the electronic structure and properties of pristine and protonated 1-azapolyacetylenes

The electronic structure and properties of pristine and protonated

1-azapolyacetylenes

M. Schwartza,*, A.N. Davisb, A.T. Yeatesb, R.J. Berryb, D.S. Dudisb

aDepartment of Chemistry, University of North Texas, P.O. Box 305070, Denton, TX 76203-5070, USAbPolymer Branch, Materials Directorate, Wright-Patterson AFB, OH 45433, USA

Received 20 December 2002; revised 17 February 2003; accepted 4 March 2003

Abstract

Ionization energies (IEs), electron affinities (EAs) and the lowest p ! pp transition energies, DEðS0 2 S1Þ and DEðS0 2 T1Þ;

for a series of polyacetylenes (PA), 1-azapolyacetylenes (APA) and the protonated azapolyacetylenes (HAPA) were computed

at the CASSCF/6-31G(d) and ROHF/6-31G(d) levels.

Whereas introduction of the terminal imino group has almost no effect on any of the electronic properties, changes induced

by protonation of the group are dramatic. As expected, IEs are greatly increased; the effect persists in systems with as many as

10–15 double bonds. EAs and excitation energies are lowered markedly, and large structural differences from the non-

protonated species are found, even in the asymptotic large molecule limit.

Observed trends are consistent with a simple qualitative picture, in which the HOMOs of the HAPAs are localized in regions

far from the NH2þ terminus, whereas LUMOs have maximum electron density in regions close to this moiety.

The results offer the possibility that one may fine tune the electronic properties of azapolyacetylenes by variation of the N:C

ratio and subsequent selective protonation (via pH adjustment) of a fraction of the nitrogens.

q 2003 Elsevier B.V. All rights reserved.

Keywords: Substituted polyacetylenes; Electronic structure; Ionization energy; Electron affinity; p–pp electronic transition energy

1. Introduction

Since the discovery, 25 years ago, that doped trans-

polyacetylene exhibits metal-like conductivity [1],

numerous applications have been found for conduct-

ing polymers in diverse fields including microelec-

tronics, energy, and medicine [2–7]. Among the

research interests in our laboratory has been investi-

gation of the effects of structure modification and

doping upon the electronic properties and electron

transport in polyacetylenes [8–11] with the goal of

expanding their utilization in the next generation of

photovoltaic devices.

A simple, yet appealing, modification in poly-

acetylene structure which has been of interest is the

incorporation of nitrogen atoms into the molecular

skeleton to create polyazene/polyacetylene copoly-

mers [11,12]. One can thus, in principle, tune the

electronic properties by varying the frequency and

topology of nitrogen substitution, while maintaining

the chain conjugation. In addition, one may enhance

0166-1280/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0166-1280(03)00192-1

Journal of Molecular Structure (Theochem) 629 (2003) 285–293

www.elsevier.com/locate/theochem

* Corresponding author. Tel.: þ1-940-565-2713; fax: þ1-940-

565-4318.

E-mail address: [email protected] (M. Schwartz).

http://www.elsevier.com/locate/theochem

the effect of the substitution via protonation of one or

more nitrogens.

With a view toward determining the magnitude and

spatial extent to which nitrogen incorporation (and

protonation) affects the electronic transition energies,

ionization energies (IEs), electron affinities (EAs) and

structure of a conjugated polyene, we have undertaken

a computation investigation of a series of trans-

polyacetylenes, trans-1-azapolyacetylenes, and their

protonated analogues.

2. Systems and computational methods

The following three series of conjugated polyenes

(in the trans conformation) were investigated (n

represents the number of double bonds): (1) polyace-

tylenes, (–CHyCH–)n [PA, C2n]; (2) 1-azapolyacety-

lenes, HNyCH(–CHyCH–)n 2 1 [APA, C2n 2 1NH];

(3) the cationic protonated 1-azapolyacetylenes,þH2NyCH( –CHyCH–)n21 [HAPA, C2n21NH2

þ].

The acronyms and shorthand formulae in brackets

will be used below for convenience.

Calculations were performed using the Gaussian-

98 [13] suite of programs. All geometries were

optimized at the HF/6-31G(d) level. It was found

that the equilibrium structures in all three series are

planar; minima were verified by frequency calcu-

lations on several members of each series. For systems

with up to 10 double bonds [C20, C19NH, C19NH2þ],

CASSCF(4,4)/6-31G(d) [14] energies were computed

on ground state molecules (with an active space

comprising the two highest occupied and two lowest

unoccupied p orbitals) and on the first excited singlet

[S1] and triplet [T1] states (with the active space

containing the three highest occupied and one lowest

virtual p orbitals). Analogous CASSCF(3,4) and

CASSCF(5,4) energy calculations were performed

on the radical cations and anions, respectively; in

the C2n 2 1NH2þ series, these are the dicationic and

neutral species, respectively.

In order to investigate energy trends in larger

systems, ROHF/6-31G(d) energies (at the ground

state geometries) were computed for the excited state

triplets [T1] and radical cations and anions with up to

25 double bonds [C50, C49NH, C49NH2þ] [15].

UHF/6-31G(d) energy calculations were also

performed on the various species. However,

the results showed large amounts of spin contami-

nation and, although the IEs and EAs were close to the

ROHF values, the singlet–triplet transition energies

for larger systems were unrealistically close to 0.

Because it is the primary goal of this study to

determine relative qualitative trends in electronic

structure among the various species, and to afford a

comparison of CASSCF and ROHF results, energies

have not been corrected for zero point vibrational

energies.

CASSCF and ROHF results for the three series are

plotted in Fig. 1. Tables containing the absolute

CASSCF and HF energies, as well as the transition

energies, IEs and EAs in all three series are available

from the authors upon request [16].

3. Results and discussion

3.1. Comparison with earlier results

There have been a number of experimental

[17–23] and theoretical [24–31] investigations of

transition energies and IEs in the smaller conjugated

polyalkenes. Table 1 contains experimental and

theoretical data (ours and others) on the vertical

electronic excitation and IEs in systems where

accurate measured values have been reported.

One notes from the table that CASSCF and ROHF

transition energies are in qualitative agreement with

eachother,butaregenerallyhigher than thosemeasured

experimentally. This is not surprising, and results from

the lack of correlation energy corrections, which would

lower the values, but are not computationally feasible

for the larger systems investigated here. However, we

are more interested in trends in the electronic par-

ameters with size (and structure), which are similar in

the experimental and computed excitation energies.

Roos and co-workers [24,25] have published CASSCF

transition energies using a larger basis (triple zeta) and

active space (eight electrons and eight orbitals) on the

smaller polyenes, which are in qualitative agreement

with those found here. They also used Roos’ CASPT2

method [32] of correcting CASSCF results for electron

correlation. As seen in the table, the CASPT2 transition

energies are quite close to experimental values.

Although not shown in the table, Bartlett and co-

workers [27] have also obtained accurate transition

M. Schwartz et al. / Journal of Molecular Structure (Theochem) 629 (2003) 285–293286

energies in ethylene and butadiene using a modification

of his coupled cluster method designed to capture the

electroncorrelation. Head-Gordonandco-workers [28]

have also reported accurate transition energies using

computationally inexpensive TD-DFT methods. How-

ever, these calculations have been found to fail for

extended p-electron systems [30]. We computed TD-

B3LYP transition energies (not shown), but also found

unrealistic results in the larger oligomers.

As seen at the bottom of Table 1, computed IEs are

in good qualitative agreement with the limited amount

of experimental data, although somewhat lower in

magnitude. Again, this deviation arises from lack of

correlation energy corrections, which are not feasible

in oligomers of the size studied here.

3.2. Substitution effects on electronic structure

In order to assess the effects of nitrogenation and

protonation of the nitrogen on the electron distribution

and charge, Natural Population Analysis [33–35] has

been used to compute natural atomic orbital popu-

lations, as well as natural charges. Because of the

molecular planarity, it is straightforward to identify

Fig. 1. Ionization energies, electron affinities and excitation energies. (A) Ionization energy; (B) electron affinity; (C) DEðS0 2 T1Þ; (D)

DEðS0 2 T1Þ: Polyacetylenes (C2n): circles and solid line. 1-Azapolyacetylenes (C2n 2 1NH): squares and dotted line. Protonated 1-

azapolyacetylenes (C2n 2 1NH2þ): diamonds and dashed line. Open symbols: CASSCF values. Filled symbols: ROHF values.

M. Schwartz et al. / Journal of Molecular Structure (Theochem) 629 (2003) 285–293 287

the p orbital population on each atom. Values of the

atomic p orbital populations ðPpÞ and natural charges

ðqÞ at the CASSCF/6-31G(d) level for C20, C19NH and

C19NHþ are contained in Tables 2A and 2B,

respectively. The charges in the table represent the

sums of each skeletal atom with directly attached

hydrogens. To determine the extent of population/

charge migration, sums of the parameters over the two

halves of the molecule are given at the bottom of each

table. The first three entries in the tables represent

ground state values of the parameters for the

molecules.

One observes a completely uniform distribution ofp

electrons in C20 (Table 2A) Furthermore, the effect of

the terminal nitrogen in C19NH is rather small, with the

only significant change in Pp occurring on C2 as a result

ofN’sgreaterelectronegativity. Incontrast,protonation

of the terminal imino group induces a major pertur-

bation in Pp over a range of 10 skeletal atoms or more,

indicating a major delocalization of the positive charge

via p electron migration along the chain.

Similarly, one sees that the nitrogen in C19NH has

a rather minor effect on natural charge over only 3–4

skeletal atoms (Table 2B) from the terminus. On the

other hand, protonation of the NH causes a marked

redistribution of charge which extends over a range of

at least 10–12 skeletal atoms. However, one notes

that, not surprisingly, approximately 90% of the

introduced positive charge resides on the half of the

molecule containing the protonated imino group [i.e.Pð1–10Þq

Pð11–20Þ].

3.3. Ionization energies and electron affinities

IEs and EAs of the three series, computed at both

CASSCF and ROHF levels, are plotted in Fig. 1A and

B, respectively. As defined here, the EA represents

DE for the process P þ e2 ! P2; therefore, a lower

EA implies a greater tendency for the molecule to add

an electron.

In general, as seen in Fig. 1A, ROHF IEs are

fairly close to CASSCF values. ROHF and CASSCF

EAs (Fig. 1B) also agree reasonably well. Trends in

both parameters obtained by the two methods are

the same.

As displayed in Fig. 1A, APA IEs are slightly

greater than in the PAs for the shorter oligomers, as

expected on the basis of N’s greater electronegativity.

However, even this small difference diminishes with

increasing number of double bonds. Not surprisingly,

Table 1

Comparison with other results

Compound Parameter Expt. CASSCF(DZ)a ROHFb CASSCF(TZ)c CASPT2(TZ)d

trans-Butadiene (C4) DEðS0 2 T1Þ 3.22e 3.62 3.31 3.39 3.20

DEðS0 2 S1Þ 5.92e 7.14 8.54 6.23

trans-Hexatriene (C6) DEðS0 2 T1Þ 2.61f 3.73 3.15 2.70 2.55

DEðS0 2 S1Þ 4.95f 7.03 7.36 5.01

trans-Octatetraene (C8) DEðS0 2 T1Þ 2.10g 3.26 2.85 2.54 2.17

DEðS0 2 S1Þ 4.41g 6.51 6.67 4.42

trans-Butadiene (C4) Ionization energy 9.07h 8.50 7.93 8.55i

(E)-2-Propen-1-imine (C3NH) Ionization energy 9.65j 9.22 8.88

trans-Hexatriene (C6) Ionization energy 8.30k

Energies in eV.a This work: CASSCF(4,4)/6-31G(d).b This work: ROHF/6-31G(d).c Refs. [24,25]: CASSCF(8,8) with triple zeta basis.d Refs. [24,25]: CASSCF results corrected for electron correlation.e Ref. [18].f Ref. [17].g Ref. [20].h Ref. [22].i Ref. [31]: full p CASSCF.j Ref. [21].k Ref. [19].


protonation of the terminal imino group causes a

dramatic increase in IE (by as much as 6 eV in the

smallest members of the series). However, this

difference is found to decrease with rising chain

length. From the figure, one observes that IEs of the

HAPAs approach those of the other two species,

differing by ,0.5 eV in the largest systems studied.

From Fig. 1B, one observes that EA(APA) is

slightly lower than EA(PA) [by approximately

0.4 eV], again arising from the greater nitrogen

atom electronegativity. As found for IEs, protonation

of the NH group induces a dramatic change in EAs,

with EA(HAPA) ! EA(APA), EA(PA) (by amounts

ranging from 5 to 7 eV). However, as seen clearly in

the figure, unlike the IE data, EAs of the HAPAs do

not begin to approach values in the other two series,

but remain far lower (and approximately constant)

even in comparatively large systems, with as many as

25 double bonds.

One may obtain a simple qualitative description of

similarities and differences in the IEs and EAs among

the three series from inspection of the frontier orbital

energy levels of the RHF ground states, which are

plotted in Fig. 2. It is seen that both HOMOs and

LUMOs of the APAs and PAs are close to coincident

for all but the smallest oligomers, indicating that

the nitrogen’s perturbation of the electronic structure

is quite small. Thus, it is not at all surprising that both

IEs and EAs of the two species are found to be very

similar.

In contrast, both frontier orbitals of the HAPAs are

observed to deviate dramatically from those of the

non-protonated series. However, whereas the HOMOs

of the protonated imines are found to slowly approach

Table 2A

Substitution effects on electronic structure (natural atomic p orbital populations ðPpÞ)

Species C20 C19NH C19NH2þ C19NH2

þ C19NH2þ C19NH2

þ C19NH2þ

State GS GS GS Cation Anion T1 S1

Atom Pp Pp Pp DPpa DPp

a DPpa DPp

a

C1/N1 1.01 1.20 1.70 20.04 0.10 0.03 0.04

C2 0.98 0.83 0.70 20.03 0.18 0.08 0.05

C3 1.00 1.04 1.22 20.03 20.03 20.08 20.06

C4 0.99 0.94 0.72 20.01 0.24 0.23 0.01

C5 1.00 1.02 1.15 20.05 20.10 20.27 20.16

C6 0.99 0.96 0.72 0.05 0.29 0.40 0.16

C7 1.00 1.00 1.08 20.05 20.09 20.31 20.17

C8 1.00 0.98 0.84 0.03 0.20 0.30 0.19

C9 1.00 1.00 1.10 20.16 20.12 20.30 20.24

C10 1.00 0.99 0.84 0.12 0.19 0.16 0.26

C11 1.00 1.00 1.04 20.20 20.06 20.08 20.21

C12 1.00 0.99 0.92 0.10 0.10 20.06 0.21

C13 1.00 1.00 1.06 20.27 20.07 0.00 20.21

C14 1.00 0.99 0.92 0.02 0.09 20.07 0.19

C15 0.99 1.00 1.04 20.12 20.05 0.03 20.08

C16 1.00 0.99 0.94 20.19 0.06 20.06 0.04

C17 0.99 0.99 1.02 0.00 20.03 0.00 20.03

C18 1.00 1.00 0.95 20.13 0.05 20.01 0.02

C19 0.98 0.98 1.00 0.06 20.02 0.01 0.03

C20 1.01 1.01 0.98 20.13 0.04 20.01 20.05

Pð1–10Þb 9.97 9.96 10.07 20.17 0.86 0.24 0.08

Pð11–20Þb 9.97 9.95 9.87 20.86 0.11 20.25 20.09

Pp and q determined from CASSCF/6-31G(d) wavefunctions.a DPp and Dq represent deviations from ground state values.b P

ð1–10Þ andPð11–20Þ represent sums over first 10 and second 10 skeletal atoms, respectively.


those of the PAs and APAs, LUMO energies remain

far lower, even in the largest systems studied. These

latter two observations offer compelling prima facie

evidence that, within the simplified single determinant

description of electronic structure, the electron

density in the LUMOs of the HAPAs is largely

localized in the vicinity of the NH2þ moiety, whereas

the density in the HOMOs resides primarily on the

opposite side of the molecule. It should be noted that

one obtains the same qualitative picture from visual

inspection of the shapes of the frontier orbitals in

several members of the series.

Based upon this simple description, because

HOMOs of the HAPAs have primarily PA character,

it is only natural to conclude that IEs in this series

should approach those of the polyacetylenes, as found

here. Analogously, with the LUMOs primarily

localized in the vicinity of the NH2þ group, an electron

initially attracted to the LUMO will experience the

bulk of the effect of the proton’s charge, leading to

lower EAs even for the largest members of the series.

The electron distributions predicted by the ground

state HOMOs and LUMOs are consistent with those

found by visual inspection of the cations’ and anions’

SOMOs, which are found to be localized primarily on

atoms far from (cation) or near to (anion) the imine

end of the molecule.

A semi-quantitative assessment of the degree of

charge localization in the ions is furnished by natural

p orbital populations and charges, displayed in Table

2 for the representative HAPA, C19NH2þ (DPp and Dq

represent differences from the ground state values). In

the cation, one finds that approximately 85% of the

decrease in p orbital population occurs in half of the

Table 2B

Substitution effects on electronic structure (natural charges ðqÞ)

Species C20 C19NH C19NH2þ C19NH2

þ C19NH2þ C19NH2

þ C19NH2þ

State GS GS GS Cation Anion T1 S1

Atom q q q Dqa Dqa Dqa Dqa

0.01 20.33 0.07 0.04 20.12 20.04 20.05

C2 20.01 0.32 0.50 0.03 20.17 20.08 20.04

C3 0.00 20.08 20.21 0.02 0.02 0.06 0.03

C4 0.00 0.06 0.27 0.01 20.23 20.21 0.03

C5 0.00 20.02 20.14 0.05 0.08 0.25 0.13

C6 0.00 0.04 0.26 20.04 20.26 20.37 20.12

C7 0.00 20.02 20.07 0.05 0.06 0.28 0.15

C8 0.00 0.02 0.16 20.03 20.19 20.28 20.17

C9 0.00 20.01 20.09 0.15 0.10 0.27 0.22

C10 0.00 0.02 0.14 20.10 20.17 20.14 20.23

C11 0.00 20.01 20.04 0.19 0.04 0.06 0.19

C12 0.00 0.01 0.08 20.10 20.10 0.06 20.20

C13 0.00 20.01 20.05 0.25 0.06 20.01 0.19

C14 0.00 0.01 0.07 20.01 20.08 0.08 20.17

C15 0.00 0.00 20.04 0.11 0.04 20.03 0.07

C16 0.00 0.01 0.05 0.17 20.06 0.05 20.06

C17 0.00 0.00 20.02 0.01 0.03 0.01 0.05

C18 0.00 0.00 0.04 0.13 20.04 0.02 20.02

C19 20.01 20.02 20.02 20.05 0.01 20.01 20.02

C20 0.01 0.02 0.05 0.14 20.04 0.02 0.05

Pð1–10Þb 0.00 20.01 0.89 0.17 20.87 20.24 20.06

Pð1–10Þb 0.00 0.01 0.11 0.83 20.14 0.24 0.06

Natural charges represent sums of skeletal atoms and directly connected hydrogens.a DPp and Dq represent deviations from ground state values.b P

ð1–10Þ andPð11–20Þ represent sums over first 10 and second 10 skeletal atoms, respectively.


molecule far from the NH2þ group (with an equivalent

increase in positive charge in this half). Similarly, it

may be seen that ,85% of the anion’s increase in Pp

is in the portion of the molecule containing the imino

group (as is a similar percentage of the net charge

decrease).

3.4. Vertical excitation energies (DEðS0-T1Þ

and DEðS0-S1Þ)

Calculated vertical transition energies from the

ground state to the lowest excited pp triplet ðDEð

S0 2 T1ÞÞ and pp singlet ðDEðS0 2 S1ÞÞ; computed at

the CASSCF and ROHF (triplet only) levels, are

plotted in Fig. 1C and D, respectively. The apparent

scatter in CASSCF transition energies results from the

differing central bond order of oligomers with even

and odd numbers of double bonds, which has been

reported in an earlier investigation [8] from these

laboratories.

From Fig. 1C, one finds that ROHF S0 2 T1

energies are somewhat lower (by an average of 0.4–

0.5 eV) than CASSCF values, but exhibit the same

trends with increasing oligomer size. A comparison of

Fig. 1C and D reveals that triplet excitation energies

are approximately 40–50% lower than transition

energies to the pp singlet. This feature has been

routinely observed in both experimental [17,18,20,23]

and computational [24–26] investigations of con-

jugated polymers, and is attributed to exciton

formation [36,37] and the consequent stabilization

of the triplet by the quantum mechanical exchange

interaction between the unpaired spins [38].

Inspection of Fig. 1C and D reveals that DEðS0 2

S1Þ or DEðS0 2 T1Þ for the PAs and APAs are almost

indistinguishable, except for the very smallest oligo-

mers. Thus, as found above for IEs and EAs,

introduction of a terminal imino group has no effect

on transition energies. On the other hand, while

HAPA excitation energies are similar to the others in

magnitude for the very smallest members of the

series, these excitation energies diminish much more

rapidly with increasing size, and are approximately

40–50% lower in systems with four or more double

bonds. Further, the lower HAPA transition energies

appear to persist, even for the larger oligomers. This is

clear from Fig. 1C, where one finds that ROHF values

of DEðS0 2 T1Þ are near to their limiting values in

systems with as as few as 10 double bonds; the

asymptotic limits are ,2.0 eV (HAPA) vs. ,2.9 eV

(PA), ,3.05 (APA).

Both the negligible effect of initial introduction of

a terminal nitrogen and the marked impact of its

protonation may be understood qualitatively by

referring, once again, to the ground state HOMOs

and LUMOs in the three series (Fig. 2). As noted

above, the frontier orbitals in the APAs are only

insignificantly different from the parent PAs, whereas

both HOMO and LUMO are stabilized significantly in

the HAPAs. In the smallest systems, both frontier

orbitals are stabilized by approximately equal

amounts, whereas in the larger members of this series,

the LUMO exhibits a residual limiting stabilization,

resulting in a lower gap between the two orbitals. It

has been noted elsewhere [39] that, as observed here,

incorporation of p electron acceptors into conducting

polymers lowers the energy of both HOMO and

LUMO, but with a greater stabilization of the latter

orbital. Qualitatively, the markedly lower HAPA

excitation energies results from the fact that an

electron is promoted from an orbital which is

primarily PA in character to one in which the bulk

of the electron density resides close to the charged

NH2þ moiety. In semiconductor terminology, this is

analogous to an indirect band gap.

Fig. 2. Ground state HOMO and LUMO energies. Polyacetylenes

(C2n): circles and solid line. 1-Azapolyacetylenes (C2n 2 1NH):

squares and dotted line. Protonated 1-azapolyacetylenes

(C2n 2 1NH2þ): diamonds and dashed line. Filled symbols:

LUMOs. Open symbols: HOMOs.


The net migration of p electron density towards the

NH2þ upon excitation is also seen numerically in the

variation of p orbital populations, DPp; in the triplet

(Table 2A). One observes that, for C19NH2þ, there is a

net movement of approximately 1/4 electron towards

the imine half of the molecule; one finds an equivalent

decrease in natural charge on this side of the molecule

(Table 2B). One also finds p electron migration

towards the imino group in the excited singlet.

However, the effect is significantly smaller. This is

consistent with the observation that singlet excitons

tend to remain bound whereas triplet excitons separate

in long polyene chains [40].

4. Summary and conclusions

IEs, EAs and the lowest p ! pp transition

energies, DEðS0 2 S1Þ and DEðS0 2 T1Þ; for a series

of polyacetylenes (PA), 1-azapolyacetylenes (APA)

and the protonated azapolyacetylenes (HAPA) were

computed at the CASSCF/6-31G(d) and ROHF/6-

31G(d) levels.

The presence of the imino group in the APAs had

very little effect onanyof theelectronicproperties, even

for the smallest oligomers. In contrast, protonation of

the terminal nitrogen induced dramatic changes in all

properties. The IEs of the HAPAs were far greater than

in the other two series and, although the values

approached those of the PAs and APAs, the differences

were still significant in moderately large oligomers

(with 10–15 double bonds). EAs in the HAPAs were

markedly lower than in the non-protonated species,

even in the asymptotic limit. Singlet–singlet and

singlet–triplet excitation energies in HAPAs were

also far lower than in either of the other series; this

difference, too, persists in the large molecule limit.

All of the observed trends are consistent with a

simple picture in which the ground state HOMOs of

the HAPAs have maximum electron density far from

the imino terminus of the molecule, whereas the

LUMOs are localized in a region close to the NH2þ

moiety.

The marked variation in electronic structure,

extending over many double bonds, caused by

introduction of NHþ groups into polyacetylenes offers

the promising possibility that one may fine tune the

electronic properties of azapolyacetylenes to desired

values for a given application by varying the N:C ratio

and subsequent selective protonation (via pH adjust-

ment) of a fraction of the introduced nitrogens.

Similar effects might also be obtained by alkylation

in lieu of protonation.

Acknowledgements

The authors acknowledge the WPAFB Materials

Directorate, and the ASC and ARL Major Shared

Resource Centers, operated by the DoD High

Performance Computing Modernization Office. One

of the authors (M. S.) thanks Robert A. Welch

Foundation [Grant B-657] and the National Research

Council [Air Force Summer Faculty Fellowship

Program] for financial support.

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the electronic structure and properties of pristine and protonated 1-azapolyacetylenes

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