Nathan R. Pillsbury, Timothy S. Zwier, Department of Chemistry, Purdue University, West Lafayette, IN 47907; David F. Plusquellic, NIST, Gaithersburg, MD 20899; W. Leo Meerts, Radboud University Nijmegen, The Netherlands.

CONFORMATION-SPECIFIC ELECTRONIC SPECTROSCOPY OF JET-COOLED BIS(2-HYDROXYPHENYL)METHANE

O OH H

Why study Bis(2-hydroxyphenyl)methane (2HDPM)?

O OH H

1. It is similar to diphenylmethane which was recently studied in our lab which was found to have very interesting spectroscopy

2. No synthesis needed!

0.00 kcal/mole 0.69 kcal/mole

DFT Calculations

π H-bonded conformerOH…OH H-bonded conformer

2 low-energy minima

Flu

ore

sc

en

ce

Sig

na

l

36800366003640036200360003580035600Wavenumbers (cm

-1)

LIF Spectrum of 2HDPM

O OH H

Records the UV spectrum of a single conformationfree from interference from others present in the expansion

Laser Timing

50-500nsec

UVHole-burn

UVprobe

UV-UV Hole-burning Spectroscopy

UV Hole-burn laser fixed: Provides selectivity UV probe laser tuned

Boltzmann distributionof conformers in the pre-expansion

Collisional cooling to zero-point vibrational level

B*

B*

B*CA

B*

C

CB

A

A

A

CA

ABC C

AAB BBB B

B B

UV

UV

C

2HDPM* (S1)

2HDPM (S0)

Hol

e-bu

rn

Pro

be

Conformer AConformer B

Hol

e-bu

rn

Pro

beTFTF

Flu

ore

sc

en

ce

Sig

na

l

36800366003640036200360003580035600Wavenumbers (cm

-1)

LIF

A

B

LIF and UVHB Spectra of 2HDPM

Flu

ore

sce

nc

e S

ign

al

36000359003580035700Wavenumbers (cm

-1)

A

B

Origin Regions of Conformers A and B

A

B

26

41

22

Fluorescence-Dip Infrared Spectroscopy (FDIRS)

Records IR spectrum of single species free from interference from others present in the expansion

2HDPM *(S1)

2HDPM (A) (S0)

2HDPM *(S1)

2HDPM (A) (S0)

S0 FDIRS S1 FDIRS

Total Fluorescence

Total Fluorescence

IR

UV

IR

UV

Dep

leti

on

(a.

u.)

3650360035503500Wavenumbers (cm

-1)

S0 A

S0 B

S0 FDIRS

OH…OH H-bonded conformer

π H-bonded conformer

OH…OH(f)OH(d)/torsioncombination

band

OH(d)…O

OH(d)…

leak-thru from A

leak-thru from A

IR In

ten

sity

(a.

u.)

3650360035503500Wavenumbers (cm

-1)

Experiment

Calculated

S0 A

Conformer A Experimental OH-stretch vs. Calculated

IR In

ten

sity

(a.

u.)

37003650360035503500Wavenumbers (cm

-1)

Experiment

Calculated

S0 B

Conformer B Experimental OH-stretch vs. Calculated

De

ple

tio

n (

a.

u.)

370036503600355035003450340033503300Wavenumbers (cm

-1)

S1 A

S1 B

18 30

S1 FDIRS

S0

S0

OH/torsioncombination bands

Ring 1(?) Ring 2(?)

Two rings now inequivalent!Strong OH stretch/torsion coupling

leak-thru from A

leak-thru from A

VOH=1

En

erg

y

OH vibration and ring torsion are very strongly coupled in the excited state

Why do we see seven OH stretches instead of one in the excited state of B?

= dihedral angle between the two rings

ring torsion levels

S1 zero-point levelVOH=0

Flu

ore

scen

ce

In

ten

sit

y

20x103

100-10-20MHz

-800 -400 0MHz

High Resolution UV Spectrum of 2HDPM A Origin

Experimental Spectrum

Simulated Fit

D. Plusquellic (NIST)

What information can we get from the high resolution spectra?

2HDPM Conformer A Origin:

A = 1159.6B = 412.5 C = 348.1

ΔA = -39.5ΔB = 9.5ΔC = 1.2

%A = 67%B = 24%C = 9

Ground State Rotational Constants

Change in Constants Upon Excitation

Transition Moment Direction

Predicted Ground State Constants

A = 1168.7 B = 404.6 C = 336.9

DFT MP2

A = 1129.6 B = 418.7 C = 346.6

a

b

c

Flu

ore

scen

e In

ten

sity

80x103

6040200-20-40MHz

High Resolution UV Spectrum of 2HDPM B Origin Region

vibronic bands of A

B origin

Experimental Spectrum

Simulated Fit

Simulated Fit

High Res. Spectrum of 2HDPM B +22 cm-1 Vibronic Band

Flu

ore

sc

en

e I

nte

ns

ity

30x103

20100-10-20MHz

-12.5x103

-12.0 -11.5 -11.0MHz

Experimental Spectrum

Simulated Fit

2HDPM Conformer B +22 cm-1 Band:

A = 1326.7B = 402.4 C = 368.8

ΔA = 27.7ΔB = -1.2ΔC = -5.7

%A = 85%B = 15%C = 0

Ground State Rotational Constants

Change in Constants Upon Excitation

Transition Moment Direction

Predicted Ground State Constants

A = 1358.0 B = 389.7 C = 360.7

DFT MP2

A = 1302.6 B = 412.1 C = 376.9

Conformer B Vibronic Band Fit Results

a

b

c

Dispersed Fluorescence Spectra of AF

luo

resc

ene

In

ten

sity

3002001000Wavenumbers

D = 2.3

612.9852.21108.31344.3

26 cm-1

FC progressionA Origin

+ 31 cm-1

+ 62 cm-1

Flu

ore

sc

en

e I

nte

ns

ity

3002001000Wavenumbers

D = 2.3 612.9

852.2

1108.31344.3

26 cm-1

FC progression

A Origin

+ 41 cm-1

+ 83 cm-1

Dispersed Fluorescence Spectra of A Cont.

Flu

ore

scen

e In

ten

sity

3000200010000Wavenumbers

B origin

+ 22 cm-1

+ 44 cm-1

Dispersed Fluorescence Spectra of B

* * * * *

*

*

What could be causing these strange intensities?

1. There could be some vibronic coupling to the nearby S2 state

2. Duschinsky mixing

3. It is possible that we may be seeing fluorescence to another minimum in the ground state

Future Studies of 2HDPM

SEP-Population Transfer Study:

What are the barriers to isomerization between conformers A and B?

Predicted Transition States (DFT):

A

B1885 cm-1

1399 cm-1

TS

TS

Conclusions

1. The two conformations of 2HDPM have been assigned using FDIRS and high resolution UV spectroscopy

2. We observed strong coupling between the OH- stretch and inter-ring torsion in the S1 state of conformer B

3. Dispersed fluorescence spectra show interesting features which we are currently attempting to explain

Acknowledgements

Professor Timothy Zwier

The Zwier GroupJasper Clarkson*Talitha SelbyEsteban Baquero*V. Alvin ShubertTracy LeGreveJosh NewbyBill JamesChirantha RodrigoDr. Ching-Ping Liu

Collaborators

Dr. David F. Plusquellic (NIST)

Professor W. Leo Meerts (Radboud University)

FundingFrederick N. Andrews Fellowship

U.S. Department of Energy

S0

S1

Zero-point levelCB

A

IV. UV Probe, 3II. UV Dump, 2 I. UV Pump, 1

Excited vibrational

LevelA*

III. Collisionalcooling,isomerization

SEP-Population Transfer Spectroscopy