DFT and experimental study of N,N'-bis(3'-carboxy,4'-aminophenyl)-1,4- quinonediimine, a carboxyl substituted aniline trimer

Article abstract of DOI:10.1016/j.molstruc.2010.05.038

Density functional calculations were performed on N,N'-bis(3'-carboxy,4'- aminophenyl)-1,4-quinonediimine, a carboxylic acid substituted aniline trimer. Results of the calculations were compared to experimental properties of the herein synthesized trimer, as well as to the properties of the anthranilic acid/aniline co-polymer reported in the literature. The calculated LUMO levels for isomers of the title compound range from -4.45 to -5.05 eV. The calculated electron affinities range from 75.93 kcal mol^-1 to 89.04 kcal mol^-1 (3.29-3.86 eV). Both the LUMO levels and electron affinities are greatest in magnitude for the anti, syn isomer. The HOMO levels, on the other hand, range from -5.32 eV (for the trans, trans isomer) to -5.36 eV (syn, syn inner). In acetonitrile solvent, the zwitterionic form is calculated to be energetically preferred to the non-zwitterion. Experimental UV-vis and near-IR studies in acetonitrile and ethanol show little evidence for zwitterion formation, but those in water show strong evidence. The predicted electronic transitions for the non-zwitterion in acetonitrile solvent correspond closely to those seen at 533 and 416 nm. The zwitterion present in solvent corresponds to a trimer with the capability to self-dope , suggesting that the trimer would be effective at corrosion inhibition in the emeraldine base form, unlike other trimers which are only effective in the emeraldine salt form. This effectiveness in the emeraldine base form would enable the material to be utilized in corrosion inhibition applications in alkaline environments where standard oligo- and polyanilines fail.

Full text of DOI:10.1016/j.molstruc.2010.05.038

Journal of Molecular Structure 977 (2010) 220–229
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
DFT and experimental study of N,N0-bis(30-carboxy,40-aminophenyl)-1,4-quinone-
diimine, a carboxyl substituted aniline trimer
*
Lawrence T. Sein Jr. , Amanda F. Lashua
Department of Chemical and Physical Sciences, Cedar Crest College, Allentown, PA 18104, United States
a r t i c l e i n f o
a b s t r a c t
Density functional calculations were performed on N,N0-bis(30-carboxy,40-aminophenyl)-1,4-quinonedii-
mine, a carboxylic acid substituted aniline trimer. Results of the calculations were compared to experi-
mental properties of the herein synthesized trimer, as well as to the properties of the anthranilic acid/
aniline co-polymer reported in the literature. The calculated LUMO levels for isomers of the title com-
pound range from ꢀ4.45 to ꢀ5.05 eV. The calculated electron affinities range from 75.93 kcal molꢀ1 to
89.04 kcal molꢀ1 (3.29–3.86 eV). Both the LUMO levels and electron affinities are greatest in magnitude
for the anti, syn isomer. The HOMO levels, on the other hand, range from ꢀ5.32 eV (for the trans, trans
isomer) to ꢀ5.36 eV (syn, syn inner). In acetonitrile solvent, the zwitterionic form is calculated to be ener-
getically preferred to the non-zwitterion. Experimental UV–vis and near-IR studies in acetonitrile and
ethanol show little evidence for zwitterion formation, but those in water show strong evidence. The pre-
dicted electronic transitions for the non-zwitterion in acetonitrile solvent correspond closely to those
seen at 533 and 416 nm. The zwitterion present in solvent corresponds to a trimer with the capability
to ‘‘self-dope”, suggesting that the trimer would be effective at corrosion inhibition in the emeraldine
base form, unlike other trimers which are only effective in the emeraldine salt form. This effectiveness
in the emeraldine base form would enable the material to be utilized in corrosion inhibition applications
in alkaline environments where standard oligo- and polyanilines fail.
Article history:
Received 1 February 2010
Received in revised form 20 May 2010
Accepted 24 May 2010
Available online 16 June 2010
Keywords:
Anthranilic acid
DFT
Conducting polymers
Polyaniline
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
with a suitable Lewis acid – ‘‘doped” [13]. Only the doped emeral-
dine form of the polymer conducts, and only the doped emeraldine
Conducting organic polymers such as polyacetylene [1], polyan-
iline [2], polypyrrole [3], and polythiophene [4] have stimulated
numerousinvestigations. The importanceoftheir discoverywas rec-
ognized by the award of the Nobel Prize in chemistry to Alan J. Hee-
ger, Alan G. MacDiarmid, and Hideki Shirakawa in 2000. Polyaniline
(PANI) has been investigated for its utility in catalysis [5], compos-
ites [6], corrosion inhibition [7–9], nano-junctions [10], non-linear
optics [11], nanoparticles and microtubules, and solar cells [12].
PANI exists in at least three oxidation states, called leucoemeraldine
(the most reduced form, LEB), emeraldine (EB), and pernigraniline
(the most oxidized form, PB). For the trimers, each of these oxidation
states is separated from the next by a two electron oxidation or
reduction. Interconversion of oxidation states is often readily
achieved under mild conditions (Fig. 1). This ease of switching oxi-
dation states is an important reason for the interest in polyanilines.
To become suitable for either conducting polymer and corrosion
control applications, the polymer in the emeraldine oxidation state
needs to become protonated by a Brønsted–Lowry acid, or reacted
form is effective for corrosion inhibition. The applicability of the
doped forms of the oligomer or polymer to these processes de-
pends upon its redox properties (the enhanced electron affinity
of the doped state is the key to the corrosion inhibition mechanism
[7]). This interconnectivity of the acid/base properties of the poly-
mer and its redox activity adds to the complexity of polyanilines.
The base strength of the imine nitrogen atoms is often a critical
factor in the utility of these materials in alkaline environments,
since deprotonation renders the polymer or oligomer less able to
inhibit corrosion.
The relationship between the structure and the function of ani-
line oligomers has been investigated [14–22]. Aniline oligomers
have several advantages over the polymer. Aniline trimers are
more effective in corrosion inhibition than the polymer. The tri-
mers are easier materials to process than the polymer, since they
are soluble in a wide variety of common solvents, whereas PANI
is only soluble in N-methyl-2-pyrrolidone (NMP). Their precise
chain length (monodispersity) removes the complications inherent
in the polydispersity of the polymer. The presence of the carboxyl-
ate functionality on the oligomer increases the solubility, particu-
larly in more polar solvents such as ethanol or water.
* Corresponding author. Tel.: +1 610 606 4666x3615.
E-mail address: ltsein@cedarcrest.edu (L.T. Sein Jr.).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.05.038

L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
221
Fig. 1. Redox states of N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine. R1=NH2 for leucoemeraldine and emeraldine states, and NH for pernigraniline. R2=COOH for all
redox states.
Since doping is necessary for several applications, the benefit of
an aniline polymer or oligomer that can self-dope is apparent. The
quest for self-doping was a motivating rationale in the synthesis of
poly-anthranilic acid [23] and the anthranilic acid/aniline (AA/A)
co-polymers [2]. High level quantum chemical calculations are fea-
sible on aniline trimers, whereas systematic investigation of the
polymers at high level is too computationally expensive. Therefore,
comparison of experimental properties of the trimers with calcula-
tions can be used to gain insight into both the trimers and the
polymers.
ylenediamine was dissolved in 200 mL of 2 M hydrochloric acid
using magnetic stirring at room temperature. The solution was
then cooled to approximately ꢀ5 °C by immersion of the reaction
beaker in an ice/salt water solution. 2.11 g (0.0185 mol) of ammo-
nium persulfate were added slowly. Upon color change of the solu-
tion from clear to light brown, 2.54 g (0.0185 mol) of anthranilic
acid (AA) were added slowly. The stirring continued for one hour.
The solution was allowed to reach room temperature, and excess
2 M ammonium hydroxide solution was added with magnetic stir-
ring. The solution was then vacuumed filtered to recover the emer-
aldine base. The crude amine base product (mixed with
a
2. Experimental methods
considerable remainder of the hydrochloride salt) was dissolved
in absolute ethanol. Excess sodium carbonate was added with
occasional stoppering of the flask and shaking, until there was no
further evolution of carbon dioxide from reaction of the acid with
the carbonate. More highly purified amine free base was removed
from this mixture by Soxhlet extraction with ethanol, which was
then evaporated in a fume hood. Ethanol was used in preference
All reagents employed were purchased from Sigma–Aldrich and
used without further purification. The anthranilic terminated ani-
line trimer N,N0-bis(30-carboxy,40-aminophenyl)-1,4-quinonedii-
mine (hereafter anthranilic emeraldine base trimer, AEB) was
synthesized [24] as follows. 1.00 g (0.00925 mol) of 1,4-para-phen-

222
L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
to acetonitrile, the anthranilic trimer being far more soluble in eth-
anol than in acetonitrile.
UV–vis spectra were collected in acetonitrile, dimethylsulfox-
ide, ethanol, or water using 1 cm quartz cuvettes on a Beck-
Enthalpies were calculated for the lowest energy singlet and
triplet states of each isomer. Triplet energies for the EB isomers
range from 16.3 kcal molꢀ1 (0.707 eV) for the anti, syn isomer to
17.7 kcal molꢀ1 (0.768 eV) for the syn, syn inner isomer. For each
isomer, S0 was lower in energy than T1. For the dihydrochloride
salts, the triplet energies were consistently near 10.3 kcal molꢀ1
(0.447 eV), with the triplets (T1) being again higher in energy than
S0, but lower than S1. S1 energies were predicted by CIS, TD-DFT,
mann-Coulter
DU-600
spectrophotometer.
The
solution
concentrations were in the range of 10ꢀ5–10ꢀ4 M. Molar absorptiv-
ities in ethanol ranged from 12,400 L molꢀ1 cmꢀ1 (at 330 nm) to
15,600 L molꢀ1 cmꢀ1 (at 246 nm).
IR spectra were collected on a Thermo Scientific ‘‘Smart Per-
former” Attenuated Total Reflectance (ATR) attachment (diamond
crystal) to a Nicolet 380 FT-IR. IR N,N0-bis(30-carboxy,40-aminophe-
nyl)-1,4-quinonediimine (ATR, cmꢀ1): mmax 1404, 748, 823, 1218,
1483, 1544, 1658, 2360, 3033, 3120.
Near-IR spectra were collected in water, acetonitrile, or 100%
ethanol solvent, using 1 cm quartz cuvettes in a Foss 6500 system,
using transmission mode. The wavelengths examines ranged from
1100 to 2500 nm, with a resolution of 2 nm. N,N0-bis(30-carboxy,40-
aminophenyl)-1,4-quinonediimine (ethanol, nm): kmax 2114; (ace-
tonitrile, nm): kmax 1902, 1394; (water, nm): kmax 1846, 1878,
1984.
ZINDO/S, and DSCF.
Electron affinities were computed on the neutral minimum
geometries (thereby giving the vertical electron affinities) using
the 6-311+G(2d,p) basis and the B3LYP functional as implemented
by Gaussian 03 [27] by difference of the computed energies (single
point energy plus zero point correction) of the neutral molecule
and the anion.
The enthalpy of the gas phase reaction between one mole of
each trimer in the emeraldine oxidation state (EB) and two moles
of hydrogen chloride to give one mole of the emeraldine dihydro-
chloride salt (ES2)
1H NMR spectra were collected on an Anasazi EPT-90 90 MHz
NMR in DMSO-d6 with 64 scans. NMR N,N0-bis(30-carboxy,40-ami-
nophenyl)-1,4-quinonediimine (DMSO-d6, d): 2.6 (proton on imine
nitrogen), 5.5 (NH2 protons, doublet), 6.7 (quinoid H, two dou-
blets), 7.2 (aromatic protons at 30 position, two overlapping dou-
blets), 7.8 (aromatic protons at 60 position, doublet), and 8.0
(aromatic protons at 50 position, multiplet).
EB þ 2HCl ! ES2
ð1Þ
was calculated using single point energy calculations at the B3LYP/
6-311+G(2d,p) level of theory on B3LYP/6-31Gꢁ optimized geome-
tries. Zero-point energy and enthalpic corrections to 298.15 K were
calculated using frequency calculations at B3LYP/6-31Gꢁ [29(b)].
Wavelengths of electronic transitions were calculated using the
D
SCF method [30], which has already demonstrated its effective-
The melting point was impossible to determine, being greater
than 350 °C.
ness in predicting electronic transitions for a number of oligoani-
lines [16–19,24]. For comparison purposes, the transition
energies were also calculated using the Configuration Interaction
Singles (CIS) method [31], Time-dependent density functional the-
ory (TD-DFT) [32], and the ZINDO/S formalism [33] on B3LYP/6-
31Gꢁ optimized geometries. All methods were employed on both
the zwitterionic and non-zwitterionic structures.
The effects of solvation on molecular structure and the elec-
tronic spectra were assessed by use of the Self-Consistent Isoden-
sity Polarizability Continuum Model (SCIPCM) [34], with a model
acetonitrile solvent. This solvent was chosen to allow comparison
with the experimental UV–vis spectra, which were taken in aceto-
nitrile. The emeraldine trimers were geometry optimized using
B3LYP/6-31Gꢁ chemistry in the model solvent (acetonitrile), and
3. Computational methods
Optimized structures for each isomer of AEB (Fig. 2) were com-
puted using the 6-31Gꢁ [25] Pople-style Gaussian basis set (double
f quality for the valence orbitals, plus one set of polarization func-
tions on heavy atoms) and Becke’s hybrid density functional B3LYP
[26] within the Gaussian 03 suite of programs [27]. All electrons
were included in the computations. The structures were then con-
firmed as true minima by frequency calculations at the B3LYP/6-
31Gꢁ level, zero imaginary frequencies being detected. Zero point
energies were computed at B3LYP/6-31Gꢁ with no scaling factors.
Single point energies for each structure were computed using
the 6-311+G(2d,p) Pople-style Gaussian basis set [28]. This basis
is triple f quality on the valence orbitals, supplemented by one
set of diffuse functions and two additional d polarization functions
on heavy atoms, and one set of p polarization functions on hydro-
gen. The computational method employed for the single point
energies was the B3LYP hybrid density functional [26] within the
Gaussian 03 suite of programs [27]. Tight convergence criteria
were selected by use of the SCF = tight keyword option.
Different basis sets were employed in this work for geometry
optimization [6-31Gꢁ] and single point energy [6-311+G(2d,p)] cal-
culations, since experience has shown that use of large basis sets is
much less advantageous for geometry optimization than for energy
calculation. Calculated thermochemical properties of a set of mol-
ecules (the ‘‘G2” set) for which excellent experimental values are
available have been compared with the experimental values [29].
These comparisons allow an estimation of the accuracy of various
density functional and basis set combinations. Results for B3LYP/6-
31Gꢁ optimization followed by B3LYP/6-311+G(2d,p) single point
energy calculation had a mean absolute deviation from experiment
of 3.2 kcal molꢀ1, whereas results for B3LYP/6-311+G(2d,p) optimi-
zation followed by B3LYP/6-311+G(2d,p) single point energy calcu-
the electronic spectra predicted using the
DSCF method with the
orbital energies generated by a single-point calculation at B3LYP/
6-311+G(2d,p) on the B3LYP/6-31Gꢁ solvent optimized geometries.
Given the presence of both carboxylic acid and amine/imine
groups in the oligomer, the potential for zwitterions formation
was considered. Optimized structures for the emeraldine trimer
zwitterions (Fig. 3) were generated in both the presence and ab-
sence of acetonitrile solvent. Acetonitrile was selected since its
dielectric constant (
= 1) and water ( = 78).
The computations were performed on the National Science
e = 36) is roughly halfway between vacuum
(e
e
Foundation HP GS1280 system at the Pittsburgh Supercomputing
Center. Visualizations of molecular structures and orbitals were
generated by ChemCraft [35].
4. Results and discussion
Calculations (Table 1) find the syn, syn (inner) isomer of AEB to
be lowest in enthalpy, though the differences in enthalpies in the
gas phase among isomers are less than 2 kcal molꢀ1. In acetonitrile
solvent, the calculated enthalpy differences are even smaller, still
favoring the syn, syn (inner) structure. Assuming the isomers are
Boltzmann distributed, the population of the AEB trimer at
298.15 K should be 69.9% syn, syn inner (ssi), 4.8% syn, syn outer
lation had
a mean absolute deviation from experiment of
3.1 kcal molꢀ1 – a minimal improvement in accuracy at best, at a
huge increase in computational cost.

L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
223
Fig. 2. Calculated molecular structures (from top) of syn-, syn- (inner) – N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine; syn-, syn- (outer) – N,N0-(30-carboxy,40-
aminophenyl)-1,4-quinonediimine; syn-, anti- N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine; anti-, syn- N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine; and
anti-, anti- N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine, optimized at B3LYP/6-31Gꢁ.

224
L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
Fig. 3. Calculated molecular structure of the anti-, syn- N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine zwitterions, optimized at B3LYP/6-31Gꢁ.
Table 1
the ring tilt is the significantly smaller 27.46° for the syn isomer,
Relative enthalpies of anthranilic emeraldine trimers (with respect to lowest energy
isomer) calculated at B3LYP/6-311+G(2d,p) on B3LYP/6-31Gꢁ optimized geometries,
with zero-point and enthalpic corrections to 298.15 K at B3LYP/6-31Gꢁ. Solvent
effects in acetonitrile modeled using SCIPCM method.
and 25.73° for the anti. The greater conductivity of the doped emer-
aldine salt forms relative to the emeraldine base is often attributed
in part to their greater (near) planarity.
For the AEB, the ring tilts are calculated at 39.69° (ssi), 38.27°
(sso), 39.81° (sa), 39.73° (as), and 39.73° (aa). In other words, the
presence of the carboxylic acid group at the 30 position on each
outer ring has an almost non-existent effect on the calculated
ground state geometry in the gas phase. Possible geometric/steric
effects of the carboxyl on neighboring chains in the solid state were
not addressed.
Neutral
Neutral
Zwitterion
Zwitterion
(solvent)a
molecule
molecule
(gas phase)a
(gas phase)a
(solvent)a
Syn-, syn- (inner)
Syn-, syn- (outer)
Syn-, anti-
Anti-, syn-
Anti-, anti-
0.000
1.602
1.034
1.247
1.649
0.000
0.892
0.824
1.339
NA
10.122
8.792
7.479
0.000
9.345
NA
0.000
NA
15.025
11.965
Calculated ring tilts for the anthranilic emeraldine salt (AES2)
were little changed from those of the unsubstituted aniline trimer
– 28.90° (ssi), 28.71° (sso), 28.95° (sa), 28.63° (as), 26.75° (aa). For
the AEB zwitterions, however, there is a quite substantial move-
ment towards planarity. The calculated ring tilts drop to 18.40°
(ssi), 21.58° (sso), 20.67° (sa), 18.44° (as), and 20.86° (aa)
The AEB zwitterions in the gas phase are approximately
46 kcal molꢀ1 higher in enthalpy that the non-zwitterionic forms
in the gas phase. This is to be expected, since there is no mecha-
nism for stabilization of the charge separation in the gas phase.
The neutral molecules are only slightly stabilized by solvation,
about 12 kcal molꢀ1. The zwitterions, however, are greatly stabi-
lized even by solvation in the slightly polar acetonitrile, nearly
65 kcal molꢀ1 for one isomer.
a
kcal molꢀ1
.
(sso), 12.3% syn, anti (sa), 8.6% anti, syn (as), and 4.4% anti, anti
(aa).
Zwitterionic forms in the gas phase have a much different pop-
ulation distribution. The anti, syn isomer is the lowest in enthalpy
by 7.5–10.1 kcal molꢀ1, which means that if the isomers are Boltz-
mann distributed, the population of the AEB zwitterionic trimer at
298.15 K should be virtually all the anti, syn isomer.
Enthalpies of activation for isomerization of AEB were calcu-
lated at the B3LYP/6-311+G(2d,p) level after identifying the transi-
tion state for inversion at the imine nitrogen (transition state
optimized and zero-point corrected to 298.15 K at B3LYP/6-
31Gꢁ). The value (19.6 kcal molꢀ1) is virtually identical for the en-
thalpy of activation for isomerization calculated for the unsubsti-
tuted EB. Similarly, enthalpies of activation for rotation of AEB
between syn and anti isomers (relative to the carboxyl group) were
calculated after identifying the transition state for rotation of the
outer amine rings (same level of theory as for isomerization). The
value (2.18 kcal molꢀ1) is only slightly larger than the enthalpy of
activation for rotation calculated for the unsubstituted EB
(1.88 kcal molꢀ1).
The presence of both a carboxylic acid function and an imine sug-
gested the potential for zwitterion formation (in polymeric parlance
– ‘‘self-doping”). The possibility is enhanced by the inherent basicity
of quinonediimines. The zwitterionic structures herein described
are the di-zwitterions, with both carboxylates deprotonated, and
both imine nitrogen atoms protonated. In the gas phase, the anti,
syn AEB zwitterion structure is enthalpically favored by as much
as 10 kcal molꢀ1 over the other isomers. In acetonitrile solvent, the
calculated preference is as much as 12 kcal molꢀ1, though calcula-
tions for two isomers failed to converge.
For the AES2 dihydrochlorides in the gas phase (Table 2), calcu-
lated enthalpies show the as isomer as most stable – by essentially
the same amount as for the zwitterions in the gas phase. The
enthalpies of reaction of AEB with two moles of hydrogen chloride
to yield the corresponding emeraldine disalt were computed. For
each isomer, the reaction was clearly exothermic. The greatest en-
thalpy of reaction was found for the as isomer, at ꢀ3.3 kcal molꢀ1
.
The enthalpies of reaction for 30,40-diaminophenyl-1,4-quinonedii-
mine was computed to be 16.35 kcal molꢀ1, for 30-hydroxy,40-ami-
nophenyl-1,4-quinonediimine 12.26 kcal molꢀ1, and for 30,40-dihy-
droxyphenyl-1,4-quinonediimine 16.97 kcal molꢀ1 [36]. Clearly,
Table 2
Enthalpies of anthranilic emeraldine trimer
dihydrochlorides calculated at B3LYP/6-
311+G(2d,p) on B3LYP/6-31Gꢁ optimized
geometries, with zero-point and enthalpic
corrections to 298.15 K at B3LYP/6-31Gꢁ, rela-
tive to anti, syn isomer.
D
Ha
Calculations reveal characteristic structural effects of the carbox-
ylate group. In the analogous aniline EB trimer without a carboxyl
substitution, i.e., N,N0-bis(40-aminophenyl)-1,4-quinonediimine,
the tilt of the outer benzenoid rings with respect to the inner quinoid
ring (C–C–N=C) is 39.97° for the syn isomer, and 40.36° for the anti;
for the corresponding carboxylate-less dihydrochloride salt (ES2),
Syn-, syn- (inner)
Syn-, syn- (outer)
Syn-, anti-
Anti-, syn-
Anti-, anti-
1.015
3.095
2.018
0.000
0.779
kcal molꢀ1
.
a

L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
225
Table 3
Like other aniline trimers, AEB can be easily oxidized to the per-
nigraniline oxidation state (APB) by addition of concentrated
hydrogen peroxide. The calculated isomeric preferences in the
gas phase for this state are slightly different than for the emeral-
dine one. The ssi is the least stable pernigraniline isomer, whereas
the aa is the most stable (Table 3).
Enthalpies of anthranilic pernigraniline trimers cal-
culated at B3LYP/6-311+G(2d,p) on B3LYP/6-31Gꢁ
optimized geometries, with zero-point and enthalpic
corrections to 298.15 K at B3LYP/6-31Gꢁ, relative to
anti, anti isomer.
D
Ha
Syn-, syn- (inner)
Syn-, syn- (outer)
Syn-, anti-
Anti-, syn-
Anti-, anti-
1.747
0.169
0.752
0.700
0.000
4.1. IR spectra
The principal absorption bands in the IR are very close to those
reported for the AA/A co-polymer. The trimer has a benzenoid
stretch at 1483 cmꢀ1 (co-polymer at 1477 cmꢀ1), and a quinoid
stretch at 1546 cmꢀ1 (1546 cmꢀ1 in co-polymer). The protonated
imine stretch is found at 1157 cmꢀ1 (1124 cmꢀ1 in the co-poly-
mer), and the C=O stretch from the carboxylate is found at
1656 cmꢀ1 (1674 cmꢀ1 in the co-polymer).
a
kcal molꢀ1
.
Table 4
Vertical electron affinities of anthranilic emeraldine trimers calculated at B3LYP/6-
311+G(2d,p) on B3LYP/6-31Gꢁ optimized geometries, with zero-point and enthalpic
corrections to 298.15 K at B3LYP/6-31Gꢁ.
4.2. 1H NMR
Emeraldine
basea
Emeraldine
Emeraldine
zwitteriona
dihydrochloridea
Indirect effects of the carboxyl group on the geometry of the AE
trimer can be seen in the NMR. The 1H NMR of the aniline trimer
(as well as 30-hydroxy and 30-amino substituted aniline trimers)
shows very broad resonances. This can be attributed to torsion of
the outer rings relative to the central quinoid ring. For the AEB tri-
mer, the proton resonances are at quite distinct chemical shifts,
and the splitting patterns are easily deciphered – quite unlike sev-
eral similar aniline trimers in the same (DMSO) solvent.
Syn-, syn- (inner)
Syn-, syn- (outer)
Syn-, anti-
37.78
38.84
38.28
38.40
38.78
80.38
79.13
79.56
79.69
79.19
82.64
89.04
82.64
75.92
89.42
Anti-, syn-
Anti-, anti-
a
kcal molꢀ1
.
Table 5
4.3. Electronic spectroscopy
LUMO levels of anthranilic emeraldine trimers calculated at B3LYP/6-311+G(2d,p) on
B3LYP/6-31Gꢁ optimized geometries, with zero-point and enthalpic corrections to
298.15 K at B3LYP/6-31Gꢁ.
Vertical electron affinities (EA) are a key index of potential util-
ity in corrosion inhibition applications (Table 4). Unsubstituted
aniline trimers (EB) have EA’s of 1.428 (syn) and 1.434 (anti)
kcal molꢀ1. For the corresponding AE trimers (AEB), the EA’s range
from 1.638 eV for the ssi isomer, to 1.684 eV for the sso isomer. The
very small range of values suggests that the primary influence of
the carboxylate group is electronic, not steric. The difference in
EA’s between the unsubstituted trimer and AE results from the
electron-withdrawing effect of the carboxyl group by resonance.
As has been previously described, doping of aniline trimers leads
to a very substantial increase in the electron affinity, attributable
to the strong electron-withdrawing effects of hydrochloride, pri-
marily by induction (the electronegativity of chlorine). The calcu-
lated electron affinities of the AE dihydrochlorides vary from
Emeraldine
basea
Emeraldine
Emeraldine
zwitteriona
dihydrochloridea
Syn-, syn- (inner)
Syn-, syn- (outer)
Syn-, anti-
Anti-, syn-
Anti-, anti-
ꢀ2.92
ꢀ2.95
ꢀ2.93
ꢀ2.93
ꢀ2.94
ꢀ4.65
ꢀ4.58
ꢀ4.61
ꢀ4.60
ꢀ4.58
ꢀ4.77
ꢀ5.05
ꢀ4.92
ꢀ4.45
ꢀ5.05
a
eV.
the imine nitrogens in AE are far weaker bases than those in aniline
oligomers featuring a hydroxyl group (30-hydroxy,40-aminophenyl)
or amino group (30,40-diaminophenyl) in place of the carboxylate.
Table 6
Experimental and theoretical electronic transitions (in nanometers) of anthranilic emeraldine trimers calculated at B3LYP/6-311+G(2d,p)
on B3LYP/6-31Gꢁ optimized geometries, with zero-point and enthalpic corrections to 298.15 K at B3LYP/6-31Gꢁ. Solvent effects in
acetonitrile using SCIPCM method. CIS (at CIS/6-311+Gꢁ), TDDFT results (at B3LYP/6-311+Gꢁ) and ZINDO/S results are on B3LYP/6-31Gꢁ
optimized geometries (gas phase). Experimental transitions are for mixture of isomers; results listed as ‘‘experimental” are identical for
each isomer, since it was not possible to ascribe transitions to specific isomers.
Transitiona
Experimental
D
SCF
D
SCF (SCIPCM)
ZINDO
CIS
TDDFT
Syn-, syn- (inner)
Syn-, syn- (outer)
Syn-, anti-
H ? L
533
416
320
533
416
320
533
416
320
533
416
320
533
416
320
508
415
341
516
418
344
512
416
344
517
418
342
520
419
343
526
424
340
533
426
344
530
424
344
533
425
343
N/A
N/A
N/A
504
449
358
507
451
358
505
450
357
511
445
357
513
447
357
550
385
346
556
388
345
552
387
345
563
385
344
565
386
343
H ꢀ 1 ? L
H ? L + 1
H ? L
H ꢀ 1 ? L
H ? L + 1
H ? L
H ꢀ 1 ? L
H ? L + 1
H ? L
H ꢀ 1 ? L
H ? L + 1
H ? L
380
384
382
384
385
Anti-, syn-
Anti-, anti-
H ꢀ 1 ? L
H ? L + 1
a
H = HOMO, L = LUMO, H ꢀ 1 = next highest occupied molecular orbital, L + 1 = next lowest unoccupied molecular orbital.

226
L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
3.431 eV to 3.487 for the ssi isomer. This corresponds to a range of
only 0.056 eV among the isomers. Surprising, the zwitterions of AE
trimers, undoped by an external mineral acid, have an even greater
electron affinity than the mineral-acid-doped emeraldine dihydro-
chloride salt. These can be as low as 3.293 eV, or as large as
3.861 eV, demonstrating that structural effects exert subtle but
substantial effects on the overall electron affinity. The phenome-
non of ‘‘self-doping” appears to be manifest through zwitterion for-
mation, with the benefit of increasing the electron affinity of the
oligomer beyond what could be achieved by mineral acid doping.
Unsubstituted aniline trimer dihydrochlorides have calculated
LUMO’s at ꢀ4.44 eV (syn isomer), or ꢀ4.42 eV (anti). The calcu-
lated LUMO levels (Table 5) for the AE dihydrochloride range from
ꢀ4.58 to ꢀ4.65 eV (for the ssi isomer). The lower LUMO levels of
the AE dihydrochlorides can be attributed to the additional with-
draw of electron density by the two carboxylic acid groups, largely
though inductive effect of the oxygen atoms. The AE zwitterions,
lacking the strong electron-withdrawing effect of the hydrochlo-
ride dopant, nevertheless have an even lower LUMO level than
the AE dihydrochloride. The calculated levels range from
ꢀ4.45 eV to ꢀ5.05 eV for the AE aa zwitterion isomer. The proton
dopant on the imine nitrogens can be expected to withdraw far less
electron density than would hydrogen chloride. Similarly, the car-
boxylate anion of the AE zwitterions should be less effective in
withdrawing density by induction than the un-ionized carboxyl
(because of the negative charge on the carboxylate). Therefore,
the difference in LUMO levels must result from the substantial
withdrawal of density by resonance. The electronic affinities of
the various AE isomers closely parallel the calculated LUMO levels,
suggesting that the most significant determinant of electron affin-
Fig. 4. Calculated frontier orbitals of anti-, syn- N,N0(30-carboxy,40-aminophenyl)-1,4-quinonediimine at B3LYP/6-311+G(2d,p) on the B3LYP/6-31Gꢁ optimized structure:
LUMO+1 (top), LUMO (upper middle), HOMO (lower middle), and HOMOꢀ1.

L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
227
ities (and therefore, corrosion inhibition) are those structural ef-
fects that ‘‘pull down” the LUMO.
1460 for acetonitrile, which suggests that the equilibrium is shifted
more toward zwitterion formation in water than in acetonitrile.
The near-IR bands (except for the band in water at 1982 nm) are
not very broad – in sharp contrast to the situation in doped poly-
anilines [39]. The broadness of the relevant near-IR peaks in poly-
aniline has been attributed to changes in the conformations of the
polymer, which conformations are not so important in the smaller
sized oligomers. The expected transitions for the non-zwitterion
appear in less polar, non-protic solvents such as DMSO and aceto-
nitrile – precisely those solvents where zwitterion formation
would be less expected.
For the unsubstituted aniline trimer, the primary electronic
transitions observed are H (HOMO) ? L (LUMO, 544 nm) and
H ? L + 1 (second lowest unoccupied molecular orbital, 277 nm).
Oxygenated aniline trimers, on the other hand, show primarily a
H ꢀ 1 (second highest occupied molecular orbital) ? L transition.
AE shows considerable difference from either of the other two clas-
ses of aniline trimer investigated. AE shows only the H ? L + 1
transition in water, but H ? L and H ꢀ 1 ? L in other solvents.
The frontier orbitals of AE are displayed in Fig. 4 (non-zwitterion)
and Fig. 5 (zwitterion). This strongly suggests that zwitterion for-
mation dominates in water, that non-zwitterion formation domi-
nates in other solvents, and that the one transition seen in both
sets of solvents is H ? L + 1, though H ? L + 1 for the non-zwitter-
ion is much different in character from the H ? L + 1 for the
zwitterions.
Unsubstituted aniline trimer dihydrochlorides have calculated
HOMO’s at ꢀ6.00 eV (syn isomer), or ꢀ6.02 eV (anti). For the AE tri-
mer hydrochlorides, the calculated HOMO values range from
ꢀ6.21 eV (for the ssi isomer) to ꢀ6.18 (st). The effect of the carbox-
ylate groups on the outer rings is to lower the HOMO, just as they
lower the LUMO.
For the AEB trimers in the non-zwitterionic form, predicted
electronic transitions by several methods are listed in Table 6.
While there is excellent agreement between the predictions for
the shortest wavelength absorption (320 nm.) and the experimen-
tal spectrum (Fig. 6), the next two absorptions are not seen in
water. For the zwitterions, the shortest predicted wavelength is
essentially the same as for the non-zwitterion, but the next two
predicted absorptions are substantially red-shifted to 923 and
1365 nm for the ssi isomer, or 1529 nm for the as isomer. Since
the as isomer is predicted to be the predominant form if conversion
to zwitterions is complete, its presence in the near-IR would be
evidence of zwitterion formation. In fact, the presence of a near-
IR band around 1600 nm [37] or 1700 nm [38] has been cited as
evidence of doping of polyaniline by Lewis or Brønsted acid. The
strong near-IR band are found at 1902 nm in acetonitrile, and
1982 nm in water, with smaller bands in both solvents around
1460 nm. The ratio of the absorbance at 1982 nm to that at
1460 nm is much great for water than the ratio of 1902 nm to
Fig. 5. Calculated frontier orbitals of the anti-, syn- N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine zwitterions at B3LYP/6-311+G(2d,p) on the B3LYP/6-31Gꢁ
optimized structure: LUMO+1 (top), LUMO (upper middle), HOMO (lower middle), and HOMOꢀ1.

228
L.T. Sein Jr., A.F. Lashua / Journal of Molecular Structure 977 (2010) 220–229
the effective conjugation length. However, the only shift observed
for the AE trimer upon addition of acid is a slight blue shift. This is
to be expected in the AE zwitterionic case, where the imine nitro-
gens are already protonated. The only protonation events possible
are at the carboxylates. Transformation of the carboxylate to a car-
boxyl group reduces the electron withdrawing ability of the group
by resonance, which, as stated above, is largely responsible for
pulling down the energy of the LUMO.
5. Conclusions
The AE trimer has be synthesized, and its experimental and the-
oretical investigation enables a deeper understanding of subtle
electronic and steric effects in co-polymers of aniline and anthra-
nilic acid. In non-protic solvents, the non-zwitterionic form pre-
dominates, whereas in water, the zwitterion predominates. This
AE zwitterion effectively self-dopes as the emeraldine base, with
LUMO levels and electron affinity even lower than those calculated
for the carboxyl substituted aniline trimer dihydrochloride. Since
lower LUMO levels and more negative electron affinity have been
linked with superior corrosion inhibition, further investigation of
these materials is warranted. The UV–vis of the trimer is almost
identical to that of the AA/A co-polymer, while the IR of the trimer
is very similar. For the perspective of electronic and vibrational
spectroscopy, with benefit of density functional theory, N,N0-
bis(30-carboxy,40-aminophenyl)-1,4-quinonediimine is an appro-
priate model for the aniline/anthranilic acid co-polymer. As an
appropriate (and computationally accessible) model, it gives in-
sight into phenomena in the co-polymer which would be impossi-
ble to access directly.
Fig. 6. UV–vis spectra of N,N0-(30-carboxy,40-aminophenyl)-1,4-quinonediimine in
acetonitrile, ethanol, and water solvent.
For the co-polymer [2], absorptions at greater than 600 nm de-
crease rapidly as the ratio of aniline to AA decreases. This is attrib-
uted to reduction of conjugation length of the co-polymer due to
steric effect of AA. It could also be attributed to the appearance
of the zwitterions. The UV–vis of the trimer is almost identical to
that of the AA/A co-polymer. According to reference [2], the
absorption at 330 nm in the co-polymer is
p ?
pꢁ and ‘‘related to
the extent of conjugation between adjacent phenyl rings”, whereas
in the trimer it seems to be an H ? L + 1 transition, involving in-
tense charge transfer between the outer rings and the central quin-
oid ring. The authors [2] note a shift from 330 nm (in polyaniline)
to 280 nm (in the AA/A 1:1 co-polymer), which they attribute to
the ‘‘steric effect of the carboxylate groups in the polymer chain
Acknowledgements
L.T.S. wishes to thank the Pittsburgh Supercomputing Consor-
tium for an allocation of advanced computing resources with the
support of the National Science Foundation and the Common-
wealth of Pennsylvania through supercomputing grant
DMR040003P.
that causes perturbation in of the coplanarity of the
For the trimers, the change is in the opposite direction. The ani-
line trimer has its
pꢁ transition at 277 nm, which is shifted to
p-system. . .”
p
?
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