Properties, aromaticity, and substituents effects in poly nitro- and amino-substituted benzenes

ORIGINAL RESEARCH

Properties, aromaticity, and substituents effects in polynitro- and amino-substituted benzenes

Irina V. Omelchenko • Oleg V. Shishkin •

Leonid Gorb • Frances C. Hill • Jerzy Leszczynski

Received: 11 December 2011 / Accepted: 4 February 2012 / Published online: 24 February 2012

� Springer Science+Business Media, LLC 2012

Abstract Geometrical parameters, aromaticity, and con-

formational flexibility of the set of polysubstituted ben-

zenes with different number and position of nitro and

amino groups were calculated at the MP2/cc-pvdz level of

theory. The key factor for structural and energetic changes

has been identified. This is related to the presence of nitro

and amino groups in vicinal positions that forms strong

intramolecular resonance-assisted hydrogen bonds with a

binding energy of 7–14 kcal/mol. Increasing number of

such bonds facilitates a cooperative effect, inducing nota-

ble changes in molecular geometry (particularly increasing

bond alternation within H2N–C–C–NO2 fragment and

planarization of amino group), drastic increasing of con-

formational flexibility and decreasing of aromaticity. In

spite of well-known p-electron effects of nitro and amino

substituents, influence of their push–pull interaction

through aromatic moiety is negligible compared to the

effect of the hydrogen bonding. That results in great dif-

ference of the ortho-isomers as compared to meta- and

para-isomers.

Keywords Aromaticity � Push–pull effect �Intramolecular hydrogen bond � Quantum chemical

calculations

Introduction

Aromaticity is one of the most general and important

concepts in organic chemistry [1]. This phenomenon also

provides the background to understanding the influence of

substituents on reactivity and stability of the very wide

range of organic species containing aromatic and hetero-

aromatic moieties. The numerous examples of such influ-

ence are available in the literature elsewhere [2–4]. Among

them, famous Hammett’s constants describing electronic

strength of substituents were derived for aromatic mole-

cules. Using this approach, it is very easy to understand

that mechanism of such influence also including changes in

aromaticity of benzene ring.

Influenced by the Hammett equation, there have been

numerous attempts to describe substituent effect on aro-

matic moiety quantitatively [5]. The common way is the

usage of some characteristic physicochemical properties

(such as amino group basicity, catalytic activity, change of

infrared, or NMR properties of substance) as an aromaticity

measure [6]. Such characteristics are usually called aro-

maticity indexes. Recently, aromaticity indexes were used

to estimate substituent effect [7, 8]. Analysis of mono-

substituted benzenes revealed that influence of a single

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11224-012-9971-8) contains supplementarymaterial, which is available to authorized users.

I. V. Omelchenko (&) � O. V. Shishkin

STC’Institute for Single Crystals, National Academy of Sciences

of Ukraine, 60 Lenina Ave, Kharkiv 61001, Ukraine

e-mail: [email protected]

O. V. Shishkin

V.N. Karazin Kharkiv National University, 4 Svobody sq,

Kharkiv 61077, Ukraine

L. Gorb

Badger Technical Services, LLC, Vicksburg, MS, USA

F. C. Hill � J. Leszczynski

US Army ERDC, 3532 Manor Dr, Vicksburg, MS 39180, USA

J. Leszczynski

Interdisciplinary Center for Nanotoxicity, Department of

Chemistry and Biochemistry, Jackson State University,

P.O. Box 17910, 1325 Lynch Street, Jackson, MS 39217, USA

123

Struct Chem (2012) 23:1585–1597

DOI 10.1007/s11224-012-9971-8

group could be essential but aromaticity of central moiety

remains relatively high.

Much less attention has been paid to cooperative effect

of several substituents. Historically, such systems were

studied in view of transmission of some electronic effects

from one group to another through aromatic moiety

[6, 9]. This consideration was given mainly to disubsti-

tuted derivatives of benzene. It is usually discussed in

terms of push–pull interactions [10]. Substituents which

have an ability to donate (‘‘push’’) or withdraw (‘‘pull’’)

electrons from the p-system of aromatic ring are con-

sidered to interact through that ring in accordance with

internal resonance polarization of p-system, changing the

properties of aromatic ring. Unlike r-polarization, the

p-polarization depends strongly on the relative positions

of the substituents [6]. Appearance of substituents in

benzene ring always results in a decrease of aromaticity

[6]. Study of homodisubstituted benzenes also revealed

that electron-donating groups affects aromaticity much

stronger than electron-withdrawing [11] ones, in agree-

ment with the data for monosubstituted benzenes [8]. For

the molecules with different types of substituents, aro-

maticity as well as molecular geometry and some other

properties usually depends on their relative positions

[6, 12–16]. However, some studies show a negligible

distinction between position of isomers or even contro-

versial estimations of their influences by different aro-

maticity indices or molecular properties [9, 11, 17–20].

There is still lack of systematic study of heterosubstituted

systems with pronounced p-electronic effects by versatile

methods.

In the case of the presence of more than two substituents

situation becomes more complex, especially if they have

different or even opposite character of electronic effects.

Several ways for interaction between substituents are

possible including both electronic interactions through

aromatic p-system as well as non-bonding interactions

between vicinal groups.

Suitable models for investigation of cooperative effects

in the species possessing strong mesomeric effects of

opposite signs are provided by aminonitrobenzene deriva-

tives having different number and position of amino and

nitro groups. For such moieties, one could expect strong

interactions between substituents causing significant

changes of properties of aromatic system of benzene ring.

In addition some aminonitrobenzenes (e.g., triaminotrini-

trobenzene and its analogs) are objects of great practical

interest in fields of energetic materials [21, 22] and

molecular biochemistry [23, 24]. Nitroanilines and their

derivatives are of interest for photoelectronic spectroscopy

as model charge-transfer systems [25–27]. Also, they are

widely discussed in frameworks of different theoretical

methods—quantum chemical calculations as well as

molecular dynamic modeling [28–30]. Substituent effects

of nitro and amino groups on the specific molecular and

crystal properties, i.e., impact sensibility, decomposition

mechanism and its energetic barrier, biological activity,

photoelectronic properties, etc., are widely discussed in

such studies. However, there are much less studies focused

on aromaticity and cooperative substituent effects of amino

and nitro groups [11–13].

In this article, we study the dependence of structural

parameters of molecular system, aromaticity of benzene

ring, and its rigidity on the nature, amount, and the position

of substituents. The aminonitrobenzene derivatives having

different number and position of amino and nitro groups

are selected as the model systems to investigate coopera-

tive influence of substituents.

R4

R3

R2

R1

R6

R5

1

2

34

5

6

Methods of calculation

The structures of all examined molecules (Table 1) have

been optimized using the Møller–Plesset second-order

perturbation theory [31] with correlation-consistent double-

zeta basis set [32] (MP2/cc-pVDZ method). Normal

vibrational modes were founded by calculations of Hes-

sian, at the same level of theory. No negative eigenvalues

of Hessian were found for the studied species.

Aromaticity of molecules under study was described by

aromaticity indices [33]. All indices have restrictions in use

[34]; therefore, it is preferred to use several different

indices [35–38]. In this study, structural Bird (Ia) [39] and

HOMA [40] indices, and magnetic index NICS(1)zz [41]

were used. As for commonly used energetic indices, there

is no unambiguous scheme of calculation of aromatic sta-

bilization energies (ASE) for polysubstituted molecules

[34] using the most reliable homodesmotic reaction

scheme.

It was already demonstrated [8, 42–44] that aromaticity

of cyclic conjugated system is closely related with its

conformational flexibility. Energy of ring out-of-plane

deformation and other characteristics of ring flexibility

correlate well with aromaticity degree. Therefore, they also

may be used as aromaticity indices.

1586 Struct Chem (2012) 23:1585–1597

123

All aromaticity indices (Ia, NICS(1)zz, HOMA) were

calculated using MP2/cc-pVDZ level of theory. Gordy

bond order calculation scheme [45] with empirical con-

stants was applied to calculation of Ia index according to

the standard equation [33]. Nucleus-independent chemical

shifts were calculated in the point located by 1 A above the

center of the ring (NICS(1)zz) as it was recommended [46]

for obtaining more accurate data.

Conformational flexibility of rings was studied on the

same level of theory by scan of each of symmetry-inde-

pendent endocyclic torsion angles in the range ±30� with

5� steps. All remaining geometrical parameters were opti-

mized at every step of scan. For each molecule, the angle

with the smallest energy of deformation E(def) (difference

in energy between equilibrium and twisted on 30� geom-

etries) was chosen.

All calculations have been performed using the

Gaussian03 [47] and GAMESS US [48] program packages.

Electron density properties were studied using Bader’s

‘‘Atoms in Molecules’’ theory (AIM) with AIM2000 pro-

gram [49] Energy of intramolecular hydrogen bonds was

estimated following the Espinosa equation [50] based on

value of local potential energy in bond critical point for

interaction.

Results and discussion

Molecular structure

Homosubstituted molecules

Analysis of the structural parameters of homosubstituted

molecules reveals good agreement of the current data with

the previous results [8, 11, 15]. Presence of nitro substit-

uents leads to small elongation of the C–C bond lengths

through the whole ring (1.398–1.406 vs 1.394 A in ben-

zene) and to increasing of ipso bond angle (120.3–123.2).

Only the angle at the ‘‘central’’ NO2 group in the 1,2,3-

trinitrobenzene (123N) slightly decreases (118.0). As for

aminobenzenes, the geometric changes are more pro-

nounced but local elongation of bonds is limited to the

Ci–Co bonds (1.407–1.417 A). Ipso bond angles slightly

decrease (117.8�–119.8�). Thus, the homosubstitution

affects molecular structure of the benzene ring according to

the r-electronic effect model. Experimental data for the gas

phase reveal the same trend (see Tables 2, 3 and references

below), except the nitrobenzene case with notable alter-

nation of the bond length (1.375 A Ci–Co bonds, 1.403 A

Co–Cm, and 1.396 A Cm–Cp).

In the nitrobenzene, as well as in 1,3- and 1,4-dinitro-

benzenes, the nitro group tends to be coplanar with aromatic

ring through conjugation of their p-systems. 1,2-dinitro-

benzene and polynitrobenzenes with adjacent nitro groups

undergo notable steric repulsion that can be roughly esti-

mated by examination of torsion angle between planes of

base ring and substituent (Table 2). The maximum rotation

angle corresponds to ‘‘central’’ nitro group in 1,2,3-trini-

trobenzene that undergo the maximum repulsion. Rotation

of these nitro groups causes no evident structural effect on

the aromatic moiety (see Table S1 in Supplementary mate-

rial). However, one can note decreasing of the C–NO2 bond

length for adjacent nitro groups. In addition, big values of

rotation angles result in reducing the p-conjugation with

aromatic system. At the time, there is very weak but quite

clear trend for elongation of the C–N bond for substituents

located in meta-position, with respect to para-position.

As for aminobenzenes, C–NH2 bond length and pyramidality

of NH2 group increases in the ortho- and para-diaminobenzenes

in comparison with aniline and meta-diaminobenzene, and

in the ortho,para-triaminobenzene (124A) in comparison

Table 1 Molecules under consideration

Molecule R1 R2 R3 R4 R5 R6

Benzene H H H H H H

1N NO2 H H H H H

12N NO2 NO2 H H H H

13N NO2 H NO2 H H H

14N NO2 H H NO2 H H

123N NO2 NO2 NO2 H H H

135N NO2 H NO2 H NO2 H

1A NH2 H H H H H

1N2A NO2 NH2 H H H H

1N3A NO2 H NH2 H H H

1N4A NO2 H H NH2 H H

13N2A NO2 NH2 NO2 H H H

13N4A NO2 H NO2 NH2 H H

13N5A NO2 H NO2 H NH2 H

135N2A NO2 NH2 NO2 H NO2 H

12A NH2 NH2 H H H H

13A NH2 H NH2 H H H

14A NH2 H H NH2 H H

1N24A NO2 NH2 H NH2 H H

1N35A NO2 H NH2 H NH2 H

13N24A NO2 NH2 NO2 NH2 H H

13N25A NO2 NH2 NO2 H NH2 H

123N46A NO2 NO2 NO2 NH2 H NH2

135N24A NO2 NH2 NO2 NH2 NO2 H

124A NH2 NH2 H NH2 H H

135A NH2 H NH2 H NH2 H

1N246A NO2 NH2 H NH2 H NH2

13N246A NO2 NH2 NO2 NH2 H NH2

135N246A NO2 NH2 NO2 NH2 NO2 NH2

Struct Chem (2012) 23:1585–1597 1587

123

with meta,meta-isomer (135A) (Table 3). Adding extra

amino groups results in elongation of C–NH2 bond lengths in

all isomers as compared to aniline. Presence of amino groups

increases vicinal endocyclic C–C bond length up to

1.410–1.419 A (1.394 A for benzene) regardless of the rel-

ative substituents position. This effect is larger than analo-

gous increase predicted for any nitrobenzene derivatives

(1.398–1.406 A, Table S1 in Supplementary material).

Therefore, one may conclude that structure of the ben-

zene moiety is quite resistant to the perturbations caused by

the presence of several nitro groups and, in some less

extent, one or two amino groups (see Table S1 in Sup-

plementary material).

Isomeric nitroanilines

Molecules with both p-donor and p-acceptor substituents

are considered to reveal strong push–pull effect [10] that is

transmitted through aromatic system. It should change

molecular structure of ortho- and para-isomers according

to the quinoid canonical resonance structures with partial

charge transfer through p-system. Scheme 1 represents a

set of canonical structures for para-nitroaminobenzene.

Therefore, ortho- and para-nitroanilines 1N2A and 1N4A

are expected to reveal alternation in the benzene ring,

shortening of the C–NH2 and the C–NO2 bond lengths,

and flattening of the amino group, in contrast to meta-

nitroaniline 1N3A which does not share a push–pull effect

[10].

However, the results of calculation do not reveal sig-

nificant deformation of bond lengths within benzene ring in

para-isomer 1N4A as compared to meta-isomer 1N3A.

Slight changes in bond lengths may be explained by

superposition of the effects of nitro and amino groups

mentioned above (see Tables S1, S2 in Supplementary

material). There are only some shortening of the C–NH2

bond and flattening of amino group as compared to aniline.

These effects reflect conjugation interactions between

substituents (Table 4).

Table 2 Values of the C–N bond lengths (A) and the C–C–N–O

torsion angles (�) in nitrobenzenes

Molecule C–NO2 C–C–N–O

1N 1.483 0.0

Exp.a 1.492(1) 0.0(1)

12N 1.472 42.5

13N 1.487 0.0

14N 1.482 0.0

123N 1.477 (C1) 35.8

1.473 (C2) 62.4

135N 1.489 0.0

Identical values for symmetrically equivalent groups are omitteda Gas-phase electron diffraction and microwave spectroscopy

experiment, rs distances [51]

Table 3 Values of the C–N bond lengths (A), pyramidality of amino

group estimated as sum of bond angles centered at the nitrogen atom

(P

(NH2), �), and angle between planes of amino group and benzene

ring (u(NH2), �) in amino benzenes

C–NH2

P(NH2) u(NH)

1A 1.410 332.2 44.4

Exp.a 1.406(3) 44(4)

Exp.b 1.402(2) 37(2)

12A 1.416 328.7 54.6

13A 1.413 331.2 50.0

14A 1.417 329.5 51.3

Exp.c 1.422(2) 43(4)

124A 1.423 (C1) 326.8 57.0

1.415 (C2) 329 53.7

1.418 (C3) 329.2 51.8

135A 1.414 (C1, C5) 330.5 50.6

1.414 (C3) 330.9 50.3

Identical values for symmetrically equivalent groups are omitteda Gas-phase electron diffraction experiment, rg distances [52]b Gas-phase microwave and electronic spectroscopy experiment [53]c Gas-phase electron diffraction experiment, rg distances [54]

NH H

N+

OO

NH H

N+

OO

NH H

N+

OO ON

+O

NH H

ON

+O

NH H

ON

+O

NH H

ON

+O

NH H

+ +

+

+ +

-

- - +

+

Scheme 1 Canonical resonance structures for para-nitroaminobenzene (1N4A)

1588 Struct Chem (2012) 23:1585–1597

123

In ortho-isomer, the C–NH2 bond length is notably

shorter, and amino group is more flattened than in meta-

and para-isomers (Table 4). Nitro group in 1N2A is not

coplanar to benzene ring (Table 4) and the C–NO2 bond is

slightly shorter than in 1N3A and 1N4A. That is accom-

panied by alternation of endocyclic bond lengths (short-

ening of C(3)–C(4) and C(5)–C(6) bonds) and by

decreasing of the CC(NH2)C angle. Although such struc-

tural changes are typical for conjugative interaction as

well, no similar changes are observed in the para-nitro-

aniline structure. Thus, para- and meta-nitroanilines pos-

sess very close geometrical parameters that differ from

ortho-nitroaniline. This set of data does not match the

values expected due to push–pull effect but may be caused

by direct local interaction between vicinal substituents.

These results do not match experimental results obtained

from X-ray diffraction study of the crystalline state [56–59]

in which noticeable structural changes that correspond to

the push–pull effect are clearly observed in both ortho-

[56] and para- [57, 58] nitroanilines as compared to meta-

nitroaniline [59]. However, effect of the polar medium and

strong intermolecular hydrogen bonding that are observed

in crystals of all isomeric nitroanilines can affect properties

related to the p-electron system dramatically [60, 61].

These effects are a subject of particular study and therefore

crystal structure data as well as data obtained in polar

solvents cannot be a reference for molecules examined in

vacuum. Available gas-phase data exist only for para-

nitroaniline. Experimental C–NH2 and C–NO2 bond

lengths are some smaller than calculated (Table 4) but

precision of their determination is rather low, and only

averaged endocyclic bond length was given (rg(C–C) =

1.402(4) A). Thus, push–pull effect cannot be revealed

from these data.

Analysis of the electron density distribution using

Bader’s AIM theory demonstrated existence of the

N–H���O hydrogen bond between substituents bonding

(H���O 1.957 A, N–H���O 125.6�). Taking into account that

proton donating and withdrawing sites of this bond are

connected by benzene ring as resonant spout it is possible

to consider it [62–66] as resonance-assisted hydrogen bond

(RAHB) (Scheme 2). Considerable contribution of the

resonance structure C in the total structure of the 1N2A

molecule should cause shortening of the C–N bonds, flat-

tening of amino group mentioned above, and elongation of

the C1–C2 endocyclic bond which is 1.420 A. Taking into

account pyramidal configuration of amino group rotation of

the nitro group creates more favorable conditions for for-

mation of the N–H���O hydrogen bond. The N–O bond of the

nitro group of 1N2A that participates in RAHB is slightly

longer (1.240 A) than the other N–O bond (1.231 A). In all

other nitroanilines and homosubstituted molecules under

study, length of the N–O bonds of nitro groups are the same.

Therefore, some deformation of geometry of 1N2A may be

explained by the formation of intramolecular RAHB rather

than push–pull p-electronic effects.

Polyheterosubstituted molecules

Geometrical parameters of polynitroaminobenzenes vary in

a much more wide range than parameters of species dis-

cussed above. The specific structural feature of highly

substituted molecules with adjacent nitro and amino sub-

stituents is reflected by a minor non-planarity of benzene

rings in equilibrium geometry (endocyclic torsion angles

vary up to 6�, see Table 5). In the previous theoretical

studies of 2,4,6-triamino-1,3,5-trinitrobenzene (135N246A,

TATB), it was found that degree of non-planarity of mol-

ecule strongly depends on the level of theory and quality

of basis applied to chemical modeling [15, 29, 67–69].

High-quality calculations reveal non-planarity of TATB

Table 4 Values of the C–N bond lengths (A), pyramidality of amino

group estimated as sum of bond angles centered at the nitrogen atom

(P

(NH2), �) and the C–C–N–O torsion angles (�) in nitroanilines,

nitrobenzene, and aniline

Molecule C–NH2

P(NH2) C–NO2 C–C–N–O

1N – – 1.483 0.0

1A 1.410 332.2 – –

1N2A 1.387 341.3 1.473 16.1

1N3A 1.400 335.8 1.475 0.9

1N4A 1.400 336.2 1.481 0.3

Exp.a 1.36(3) (360) 1.47(1) (0)

a Gas-phase electron diffraction experiment, rg distances [55]. Values

in parenthesis were fixed during experiment analysis so they are non-

informative

N+

NH

OOH N

+

N+

O OH

H

N+

N

H

H

OO

A B C

Scheme 2 Resonance

structures proposed for

hydrogen bonding of ortho-

nitroaniline

Struct Chem (2012) 23:1585–1597 1589

123

molecule in equilibrium geometry that arises from an

extremely low energy of out-of-plane deformation of

molecule [12]. Experimental values of torsion angles [70]

measured by XRD method cannot be hold as reference due

to the strong influence of polar medium and intramolecular

interactions in crystal.

As may be seen from Table 5, non-planarity of benzene

ring is observed mainly for molecules with at least two

nitro groups and two vicinal locations of nitro and amino

groups. This allows to assume that non-planarity is caused

by interaction between these substituents.

Analysis of bond lengths within benzene ring in poly-

substituted molecules indicates only one type of deformation

namely elongation of bonds. No alternation of bonds is

observed. The most significant elongation is found for mol-

ecules with vicinal arrangement of nitro and amino groups

(Table 6). In these cases, length of the C–C bond between

substituents is within 1.421–1.446 A. These values are dis-

tinct from the value of bond length in benzene (1.394 A).

This effect is promoted by increase of number of vicinal nitro

and amino groups and it becomes maximal in symmetric

trinitrotriaminobenzene 135N246A (Table 6).

Geometrical parameters of substituents reveal the same

tendency as in the case of nitroanilines. Vicinal location of

nitro and amino groups results in shortening of the C–NH2

bonds, flattening of the NH2 group, and nitro group rotation

with respect to the ring plane (Table 7). In the series of

isomeric dinitroanilines, diaminonitrobenzenes, and diam-

inodinitrobenzenes, amino groups adjacent to the two nitro

groups (13N2A and amino group at the C2 atom of

13N24A) reveals the shortest C–NH2 bond (1.360–1.364 A)

and the most planar amino group (pyramidality degree

described as sum of bond angles centered at the nitrogen

atom is within 350.5�–352.6�). Amino groups in the 13N4A,

123N46A, at the C2 atom of the 1N24A and at the C4 atom of

13N24A (ortho-positions, with one nitro group adjacent to

amino) possess intermediate structure (C–NH2 1.375–1.386

A, pyramidality 341.6�–346.8�). The amino groups in

13N5A, 1N35A, at the C5 atom of 13N25A (meta-positions),

and at the C4 atom of the 1N24A (para-position) that do not

Table 5 Maximal values of endocyclic torsion angles (�) in mole-

cules with non-planar benzene ring according to quantum chemical

calculations and experimental data

Molecule Torsion angle Value Method

13N2A C1–C2–C3–C4 3.2 MP2/cc-pvdz

1.2 Experiment [70]

13N24A C6–C1–C2–C3 2.6 MP2/cc-pvdz

13N25A C6–C1–C2–C3 2.6 MP2/cc-pvdz

123N46A C6–C1–C2–C3 2.9 MP2/cc-pvdz

135N2A C6–C1–C2–C3 2.6 MP2/cc-pvdz

0.0 B3LYP/DNP [15]

2.8 Experiment [70]

135N24A C6–C1–C2–C3 3.5 MP2/cc-pvdz

0.0 B3LYP/DNP [15]

4.2 Experiment [70]

13N246A C4–C5–C6–C1 4.0 MP2/cc-pvdz

135N246A C1–C2–C3–C4 5.7 MP2/cc-pvdz

4.6 MP2/6-311G(d,p) [67]

3.4 B3LYP/6-311G(d,p) [67]

0.1 MP2/6-311(2d,2p) [12]

0.0 B3LYP/6-311(2d,2p) [12]

0.8 MP2/aug-cc-pvdz [12]

0.0 B3LYP/aug-cc-pvdz [12]

5.0 Experiment [70]

Only molecules with torsion angle values larger than 2� are listed

Table 6 Values of the C–C bond lengths (A) within benzene ring in heteropolysubstituted molecules

Isomer Molecule C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C1–C6

Ortho, ortho 13N2A 1.432 1.432 1.398 1.398 1.398 1.398

Ortho, para 13N4A 1.390 1.401 1.422 1.422 1.392 1.403

Meta, meta 13N5A 1.399 1.399 1.397 1.414 1.414 1.397

Ortho, para 1N24A 1.421 1.414 1.401 1.416 1.391 1.407

Meta, meta 1N35A 1.399 1.410 1.410 1.410 1.410 1.399

Ortho, ortho/ortho, para 13N24A 1.427 1.438 1.420 1.417 1.384 1.403

Ortho, ortho/meta, meta 13N25A 1.428 1.428 1.399 1.406 1.406 1.399

Ortho, ortho/meta, meta 123N46A 1.398 1.398 1.420 1.408 1.408 1.420

Ortho, ortho/ortho, para/para, para 135N2A 1.437 1.437 1.393 1.392 1.392 1.393

135N24A 1.437 1.442 1.442 1.437 1.386 1.386

1N246A 1.426 1.406 1.404 1.404 1.405 1.426

13N246A 1.446 1.446 1.436 1.401 1.401 1.436

135N246A 1.443 1.443 1.443 1.443 1.443 1.443

The types of isomers are listed

1590 Struct Chem (2012) 23:1585–1597

123

have adjacent nitro group reveals the longest C–NH2 bonds

(1.390–1.404 A) and the highest pyramidality (334.3�–

339.8�). The C–NO2 bond lengths depend on the presence

of adjacent amino group generally in the same way (the

shortest bond of 1.463 A is observed in the 13N24A, while

the longest bond of 1.485 A–in the 1N35A). Nitro group of

13N24A that has two vicinal amino groups undergo maxi-

mum rotation (29.6� deviation from the ring plane).

Molecular structures of trinitro- and triaminosubstituted

species (excluding 123N46A) agree with the above-men-

tioned trends. In general, the C–NH2 and the C–NO2 bonds

are shorter and amino group becomes more planar with the

increasing number of adjacent substituents. For example, the

C–NH2 bond length decreases from 1.364 A in 13N2A to

1.343 A in 13N246A, but it is much longer in the molecules

that do not contain adjacent nitro and amino groups (1.390 A

in 13N5A, 1.404 A in 1N35A). Correspondingly, the C–NO2

bond length decreases from 1.478 to 1.444 A (it is equal

1.485 A in 13N5A and 1.479 A in 1N35A), and amino group

becomes planar (pyramidality degree is 350.5� in 13N2A and

360.0� in 13N246A, and is much smaller in both meta,meta-

isomers—339.8� in 13N5A and 334.3� in 1N35A). The

remarkable exception is the 2,4,6-triaminonitrobenzene

(1N246A). The C–NH2 bonds 1.392–1.402 A and pyrami-

dality degree of amino groups 335.3�–338.4� are much clo-

ser to the values for mononitroanilines than for other

polysubstituted molecules. The molecular parameters of this

compound do not match the general trend in changing of

geometrical parameters with increasing number of amino

group. At the time, changes in the C–C bonds in 1N246A are

very close to those in 135N2A. Considerations of electron

density in the 1N246A molecule reveal that donating of the

extra electrons by amino groups into p-orbitals of the ben-

zene ring is energetically unfavorable for aromatic system.

Interestingly, this effect is compensated by electron-with-

drawing effect of the second and third nitro groups in all

other molecules.

As well as in mononitroanilines, direct interactions

between vicinal nitro and amino substituents play the

leading role in the all structural changes mentioned above.

Table 7 Values of the C–N bond lengths (A), pyramidality of amino groups estimated as sum of bond angles centered at the nitrogen atom

(P

(NH2), �) and the mean values of the two C–C–N–O torsion angles (�) for nitro groups in polynitroaminobenzenes

Isomer Molecule C–NH2

P(NH2) C–NO2 C–C–N–O

Ortho, ortho 13N2A 1.364 350.5 1.478 18.1

Ortho, para 13N4A 1.375 346.8 1.476 (C1) 0.4

1.476 (C3) 13.0

Meta, meta 13N5A 1.390 339.8 1.485 1.0

Ortho, para 1N24A 1.386 (C2) 341.6 (C2) 1.465 9.8

1.401 (C4) 335.6 (C4)

Meta, meta 1N35A 1.404 334.3 1.479 1.5

Ortho, ortho/ortho, para 13N24A 1.360 (C2) 352.6 (C2) 1.466 (C1) 1.5 (C1)

1.376 (C4) 345.3 (C4) 1.463 (C3) 29.6(C3)

Ortho, ortho/meta, meta 13N25A 1.377 (C2) 344.2 (C2) 1.478 (C1) 19.6 (C1)

1.400 (C5) 335.1 (C5) 1.478 (C3) 19.3 (C3)

Ortho, ortho/meta, meta 123N46A 1.381 342.1 1.465 (C1) 39.6 (C1)

1.475 (C2) 59.2 (C2)

Ortho, ortho/ortho, para/para, para 135N2A 1.350 358.0 1.482 (C1) 10.8 (C1)

1.482 (C3) 10.4 (C3)

1.477 (C5) 0.1 (C5)

135N24A 1.343 (C2) 359.6 (C2) 1.468 (C1) 2.0 (C1)

1.343 (C4) 359.8 (C4) 1.458 (C3) 16.4 (C3)

1.468 (C5) 3.8 (C5)

1N246A 1.392 (C2) 338.9 (C2) 1.454 27.9

1.402 (C4) 335.3 (C4)

1.392 (C6) 338.4 (C6)

13N246A 1.343 (C2) 360.0 (C2) 1.444 (C1) 7.0 (C1)

1.364 (C4) 352.7 (C4) 1.444 (C3) 6.5 (C3)

1.364 (C6) 352.5 (C6)

135N246A 1.336 360.0 1.445 11.2

Values are listed consequently from position 1 to position 6 of substituent. Identical values for symmetry equivalent groups are skipped

Struct Chem (2012) 23:1585–1597 1591

123

Repulsion of nitro groups affects them only slightly so

geometrical parameters of 123N46A with nitro group

rotated at almost 60� are close to the 13N25A (both

ortho,meta-isomers), but not to 135N24A (ortho,ortho-

isomer). Some minor changes can be referred to the push–

pull interaction with amino group in the para-position as

well. In the 135N2A, the C(5)–NO2 bond that does not

have adjacent substituents but has an amino group in the

para-position, is shorter than C(1)–NO2 and C(3)–NO2 by

0.005 A. In 13N4A, both C–NO2 bonds having amino

group in ortho- and para-positions are of the equal length.

No such effect was observed in the nitroanilines. Thus, in

polysubstituted molecules, C–NO2 bond length is little

more sensitive to the p-conjugation with amino group in

para-position. One may suppose that perturbative effect of

the intramolecular RAHB makes the aromatic system more

responsive to the push–pull effect. However, that does not

affect the endocyclic bond lengths of the benzene rings in

the corresponding molecules (see Table 6).

Thus, based on these data, it is possible to conclude that

effect of the p-electron interactions between substituents

through aromatic ring on the geometry of polysubstituted

nitroaminobenzenes is negligible as compared to direct

non-bonded interactions between vicinal substituents. The

character of changes in bond lengths indicates that this is

due to formation of resonance-assisted N–H���O hydrogen

bonds rather than to conjugation between substituents.

Lengths of the N–H���O–N hydrogen bonds vary from

1.957 A in 2-aminonitrobenzene 1N2A to 1.703 A in

135N246A (Table 8). Analysis of these molecules using

Bader’s AIM theory revealed existence of corresponding

(3; -1) bond critical points (BCP) for all N–H���O–N

contacts. Energy estimated from BCP properties indicates

rather strong bonding (Table 8).

The N–H���O bond strength grows noticeable with the

increase of the number of adjacent nitro and amino substit-

uents. For example, bond energy is about 8–9 kcal/mol in all

molecules with one to three corresponding bonds,

10–13 kcal/mol for molecules with four contacts (and

13N246A), and it is more than 14 kcal/mol in 135N246A

with six contacts. Therefore, such hydrogen bonding reveals

a cooperative effect [66, 71, 72] on the aromatic moiety.

Forming of the first bond disturbs the p-system and such

transformation facilitates its further alterations by other

RAHB. In addition, this makes benzene ring moiety more

sensible not only to the p-system disturbance but also to

r-electron perturbation, due to the polarization of the

r-skeleton by charges with opposite signs (Schemes 2, 3).

Extra elongation of the C–C bonds in 135N24A, 13N246A,

and 135N246A (see Table 8) provides the immediate

example.

Interestingly, AIM analysis revealed also the formation

of some C–H���O hydrogen bonds in 135N24A and

13N24A (Table 8). These bonds are significantly weaker

than the N–H���O–N bonds. However, formation of the

C–H���O bonds is observed only in molecules with at least

three N–H���O bonds.

In the extreme case of 135N246A, total energy of

hydrogen bonding is very high (about 87 kcal/mol) leading

to significant violation of structure of aromatic ring which

corresponds rather to derivative of cyclohexane with six

exocyclic double bonds than to hexasubstituted benzene

(Scheme 3). Probably, this significantly influences the

mechanism of decomposition of 135N246A. [73–78].

Conformational flexibility of benzene ring

Earlier it was demonstrated [42] that benzene and other six-

membered aromatic rings possess a notable conformational

flexibility. This property strongly depends on aromaticity

degree of cyclic conjugated system and presence of sub-

stituents. In particular, the presence of both p-electron-

donating and withdrawal groups in monosubstituted

derivatives of benzene results in increase of conformational

flexibility of ring due to decrease of aromaticity degree of

cyclic conjugated system [8].

In agreement with previous findings [8], energies of

benzene ring out-of-plane deformation in nitrobenzene 1N

and aniline 1A are smaller as compared to unsubstituted

benzene. Presence of several nitro groups results in a negli-

gible increase of flexibility, but existence of one or several

amino groups leads to more pronounced effect (Table 9).

Both these effects are non-additive: ring out-of-plane

deformation energy E(def) decreases by 0.67–0.98 kcal/mol

for nitro substituents and by 1.08–1.93 kcal/mol for amino

substituents and it does not depend on the number of

substituents.

Changes of the E(def) of the heterosubstituted molecules

clearly follow the revealed trends in structural changes.

The main factor that diminishes deformation energies is the

presence of vicinal nitro and amino groups that form

hydrogen bonds accompanied by perturbation of aromatic

system. Conformational flexibility of benzene ring in iso-

mers containing the N–H���O hydrogen bonds (for example,

1N2A, 1N24A) is always notably higher than for other

isomers (for example, 1N3A, 1N4A, 1N35A). Increase of

number of the N–H���O hydrogen bonds results in signifi-

cant decrease of E(def) which becomes minimal for

135N246A (less than 1 kcal/mol, Table 9). Thus, presence

of nitro and amino groups makes benzene ring very flexible

which does not match its aromatic character. This is

another evidence of significant violation of aromatic sys-

tem especially in trinitrotriaminobenzene 135N246A.

It should be noted that the value of ring out-of-plane

deformation energy is more sensitive to the electronic push–

pull effects than the molecular geometry of benzene ring.

1592 Struct Chem (2012) 23:1585–1597

123

The values of E(def) for 1N3A and 1N4A are slightly lower

than those for nitrobenzene and aniline (Table 9), in agree-

ment with polarization of p-system by push–pull effects.

It is known [42] that, for overwhelming majority of

simple aromatic molecules, including heterocycles, the

energy of ring out-of-plane deformation reveal strict har-

monic dependence on endocyclic torsion angle, at least in

the range from -30� to 30�. However, the results of cal-

culations demonstrate some deviations from that rule in

some of the highly substituted nitroaminobenzenes

Table 8 Geometrical param-

eters (H���O, A, D–H���O, �, O–N,

A) and AIM derived energy

(EAIM, kcal/mol) of

intramolecular hydrogen bonds

in nitroamino derivatives of

benzene

Molecule D–H���O–N H���O D–H���O O–N EAIM

1N2A N(10)–H���O(9)–N(7) 1.957 125.6 1.240 7.73

1N24A N(10)–H���O(9)–N(7) 1.934 126.8 1.242 8.09

1N246A N(10)–H���O(9)–N(7) 1.938 125.8 1.240 8.01

13N2A N(10)–H���O(9)–N(7) 1.893 129.3 1.240 8.69

N(10)–H���O(12)–N(11) 1.893 129.3 1.240 8.69

13N4A N(13)–H���O(12)–N(10) 1.943 125.6 1.239 7.91

13N246A N(10)–H���O(9)–N(7) 1.734 133.4 1.247 13.20

N(10)–H���O(12)–N(11) 1.733 133.5 1.247 13.22

N(14)–H���O(13)–N(11) 1.816 127.2 1.245 10.79

N(15)–H���O(8)–N(7) 1.817 127.2 1.245 10.76

135N2A N(10)–H���O(9)–N(7) 1.866 129.3 1.240 9.21

N(10)–H���O(12)–N(11) 1.863 129.4 1.240 9.26

135N24A N(10)–H���O(9)–N(7) 1.804 133.1 1.244 10.61

N(10)–H���O(12)–N(11) 1.797 128.8 1.242 11.10

N(14)–H���O(13)–N(11) 1.798 128.6 1.242 11.07

N(14)–H���O(16)–N(15) 1.805 133.0 1.244 10.59

C(6)–H���O(17)–N(15) 2.281 97.6 1.230 5.54

C(6)–H���O(8)–N(7) 2.280 97.6 1.230 5.53

135N246A N(10)–H���O(9)–N(7) 1.703 133.8 1.248 14.50

N(10)–H���O(12)–N(11) 1.705 133.6 1.248 14.38

N(14)–H���O(13)–N(11) 1.703 133.8 1.248 14.48

N(14)–H���O(16)–N(15) 1.704 133.6 1.248 14.40

N(18)–H���O(17)–N(15) 1.702 133.8 1.248 14.50

N(18)–H���O(8)–N(7) 1.704 133.6 1.248 14.40

13N24A N(10)–H���O(9)–N(7) 1.835 131.9 1.243 9.94

N(10)–H���O(12)–N(11) 1.888 128.0 1.239 8.77

N(14)–H���O(13)–N(11) 1.925 125.1 1.239 8.18

C(6)–H���O(8)–N(7) 2.281 98.3 1.232 5.36

13N25A N(10)–H���O(9)–N(7) 1.917 128.9 1.239 8.28

N(10)–H���O(12)–N(11) 1.916 128.9 1.239 8.28

123N46A N(16)–H���O(15)–N(13) 2.018 125.3 1.238 6.50

N(17)–H���O(8)–N(7) 2.018 125.3 1.238 6.50

N+

N+

O OH

H

Ν Η2

ΝΗ2

ΝΟ2Ο2Ν

N+

H

H

Ν Η2

ΝΗ2

ΝΟ2Ν+Ο

Ο

ΝΟ2

N+

H

H

Ν+

Ν+

Ν+

Ν+Ο

Ο

Ν+

Ο Ο

Η Η

Η

Η

Ο

Ο

Scheme 3 Some of resonance structures of TATB with single and multiple charge transfer that are stabilized by RAHB

Struct Chem (2012) 23:1585–1597 1593

123

(Table 9). Changes of endocyclic torsion angle in opposite

directions leads to different values of E(def). Asymmetrical

character of the PES in highly aromatic aniline and

polyaminobenzenes is caused by non-planar geometry of

the amino groups. Energy of non-bonded interactions of the

hydrogen atoms benzene ring with amino group is different

for out-of-plane deformations of the ring in the opposite

directions. Therefore, strict harmonic dependence is

maintained for both directions of PES scans (from -30� to

0� and from 0� to ?30�), but coefficients of these frag-

ments of parabolas are slightly different.

In molecules with two and more N–H���O hydrogen

bonds formed between vicinal nitro and amino groups, this

effect becomes considerably stronger because of the non-

coplanarity between the N–H���O hydrogen bonds and

benzene ring. Therefore, change of ring conformation

results in strengthening of hydrogen bonds during ring

out-of-plane deformation in one direction and weakening

of H bonds due to ring deformation in an opposite direc-

tion. This leads to notable asymmetry of potential energy

surface.

Aromaticity

Earlier, it was demonstrated [34] that appearance of any

substituent in benzene ring results in decrease of aroma-

ticity degree of cyclic conjugated system due to violation

of symmetry of p–p interactions within the ring. Therefore,

it is possible to expect that presence of several strong

p-electron-donating and withdrawing groups should sig-

nificantly decrease the aromaticity of benzene ring.

The results of present calculations demonstrate that

degree of aromaticity of molecules under study vary in quite

a wide range. It amounts to 8–96% for NICS(1)zz, 18–86%

for E(def), 74–100% for Ia, and 22–98% for HOMA, as

compared to benzene (Table 10). Some of molecules under

study, namely 2,4,6-triamino-1,3-dinitrobenzene 13N246A,

2,4-diamino-1,3,5-trinitrobenzene 135N24A, and 2,4,6-

triamino-1,3,5-trinitrobenzene 135N246A, possess extre-

mely low degree of aromaticity and may be considered even

as non-aromatic (Table 10).

Values of all aromaticity indices are quite consistent and

demonstrate reasonable correlation with each other

(Table 11). It is not a common result for aromatic systems,

e.g., monosubstituted benzenes and different heterocycles

[34, 35]. Correlation is observed in areas of both high and

low aromaticity. Therefore, it is possible to conclude that

molecules having high (small) values of all indices are

clearly aromatic (non-aromatic) without detailed discus-

sions regarding validity of the used index. As it was

demonstrated earlier [8], for highly aromatic systems,

structural indices are not very sensitive but the total cor-

relation is very good.

The remarkable result is that, in spite of above-men-

tioned features of potential energy surface for highly

substituted molecules, the values of ring out-of-plane

deformation energy describe changes of aromaticity degree

of the molecules under study in the agreement with general

trend. Moreover, correlation coefficients of E(def) with

other indices exceed 90% (Table 11). This confirms

applicability of ring out-of-plane deformation energy for

quantitative description of aromaticity even in the case of

quite complex molecules.

The Bird’s Ia index classifies all molecules as highly aro-

matic, while some of them possess clearly low degree of

aromaticity according to all other indices. This is well-known

limitation of the Bird’s index [79] for highly symmetrical

molecules. Therefore, 2,4,6-triamin-1,3-dinitrobenzene, 2,4-

diamino-1,3,5-trinitrobenzene and 2,4,6-triamin-1,3,5-trini-

trobenzene are outliers in all correlations. The HOMA index is

Table 9 Energies of ring out-of-plane deformations E(def) (kcal/mol)

upon change of the softest endocyclic torsion angle by ?30� (E?) and

-30� (E-) in molecules under consideration

Molecule Torsion angle E- E?

Benzene C1–C2–C4–C4 7.22 7.22

1N C5–C6–C1–C2 6.42 6.42

12N C1–C2–C4–C4 6.55 6.55

13N C2–C3–C4–C5 6.39 6.39

14N C5–C6–C1–C2 6.24 6.24

123N C3–C4–C5–C6 6.41 6.41

135N C1–C2–C4–C4 6.45 6.45

1A C5–C6–C1–C2 6.14 5.90

12A C1–C2–C4–C4 5.84 5.29

13A C1–C2–C4–C4 5.68 5.57

14A C1–C2–C4–C4 6.09 5.68

124A C1–C2–C4–C4 5.29 5.29

135A C1–C2–C4–C4 5.73 5.53

1N2A C5–C6–C1–C2 4.82 4.82

1N3A C3–C4–C5–C6 5.72 5.72

1N4A C1–C2–C4–C4 5.66 5.66

13N2A C6–C1–C2–C3 5.32 3.21

13N4A C4–C5–C6–C1 4.80 4.80

13N5A C3–C4–C5–C6 5.65 5.65

1N24A C6–C1–C2–C3 4.89 4.89

1N35A C2–C3–C4–C5 5.49 5.49

13N24A C6–C1–C2–C3 4.36 3.16

13N25A C6–C1–C2–C3 5.59 3.66

123N46A C3–C4–C5–C6 4.68 4.68

135N2A C3–C4–C5–C6 5.74 3.23

135N24A C5–C6–C1–C2 3.78 2.10

1N246A C3–C4–C5–C6 5.12 4.79

13N246A C6–C1–C2–C3 3.46 2.03

135N246A C3–C4–C5–C6 2.34 0.58

1594 Struct Chem (2012) 23:1585–1597

123

much more sensitive to structure deformation due to presence

of EN and GEO components while Ia index takes into con-

sideration only GEO part [40].

General trends in changes of aromaticity indices agree

well with the deformations of structure of benzene ring

mentioned above. Polysubstitution by amino groups affects

aromaticity stronger than polysubstitution by nitro groups,

and with no additive features of this effect (Table 10).

Steric repulsion of nitro groups causing their rotation does

not lead to considerable changes of aromaticity. Interest-

ingly, the values of aromaticity indices for sterically

strained 1,2,3-trinitrobenzene 123N and 1,2-dinitrobenzene

12N remains are quite high.

The most pronounced change of aromaticity is caused

by vicinal arrangement of nitro and amino groups.

Therefore, aromaticity of benzene ring in ortho-nitroaniline

1N2A is considerably lower as compared to meta- and

para-isomers 1N3A and 1N4A (Table 10). Aromaticity

Table 10 Aromaticity indices

of nitro-and amino-substituted

benzenes arranged by numbers

of nitro and amino substituents

Number of H bonds Molecule E(30), kcal/mol NICS(1)zz Ia HOMA

0 Benzene 7.22 -30.45 100.00 99.17

0 nitro groups

0 1A 5.90 -26.99 96.62 90.82

12A 5.29 -26.18 94.80 89.41

13A 5.57 -23.89 96.81 90.61

14A 5.68 -24.56 96.80 90.02

124A 5.29 -23.82 95.15 89.18

135A 5.53 -20.58 99.94 90.85

1 nitro group

0 1N 6.42 -29.10 97.78 93.73

1N3A 5.72 -25.46 94.48 91.57

1N4A 5.66 -25.71 93.88 92.22

1N35A 5.49 -21.81 94.73 90.84

1 1N2A 4.82 -24.99 90.59 87.83

1N24A 4.89 -20.97 90.07 86.84

1N246A 4.79 -16.36 89.93 82.63

2 nitro groups

0 12N 6.55 -27.72 98.03 94.48

13N 6.39 -28.84 97.34 95.67

14N 6.24 -27.69 99.52 95.17

13N5A 5.65 -24.25 92.34 92.33

1 13N4A 4.80 -23.88 87.46 88.32

2 13N2A 3.21 -22.49 84.60 81.94

13N25A 3.66 -20.49 87.90 82.65

3 13N24A 3.16 -16.32 82.43 73.53

4 13N246A 2.03 -8.25 80.31 49.57

3 nitro groups

0 123N 6.41 -27.01 98.32 95.29

135N 6.45 -28.74 100.00 97.60

2 123N46A 4.68 -17.63 91.24 87.00

135N2A 3.23 -21.27 79.75 79.25

4 135N24A 2.10 -11.65 74.28 54.71

6 135N246A 1.20 -2.46 99.79 21.52

Table 11 Correlation coefficients (%) between aromaticity indices

excluding outlying points (135N246A, 135N24A, 135N2A,

13N246A)

NICS(1)zz Ia HOMA

E(def) -91 93 (66) 92

NICS(1)zz -77 (-48) -93

Ia 91 (41)

Values in parenthesis correspond to all dataset including outlying

points

Struct Chem (2012) 23:1585–1597 1595

123

degree of the last two molecules is very similar, indicating

almost negligible push–pull effect.

Analysis of aromaticity degree of benzene ring in het-

eropolysubstituted molecules clearly indicates dependence

of aromaticity indices on number of intramolecular

N–H���O hydrogen bonds. For example, degree of varia-

tion of HOMA index stretches between 89.18–97.6 for

molecules without H bonds, 86.84–88.32 for molecules

with 1 H bond, 79.25–87.00 for molecules with 2 H

bonds, 73.53 for 13N24A with 3 H bonds, and

49.57–54.71 for molecules with 4 H bonds and only 21.52

for 135N246A containing 6 H bonds. This agrees well

with resonance-assisted character of hydrogen bonds

leading to specific deformation of geometry of benzene

ring mentioned above.

It is interesting to note that variation of aromaticity

degree within group of molecules with the same number of

intramolecular hydrogen bonds almost does not depend on

number of nitro and amino groups. For example, aroma-

ticity of benzene ring in 1N3A, 1N4A is very close to

aromaticity of 1N35A containing two amino groups or

13N5A containing two nitro groups. This also confirms

negligible influence of p–p interactions between substitu-

ents on aromaticity of cyclic conjugated system as com-

pared to formation of RAHB.

Conclusions

An analysis of structural properties and aromaticity of

nitro- and amino-substituted derivatives of benzene dem-

onstrates that the conjugation interactions between sub-

stituents (push–pull effect) is very small and almost does

not influence properties of investigated molecules. The

main factor governing molecular structure and aromaticity

of benzene ring is the formation of intramolecular N–H���Ohydrogen bonds between vicinal nitro and amino groups.

Resonance-assisted characteristics of these hydrogen bonds

result in significant elongation of the C–C bond between

nitro and amino groups accompanied by decrease of aro-

maticity of cyclic conjugated system and increase of con-

formational flexibility of ring. Degree of such deformations

depends on number of the N–H���O bonds. The most sig-

nificant deformation is observed in symmetric 1,3,5-trini-

tro-2,4,6-triaminobenzene (135N246A) in which structure

should be described as derivative of cyclohexane with six

exocyclic double bonds, rather than hexasubstituted ben-

zene. This is confirmed by very low values of aromaticity

indices and high conformational flexibility of benzene ring

corresponding to non-aromatic character of cyclic conju-

gated system. All other polysubstituted derivatives of

benzene have intermediate properties between nitroben-

zene or aniline and 135N246A.

Acknowledgments The use of trade, product, or firm names in this

report is for descriptive purposes only and does not imply endorse-

ment by the U.S. Government. Results in this study were funded and

obtained from research conducted under the Environmental Quality

Technology Program of the United States Army Corps of Engineers

by the USAERDC. Permission was granted by the Chief of Engineers

to publish this information. The findings of this report are not to be

construed as an official Department of the Army position unless so

designated by other authorized documents.

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