; Federal Institute for Materials Research and Testing: Berlin,

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(94) Kraus, W.; Nolze, G. Journal of Applied Crystallography 1996, 29, 301-303.

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Appendix A Experimental details for the synthesis of un-published 77Se heterocyliccarbenoid compounds.

Compound 1. Me3SiOSO2CF3 (0.011 mL, 0.0608 mmol) was added to a colourless

stirred CH2Cl2 (5 mL) solution of 4 (0.0137 g, 0.0606 mmol) at RT. A white precipitate

formed immediately and the mixture was allowed to stir for 10 minutes. Hexanes (10 mL)

were added and the resulting suspension was centrifuged. The solids were washed with

hexanes (3 x 10 mL) and the volatiles removed in vacuo to give 1 as a white powder. 0.0204

g; 99% yield.

Compound 5. Me3SiOSO2CF3 (0.055 mL, 0.304 mmol) was added to an orange

CH2Cl2 (5 mL) solution of 3 (0.205 g, 0.304 mmol) at RT giving an orange solution. After

stirring for 30 minutes the solvent was removed in vacuo giving an orange resin. Di-ethyl

ether (5 mL) was added giving a yellow slurry. The mixture was centrifuged and the solution

was decanted. The solids were washed with di-ethyl ether (2 x 5 mL) and volatiles removed

in vacuo giving 5 as a yellow solid. 70% yield .

Compound 6. Diethyl ether (10 mL) was added to 7 giving an orange solution. After

120 seconds an orange solid precipitated. The mixture was centrifuged and the solution was

decanted. The solids were washed with di-ethyl ether (2 x 5 mL) and volatiles removed in

vacuo giving 6 as an orange solid. 65% yield.

Compound 8. Dipp-DAB (2.89 g, 7.33 mmol) was dissolved in THF (15 mL). A

THF solution of SeCl2 (0.62 mL, 7.33 mmol, 0.4738 mol/L) was added to the stirred solution

of dipp-DAB at -78°C, immediately giving a neon red mixture. After 1 hour, the red solution

was warmed to RT and reduced in vacuo to give 8 as a bright orange-red powder. 3.98 g;

-80-

96% yield.

Compound 9. Dipp-BIAN (0.407 g, 0.626 mmol) was dissolved in THF (15 mL).

A THF solution of SeCl2 (1.2 mL, 0.626 mmol, 0.5484 mol/L) was added to the stirred

solution of dipp-BIAN at RT, immediately resulting in a deep red mixture. After 20 minutes,

the red solution was reduced in vacuo to give 9 as a red powder. 0501 g; 100% yield.

-81-

Appendix B 77Se VACP/MAS NMR experimental parameters.

Compound SpectraFreq.

νrot pw90(H) ν2 (CP) contacttime

spectralwidth

recycledelay

# scans

(MHz) (kHz) (Fs) (kHz) (ms) (kHz) (s)

1 76.350 4.5 2.0 67.0 17 200 1 45900

76.350 7.3 2.0 67.0 17 200 1 34700

2 76.260 4.5 1.6 57.9 17 200 1 16300

76.260 5.7 1.6 57.9 17 200 1 45600

3 76.330 4.5 1.6 57.9 17 200 3 7732

76.330 7.3 1.6 57.9 17 200 3 15676

4 76.346 4.5 1.6 57.9 17 200 1 5000

76.346 5.7 1.6 57.9 17 200 1 6400

5 76.362 4.5 1.6 71.6 17 300 1 12000

76.362 6.3 1.6 71.6 17 300 1 73500

6 76.362 4.5 1.6 71.6 17 300 1 57700

76.362 7.3 1.6 71.6 17 300 1 19800

7 76.260 11 4.0 62.5 15 250 5 35360

76.260 12 4.0 62.5 15 250 5 13544

8 76.362 4.5 1.6 71.6 17 300 4 8100

76.362 5.8 1.6 71.6 17 300 4 11700

9 76.260 4.5 1.6 71.6 17 300 4 18900

76.260 5.8 1.6 71.6 17 300 4 19712

10 76.283 5.6 2.0 87.0 15 250 30 232

76.283 7.185 2.0 87.0 15 250 30 164

11 76.331 4.5 1.8 53.0 22 300 4 13136

76.331 7.3 1.8 53.0 22 300 4 2664

76.331 10 1.8 53.0 22 300 4 2076

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Appendix C 13C CP/MAS NMR experimental parameters.

Compound SpectraFreq.

νrot pw90(H) ν2 (CP) contacttime

spectralwidth

recycledelay

# scans

(MHz) (kHz) (Fs) (kHz) (ms) (kHz) (s)

1 100.524 5 1.5 35.0 10 50 3 500

100.524 6.5 1.5 35.0 10 50 3 500

2 100.515 4 1.5 87.6 9 50 3 352

100.515 5.5 1.5 87.6 9 50 3 264

3 100.524 7 1.5 35.0 10 50 3 344

4 100.515 4 1.6 87.6 9 50 3 316

100.515 5.5 1.6 87.6 9 50 3 300

5 100.524 4 1.5 35.0 10 50 3 500

100.524 5.5 1.5 35.0 10 50 3 500

6 100.524 5 1.5 35.0 10 50 3 400

100.524 6.5 1.5 35.0 10 50 3 400

7 100.524 5 1.5 35.0 10 50 3 3368

100.524 6.5 1.5 35.0 10 50 3 1904

8 100.515 4 1.6 87.6 9 50 4 800

100.515 7 1.6 87.6 9 50 4 848

9 100.515 4 1.6 87.6 9 50 4 400

100.515 6.5 1.6 87.6 9 50 4 400

10 100.524 1.95 1.6 89.9 15 50 60 132

100.524 6 1.6 89.9 15 50 60 88

11 100.524 5 1.6 39.6 16 100 4 776

100.524 7.9 1.6 39.6 16 100 4 776

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350 300 250 200 150 100 50 0 -50 ppm

<rot = 5000 Hz

<rot = 6500 Hz

N N

SeCB

H3CA

CAH3

CAH3

CC CD

CF3SO3-

Solid-State Solution NMR Assignment (ppm) (ppm)

157.5 155.8/153.5 CC/CD

121.2 118.9 CF3 71.5 69.2 CB

32.5 31.9 CA

Figure D.1 13C CP/MAS NMR spectra of (1) at two different spinning speeds.

<rot = 4000 Hz

<rot = 5500 Hz

250 200 150 100 50 0 -50 -100 -150 ppm


155.7 156.8 CC/CD

71.0 71.0 CB

32.9 31.9 CA

N N

SeCB

H3CA

CAH3

CAH3

CC CD

GaCl4-


Appendix D 13C CP/MAS spectra of 1-11.

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200 150 100 50 0 ppm

75 70 65 ppm

40 35 30 25 20 ppm


160.7 - *157.1 153.7 CD

153.8 152.8 CC

71.7/70.4 - * 69.1 68.0 THF 67.5 65.3 CB

33.3 - * 32.7 31.7 CA

26.5 25.9 THF

N N

SeCB

H3CA

CAH3

CAH3

CC CD

SeCl62-

<rot = 7000 Hz

Figure D.3 13C CP/MAS NMR spectra of (3) at 7000 Hz.

250 200 150 100 50 0 -50 -100 -150 ppm

<rot = 4000 Hz

<rot = 5500 Hz


157.1 153.7 CD

154.3 152.8 CC

67.2 65.2 CB

30.3 31.7 CA

N N

SeCB

H3CA

CAH3

CAH3

CC CD

Cl


-85-

350 300 250 200 150 100 50 0 -50 -100 ppm

<rot = 4000 Hz

<rot = 5500 Hz

Solid-State Solution NMR (ppm) (ppm)164.4 -143.6 143.2141.9 -135.4 133.5131.0 132.7124.8 125.3122.4 125.2119.2 124.3 28.7 28.9 25.1 24.0 24.2 23.5


Solid-State Solution NMR (ppm) (ppm)

162.1 160.7143.1 143.1

136.0 133.8/132.8 125.4 125.1 29.9 29.7

26.7 25.3 24.2 24.9

250 200 150 100 50 0 ppm

<rot = 5000 Hz

<rot = 6500 Hz


-86-

350 300 250 200 150 100 50 0 -50 -100 ppm

<rot = 6500 Hz

<rot = 5000 Hz

Solid-State Solution NMR (ppm) (ppm)

143.8 143.1; 160.7125.3 125.1; 132.8; 133.8 71.6 70.8

- 53.6 25.6 24.1; 24.9; 25.3; 29.7


350 300 250 200 150 100 50 0 -50 ppm

Solid-State (ppm)159.6159.0

141.4140.7139.9139.1137.4132.3130.6126.1124.6123.7

30.2 29.5 27.7 26.5 25.6 23.7 23.0 22.7

140 120

40 20 ppm

<rot = 7000 Hz

<rot = 4000 Hz


-87-

350 300 250 200 150 100 50 0 -50 ppm

Solid-State Solution NMR(ppm) (ppm)164.8 -164.1 163.3

142.9 -142.4 -140.3 -138.5 138.9133.8 132.4131.6 131.2130.6 -129.2 128.9128.2 128.6127.1 127.1124.9 125.8124.4 124.6

- 68.2 30.3 - 29.4 29.5 28.1 - 25.9 25.8 25.4 - 24.8 - 24.3 24.1 23.4 - 22.5 - 21.2 -

<rot = 6500 Hz

<rot = 4000 Hz

150 140 130 120 ppm


100150 50 0 ppm

<rot = 6000 Hz

<rot = 1950 Hz

C2

C1

C7

C8 C3

C9

C10 C5

C4C6

Solid-State Assignment(ppm)131.4 C3/C8 129.7 C2/C7 99.7 C1/C6 18.7/18.5 C4/C6/C9/C10


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25 0 20 0 15 0 10 0 50 0 -5 0 pp m

<rot = 5000 Hz

<rot = 7900 Hz

Solid-State Assignments(ppm)161.63 5/5'/6/6'158.57 5/5'/6/6'134.37 5/5'/6/6'131.51 5/5'/6/6'122.54 4119.26 7 45.15 1/2/3/? 24.02 1/2/3/? 13.26 1/2/3/? 2.22 1/2/3/?

N6'

5'4

5

6

SSe

NN

SeS

Et

Me

+

OTf

1,2

3

7


-89-

5 10 15 20 25 30 35 40 45

1526

763

0

(simulation)(experimental)

Figure E.1 Powder XRD pattern of 3

5 10 15 20 25 30 35 40

1788

894

0



Appendix E Powder X-ray diffraction pattern for 3, 6 and 7.

-90-

5 10 15 20 25 30 35 40

1372

686

0



-91-

Appendix F

Table F.1 Calculated 77Se NMR parameters for [tBu-N2C2Se][GaCl4] (2)

Method Basis Setb σ11 σ22 σ33 σiso δiso (ppm) Ω (ppm) κexperimental 1565 (15) 712 (10) -0.32 (2)

B3LYP 13s10p4d -32.2 -26.0 -4.4 -20.9 2012.3 27.8 0.56

14s10p5da 121.6 475.2 739.7 445.5 1327.9 618.0 -0.14

17s13p6d 30.7 406.3 627.2 354.7 1352.6 596.4 -0.26

RHF 13s10p4d -27.4 -25.2 17.7 -11.6 2360.0 45.2 0.90

14s10p5d 57.0 908.1 1064.8 676.7 1380.2 1007.8 -0.69

17s13p6d -11.6 866.4 981.9 612.2 1385.1 993.6 -0.77

a all the numbers in italic indicate the best agreement with experimental results.b The basis set indicated in the table was only applied to the Se atom, while 6-311G** basis set was applied to all theother atoms.

Table F.2 Calculated 77Se NMR parameters for [tBu-N2C2Se]2[SeCl6] (3)

Method Basis Setb σ11 σ22 σ33 σiso δiso (ppm) Ω (ppm) κ

experimental site 1 1522 (10) 539 (20) -0.77 (4)

site 2 1507(10) 559 (20) -0.67 (3)

consider dianion (SeCl62-)

B3LYP 6-31G** 175.3 680.8 816.8 557.6 1078.6 641.5 -0.58

6-311G** -300.4 171.3 401.0 90.6 1545.6 701.4 -0.34

13s10p4da 276.4 716.3 867.2 620.0 1371.4 590.9 -0.49

14s10p5d -71.9 398.9 552.4 293.2 1480.2 624.3 -0.51

17s13p6d -173.0 259.0 465.0 183.7 1523.6 638.1 -0.35

24s20p10d -243.9 214.7 444.2 138.4 1514.9 688.1 -0.33

RHF 6-31G** 139.3 727.0 1123.9 663.4 972.8 984.6 -0.19

6-311G** -303.4 268.2 823.9 262.9 1654.4 1127.3 -0.01

13s10p4d 187.7 731.7 1152.2 690.5 1657.8 964.5 -0.13

14s10p5d -120.1 437.7 907.4 408.3 1648.5 1027.5 -0.09

17s13p6d -194.4 327.1 855.2 329.3 1668.0 1049.6 0.01

-92-

not consider dianion

B3LYP 6-31G** -160.3 542.4 821.2 401.1 1235.1 981.4 -0.43

6-311G** -696.9 147.8 364.4 -61.6 1697.8 1061.3 -0.59

3-21G** -107.3 486.2 774.1 384.3 1511.1 881.4 -0.35

13s10p4d -93.7 553.7 824.4 428.1 1563.3 918.2 -0.41

14s10p5d -395.8 287.0 564.1 151.8 1621.6 960.0 -0.42

17s13p6d -502.8 214.0 461.8 57.7 1649.6 964.6 -0.49

24s20p10d -639.2 198.9 406.4 -11.3 1664.6 1045.6 -0.60

RHF 6-31G** -398.6 992.0 1104.0 565.8 1070.5 1502.6 -0.85

6-311G** -903.1 657.9 744.0 166.3 1751.0 1647.1 -0.90

3-21G** -341.9 912.7 1000.1 523.6 1606.0 1342.1 -0.87

13s10p4d -364.0 1028.2 1112.5 592.3 1756.1 1476.5 -0.89

14s10p5d -623.4 781.8 892.0 350.1 1706.7 1515.4 -0.85

17s13p6d -693.2 728.7 817.2 284.2 1713.1 1510.4 -0.88


Table F.3 Calculated 77Se NMR parameters for [tBu-N2C2Se-Cl] (4)


experimental 1452 (5) 537 (10) -0.76 (4)

B3LYP 6-311++G**a -319.3 317.2 439.8 145.9 1490.4 759.1 -0.68

13s10p4d -24.9 -12.5 15.4 -7.3 1998.7 40.3 0.38

14s10p5d -24.3 -12.1 15.2 -7.1 1780.5 39.6 0.38

17s13p6d -24.3 -12.1 15.2 -7.1 1714.4 39.6 0.38

RHF 6-311++G** -127.2 490.4 924.1 429.1 1488.2 1051.3 -0.17

13s10p4d -24.9 -8.6 16.8 -5.6 2353.9 41.7 0.22

14s10p5d 111.6 675.8 1060.3 615.9 1440.9 948.7 -0.19

17s13p6d 48.3 579.2 1003.1 543.5 1453.8 954.7 -0.11


-93-

Table F.4 Calculated 77Se NMR parameters for [SeN2C26H36Cl][OTf] (5)


experimental 1514 (5) 1216 (15) 0.75 (4)B3LYP 6-311++G**a 1506.4 191.2 691.0 -208.1 1844.3 2197.4 -0.55

14s10p5d -1858.4 195.2 1744.4 27.1 1868.3 3602.8 -0.14

17s13p6d -1903.0 266.5 1756.3 39.9 1803.3 3659.3 -0.19

RHF 14s10p5d -3365.6 96.2 1658.3 -537.1 2310.4 5023.9 -0.38

17s13p6d -2372.5 -118.0 1532.2 -319.4 2026.7 3904.8 -0.15

a all the numbers in italic indicate the best agreement with experimental results.b 6-311G** basis set was applied to the 2N and 2C atoms on the ring, while smaller basis set (3-21G**) was applied tothe rest of the atoms.

Table F.5 Calculated 77Se NMR parameters for [SeN2C26H36Cl]2[SnCl6] (6)


experimental 1527 (5) 1269 (15) 0.62 (3)B3LYP 14s10p5da -353.9 1194.9 1603.4 814.8 1102.5 1957.4 -0.58

17s13p6d -792.6 922.6 1356.2 495.4 1421.9 2148.9 -0.60

RHF 14s10p5da -631.1 779.8 1112.3 420.4 1215.9 1743.4 -0.62

17s13p6d -1134.0 422.8 772.4 20.4 1615.9 1906.4 -0.63


Table F.6 Calculated 77Se NMR parameters for [SeN2C26H36]2[SnCl6] (7)


experimental 1526 (10) 1343 (25) 0.37 (2)

B3LYP 13s10p4d -45.8 3.3 10.9 -10.6 2002.0 56.8 -0.73

14s10p5da -2173.5 -2.2 1536.6 -213.0 1986.4 3710.1 -0.17

17s13p6d -2302.5 -105.3 1529.6 -292.8 2000.1 3832.1 -0.15

RHF 14s10p5d -2520.9 494.1 1372.4 -218.2 2275.0 3893.3 -0.55

17s13p6d -2247.9 431.0 1379.4 -145.8 2143.1 3627.4 -0.48


-94-

Table F.7 Calculated 77Se NMR parameters for [SeN2C36H40Cl2] (9)

Method Basis Setb σ11 σ22 σ33 σiso δiso (ppm) Ω (ppm) κexperimental 1782 (15) 1386 (25) 0.49 (3)

B3LYP 13s10p4d -30.4 -18.1 -7.5 -18.7 2010.1 22.9 -0.07

14s10p5da -227.2 191.5 455.0 139.8 1633.6 682.2 -0.23

17s13p6d -320.7 112.2 400.4 64.0 1643.3 721.2 -0.20

a all the numbers in italic indicate the best agreement with experimental results.b 6-311G** basis set was applied to the 2N and 2C atoms on the ring, while smaller basis set (3-21G**) was applied to

the rest of the atoms.

Table F.8 Calculated 77Se NMR parameters for TMTSF (10)


experimental site 1 670 (5) 785 (10) 0.64 (6)

site 2 658 (5) 784 (10) 0.57 (3)

solid phase

B3LYP 3-21G**a 1050.2 1164.3 1906.1 1373.5 521.9 855.8 0.73

6-31G** 1055.5 1134.2 1951.2 1380.3 462.9 895.7 0.82

6-311G** 651.2 861.6 1634.6 1049.2 587.1 983.5 0.57

RHF 3-21G** 1233.4 1452.2 2190.1 1625.2 504.4 956.8 0.54

6-31G** 1256.4 1455.3 2251.6 1654.4 426.4 995.2 0.60

6-311G** 924.4 1246.4 2009.4 1393.4 523.9 1085.0 0.41

gas phase

B3LYP 3-21G** 977.1 1029.2 1941.4 1315.9 579.5 964.4 0.89

6-31G** 937.4 1042.6 1995.9 1325.3 517.9 1058.5 0.80

6-311G** 646.4 669.1 1685.4 1000.3 636.0 1038.9 0.96

RHF 3-21G** 1241.2 1296.8 2221.0 1586.3 543.3 979.8 0.89

6-31G** 1270.1 1296.2 2288.7 1618.3 462.5 1018.6 0.95

6-311G** 948.4 1094.9 2050.0 1364.4 552.9 1101.6 0.73

a all the numbers in italic indicate the best agreement with experimental results.b The basis set indicated in the table was applied to all the atoms.

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Vita Auctoris

Andre Sutrisno was born in Jakarta, Indonesia on March 8, 1985. He graduated from Canisius

College High School in June 2002. He is currently a candidate for a BSc.[H] Degree in

Biochemistry at the University of Windsor and hopes to graduate in Spring 2007.

experimental and theoretical investigations of selenium...

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