A complete vibrational study on a potential environmental toxicant agent, the 3,3′,4,4′-tetrachloroazobenzene combining the FTIR, FTRaman, UV-Visible and NMR spectroscopies with DFT calculations

Article abstract of DOI:10.1016/j.saa.2014.07.032

In this study 3,3′,4,4′-tetrachloroazobenzene (TCAB) was prepared and then characterized by infrared, Raman, multidimensional nuclear magnetic resonance (NMR) and ultraviolet-visible spectroscopies. The density functional theory (DFT) together with the 6-

Full text of DOI:10.1016/j.saa.2014.07.032

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A complete vibrational study on a potential environmental toxicant
agent, the 3,30,4,40-tetrachloroazobenzene combining the FTIR, FTRaman,
UV–Visible and NMR spectroscopies with DFT calculations q
María V. Castillo a, Jorgelina L. Pergomet b, Gustavo A. Carnavale b, Lilian Davies c, Juan Zinczuk b,1
,
Silvia A. Brandán a,
⇑
a Cátedra de Química General, Instituto de Química Inorgánica, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, 4000 San Miguel
de Tucumán, Tucumán, Argentina
b Instituto de Química Rosario (CONICET-UNR), Facultad de Ciencias Bioquímicas y Farmacéuticas, Suipacha 531, 2000 Rosario, Santa Fé, Argentina
c Instituto de Investigaciones para la Industria Química (INIQUI, CONICET), Universidad Nacional de Salta, Av. Bolivia 5150, 4400 Salta, Argentina
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3,30,4,40-Tetrachloroazobenzene was
characterized by using spectroscopic
techniques.
Two pairs of conformers of TCAB were
theoretically determined in the gas
phase.
B3LYP calculations were used to
study the two cis and trans isomers of
TCAB.
A complete vibrational assignment
for 3,30,4,40-tetrachloroazobenzene
was performed.
The cis and trans isomers exhibit
different structural and vibrational
properties.
a r t i c l e i n f o
a b s t r a c t
In this study 3,30,4,40-tetrachloroazobenzene (TCAB) was prepared and then characterized by infrared,
Raman, multidimensional nuclear magnetic resonance (NMR) and ultraviolet–visible spectroscopies.
The density functional theory (DFT) together with the 6-31G* and 6-311++G** basis sets were used to
study the structures and vibrational properties of the two cis and trans isomers of TCAB. The harmonic
vibrational wavenumbers for the optimized geometries were calculated at the same theory levels. A com-
plete assignment of all the observed bands in the vibrational spectra of TCAB was performed combining
the DFT calculations with the scaled quantum mechanical force field (SQMFF) methodology. The molec-
ular electrostatic potentials, atomic charges, bond orders and frontier orbitals for the two isomers of TCAB
were compared and analyzed. The comparison of the theoretical ultraviolet–visible spectrum with the
corresponding experimental demonstrates a good concordance while the calculated 1H and 13C chemicals
shifts are in good conformity with the corresponding experimental NMR spectra of TCAB in solution.
The npÒp* transitions for both forms were studied by natural bond orbital (NBO) while the topological
Article history:
Received 30 August 2013
Received in revised form 15 July 2014
Accepted 19 July 2014
Available online xxxx
Keywords:
3,30,4,40-Tetrachloroazobenzene
Vibrational spectra
Molecular structure
Force field
DFT calculations
q
Selected paper presented at XIIth International Conference on Molecular
spectroscopy, Kraków – Bialka Tatrzanska, Poland, September 8–12, 2013.
⇑
Corresponding author. Tel.: +54 381 4247752; fax: +54 381 4248169.
E-mail address: sbrandan@fbqf.unt.edu.ar (S.A. Brandán).
Member of the Carrera de Investigador Científico, CONICET, R. Argentina.
1
http://dx.doi.org/10.1016/j.saa.2014.07.032
1386-1425/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

2
M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
properties were calculated by employing Bader’s Atoms in the Molecules (AIM) theory. This study shows
that the cis and trans isomers exhibit different structural and vibrational properties and absorption bands.
Ó 2014 Published by Elsevier B.V.
Introduction
Experimental methods
3,30,4,40-Tetrachloroazobenzene (TCAB) is a contaminant of
dichloroaniline-derived herbicides whose IUPAC chemical name
is (3,4-dichlorophenyl)-(3,4-dichlorophenyl)imino-oxidoazanium.
Several aniline herbicides yield azobenzenes as products in soil.
Soil microorganisms are responsible for the transformation of
30,40-dichloropropionanilide (propanil) to 3,4-dichloroaniline and
subsequently TCAB [1]. Recent studies have demonstrated that
mice chronically exposed at dose levels of tetrachloroazobenzene
developed a number of neoplastic and nonneoplastic lesions,
including carcinoma of the urinary tract [2]. The mechanism of
tumor induction is uncertain, but the high frequency of tumors in
the proximal urethra of male mice suggests that the neoplasms
result from the exposure of a susceptible population of urothelial
cells to a carcinogenic metabolite of TCAB [2]. TCAB in itself is not
a pesticide [3] but some herbicides are connected directly or indi-
rectly with the compound, hence, the environmental safety of this
chemical is extremely important from the viewpoint of human
health. The crystal structure of azobenzene [4] and the structural
studies of the systems cis and trans-azobenzene were reported
[5–7]. The photophysical and photochemical properties of these
azobenzene compounds and its derivatives are of industrial interest
as light-triggered switches, constituents of erasable holographic
data, image storage devices and materials with photomodulable
properties, and as a possible basis for a light-powered molecular
machine [8]. Computational studies on the potential energy surface
(PES), the excited electronic states, the use of quantitative-structure
property relationship (QSPR) and artificial neural network (ANN)
and, the photoisomerization of the azobenzene compounds and
its derivatives were already published [9–12]. So far, the vibrational
spectra of TCAB still remain unassigned and, to identify it com-
pound in all the systems by means of vibrational spectroscopy, a
complete characterization of the infrared and Raman spectra in
solid phase is necessary. Actually, for TCAB there are not theoretical
studies on the molecular electrostatic potentials, atomic charges
derived from the Merz et al. [13], bond orders, natural atomic
charges, force constants and topological analysis. Hence, in this
work we prepared and characterized the compound by means of
infrared, Raman, multidimensional nuclear magnetic resonance
(NMR) and ultraviolet–visible spectroscopies. The structures and
vibrational properties for all the isomers of TCAB were studied
by using B3LYP level of theory together with the 6-31G* and
6-311++G** basis sets. The optimized geometries and the corre-
sponding frequencies for all the stable conformers of TCAB were
calculated by using the same levels of theory. Then, the structural
properties and the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) energy gaps of
TCAB were compared and analyzed. The electronic delocalizations
for TCAB were calculated by means of the natural bond orbital
(NBO) study [15,16] while the topological properties were analysed
by employing Bader’s Atoms in the Molecules theory (AIM) [17,18].
Here, a detailed study of the vibrational spectra of TCAB based on
the normal coordinate analysis and the complete assignments of
the 66 normal modes of vibration are reported. The corresponding
force fields and force constants were obtained using scaling factors
[14]. The comparisons of the theoretical ultraviolet–visible spec-
trum and the calculated 1H and 13C chemicals shifts with the corre-
sponding experimental ones demonstrate a good concordance.
Synthesis
3,30,4,40-Tetrachloroazobenzene was obtained according to
Mehta and Vakilwala [19] using the following procedure.
To a stirred solution of 3.15 g (0.020 mol) of sodium perborate
tetrahydrate in glacial acetic acid (25 mL) was added 0.62 g
(0.010 mol) of boric acid and a solution of 1.95 (0.012 mol) of
3,4-dichloroaniline in acetic acid (25 mL). The mixture was heated
to 60 °C for 6 h. At this time, no 3,4-dichloroaniline was detected
by TLC. The mixture was cooled to room temperature and the yel-
low product was collected on a Büchner funnel, washed with water
to remove acetic acid, dried, and crystallized from dichlorometh-
ane. Yield 1.04 g (27%), m.p. = 155.5–156 °C. (Lit. 158 °C).
Equipments
The infrared spectrum of the TCAB solid in KBr pellets from
4000 to 400 cmꢁ1 was recorded on an FTIR GX1 spectrophotome-
ter, equipped with a globar source and a DGTS detector at a reso-
lution of 1 cmꢁ1 and 64 scans. The Raman spectrum (resolution
1 cmꢁ1, 200 scans) was recorded 2000–200 cmꢁ1 with a Bruker
RF100/S spectrometer equipped with a Nd:YAG laser source (exci-
tation line 1064 nm, 800 MW power) and a Ge detector.
Nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker 300 AVANCE spectrometer at 300 MHz for 1H and 75 MHz
for 13C in CDCl3 solutions containing 0.03 vol.% TMS as internal
standard. GC–MS spectrum was recorded on a 5973 Hewlett–
Packard selective mass detector coupled to a Hewlett Packard
6890 gas chromatograph equipped with a Perkin–Elmer Elite-
5MS capillary column (5% phenyl methyl siloxane, length = 30 m,
inner diameter = 0.25 mm, film thickness = 0.25 lm); ionization
energy, 70 eV; carrier gas: helium at 1.0 mL/min. UV spectra were
collected on a UV–Visible 160 A Shimadzu spectrophotometer.
Computational details
The potential energy curves associated with rotation around the
C1AN12 bond, described by the C2AC1AN12AN13 dihedral angles
for the cis and trans forms of TCAB are observed in Figs. S1 and S2
(Supporting Material) at the B3LYP/6-31G* levels of theory [20,21].
The DFT calculations show, for each form, the presence of two sta-
ble conformers with geometries C1, named Cis-I and Cis-II and
Trans-I and Trans-II, respectively. The initials geometries for the
two pairs of conformers of TCAB were modeled by means of Gauss-
View program [22] and all the structures were optimized at the
B3LYP/6-31G* and 6-311++G** levels of theory. The structures and
labeling of the atoms for all the structures of TCAB can be seen
in Fig. 1. The topological analysis and the NBO calculations for
the four structures were performed by using the AIM200 program
package [18] and the NBO 3.1 [16] program, as implemented in the
Gaussian 03 package [23]. The natural internal coordinates for the
cis and trans forms of TCAB are listed in Tables S1 and S2 (Support-
ing Material) and were defined as those reported in the literature
[24–37]. The MOLVIB program [38] was used to transform the
resulting force fields in Cartesian coordinates to ‘‘natural’’ internal
coordinates. The harmonic force fields for all the forms were
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
3
Fig. 1. Molecular structures of 3,30,4,40tetrachloroazobenzene and atoms numbering: from CI and CII Cis conformers and CI and CII trans conformer.
evaluated at the B3LYP/6-31G* level following the SQMFF proce-
dure [14]. Then, from resulting SQMFF only the potential energy
distribution components (PED) P10% were considered to perform
the final assignment of TCAB.
computational study for azobenzene and stilbene [10]. Besides,
the B3LYP/cc-pVDZ calculations for those compounds show the
coplanarity of the N@N bond in the two trans forms because the
CAN@NAC dihedral angles are planar and, for this, the interactions
of the azo group with the phenyl ring favors the p–p interactions.
Probably, these observations only in the two trans forms of TCAB
justify the energetical stabilities of the two conformations.
Results and discussion
Geometry
Molecular electrostatic potentials and atomic charges
A comparison of the total energies together with the corre-
sponding dipole moment values for the cis and trans forms, by
using both basis sets, is given in Table S3 (Supporting Material).
The results show first, that the energies of the two trans forms
are significantly lower than the corresponding to the cis forms, as
observed by Chen and Chieh in a computational study for azoben-
zene and stilbene [10], and second, that the differences of potential
energy between trans-I and trans-II in reference to cis-I and cis-II by
using 6-31G* basis set are 0.79 and 3.41 kJ/mol, respectively. Prob-
ably, this low energies difference justify the photoinduced isomer-
ization between the two forms, as observed for azobenzene [9].
Note that the high values of the dipole moments for the cis-II
and trans-II forms of TCAB could partially explain its stability in
solid phase. A similar dependence between energy and dipole
moment was also observed in other molecules [39–41]. The calcu-
lated geometrical parameters for the cis and trans conformations of
TCAB by using both basis sets are summarized in Tables 1 and 2
and, they are compared with the experimental values reported
for cis-azobenzene [5], azobenzene [10] and (4-chlorophenyl)qui-
nazolin-4(3H)-one [42] by means of root mean square (rmsd).
The rmsd values show that the better bond length and angles val-
ues are obtained when the calculated parameters are compared
with the azobenzene compound, as expected because it is a similar
compound. Note that the N12AC1AC2 and N12AC1AC6 bond
angles and the N13AN12AC1AC2 and N13AN12AC1AC6 dihedral
angles have different values in both cis conformers. For the struc-
tures of the trans conformations a better correlation in the bond
angles is observed when their values are compared with the calcu-
lated geometrical parameters reported by Chen and Chieh in a
The electrostatic interactions and stabilities of all the TCAB con-
formers were investigated by using the molecular potentials and
the natural atomic charges and the corresponding results are given
in Tables S4 and S5, respectively. Note that the higher molecular
potentials are observed on the Cl and N atoms of both trans con-
formers and that the values are approximately the same in the I
and II conformers of the two cis and trans forms, as can be seen
in Table S4. On the contrary, the NPA charges for the cis forms
are different from those corresponding to the two trans forms, as
observed in Table S5. In this case, we observed on the N atoms
the following trend: cis I = cis II < trans I = trans II while on the Cl
atoms a contrary relation is observed, this is, trans I = trans II < cis
I = cis II. The influence of the size of the basis set is notable in both
properties and particularly on the Cl and N atoms, thus, the molec-
ular potentials and charges values increase when the 6-311++G**
basis set is used while for the remaining atoms a contrary effect
in the two properties studied is observed.
The bond orders, expressed by Wiberg’s index for all atoms cor-
responding to the four conformations of TCAB are given in Table S6.
Here, the higher values are observed in the N@NA bonds corre-
sponding to the two cis forms, being high those values calculated
by using 6-311++G** basis set. For the Cl atoms a same relation is
found, as can be seen in Table S6.
NBO study
The analysis of the stabilization energies is very important in a
molecule as TCAB because, as was explained in the introduction
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

4
M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
Table 1
Calculated geometrical parameters for the cis conformers of TCAB.
Parameter
Cis-Ia
Cis-IIa
Expb,c cis
Expc,d cis
6-31G*
6-311++G**
6-31G*
6-311++G**
Bond length (Å)
C1AC2
C2AC3
C3AC4
C4AC5
1.398
1.393
1.403
1.398
1.390
1.402
1.402
1.390
1.398
1.403
1.393
1.398
1.435
1.435
1.249
1.745
1.744
1.745
1.744
1.395
1.391
1.400
1.396
1.387
1.399
1.399
1.387
1.396
1.400
1.391
1.395
1.435
1.435
1.243
1.745
1.743
1.745
1.743
1.399
1.394
1.403
1.398
1.389
1.400
1.402
1.389
1.398
1.403
1.393
1.398
1.434
1.434
1.249
1.746
1.744
1.745
1.743
1.396
1.391
1.400
1.395
1.387
1.398
1.399
1.387
1.396
1.400
1.391
1.395
1.434
1.435
1.243
1.746
1.744
1.745
1.743
1.401
1.378
1.374
1.389
1.377
1.385
1.385
1.377
1.389
1.374
1.378
1.401
1.449
1.449
1.253
1.735
1.735
1.735
1.735
1.410
1.378
1.374
1.389
1.377
1.385
1.385
1.377
1.389
1.374
1.378
1.410
1.449
1.449
1.253
1.735
1.735
1.735
1.735
C5AC6
C6AC1
C14AC15
C15AC17
C17AC21
C21AC19
C19AC16
C16AC14
C1AN12
C14AN13
N12AN13
C3ACl10
C4ACl11
C19ACl23
C21ACl24
RMSDb
RMSDd
0.0033
0.0034
0.0030
0.0032
0.0033
0.0034
0.0030
0.0032
Bond angle (°)
C1AC2AC3
C2AC3AC4
C3AC4AC5
C4AC5AC6
120.33
119.82
119.53
120.76
119.62
119.85
115.98
123.48
124.07
124.07
123.48
119.62
120.76
119.53
119.82
120.33
115.99
119.85
118.66
121.52
121.59
118.88
121.52
118.65
118.88
121.59
120.24
119.86
119.54
120.72
119.58
119.94
116.14
123.27
124.02
124.02
123.27
119.58
120.72
119.54
119.86
120.24
116.14
119.94
118.66
121.47
121.55
118.90
121.47
118.66
118.90
121.55
119.98
120.13
119.54
120.43
119.96
119.86
122.79
116.68
124.28
124.05
123.54
119.63
120.74
119.55
119.82
120.32
115.95
119.83
118.48
121.39
121.57
118.88
121.48
118.69
118.85
121.60
119.94
120.12
119.57
120.43
119.88
119.95
122.60
116.84
124.18
123.98
123.26
119.60
120.70
119.56
119.86
120.22
116.16
119.93
118.50
121.38
121.52
118.90
121.44
118.68
118.87
121.56
119.8
118.7
121.7
119.0
120.8
120
118.7
121.7
119
120.8
120
119.8
117.3
122.5
121.9
121.9
122.5
120
120.8
119
121.7
118.7
117.3
119.8
C5AC6AC1
C6AC1AC2
N12AC1AC2
N12AC1AC6
C1AN12AN13
N12AN13AC14
N13AC14AC15
C14AC15AC17
C15AC17AC21
C17AC21AC19
C21AC19AC16
C19AC16AC14
C16AC14AN13
C16AC14AC15
C2AC3ACl10
C4AC3ACl10
C3AC4ACl11
C5AC4ACl11
C21AC19ACl23
C16AC19ACl23
C17AC21ACl24
C19AC21ACl24
122.5
121.9
121.9
122.5
120.8
119.0
121.7
118.7
119.8
120.0
119.9
119.9
119.9
119.9
RMSDb
RMSDd
0.34
0.28
0.21
0.27
0.46
0.50
0.29
0.49
Dihedral angles (°)
C1AC2AC3ACl10
C1AC2AC3AC4
ꢁ178.14
2.98
ꢁ178.04
3.15
ꢁ179.26
0.934
ꢁ179.30
0.845
C2AC3AC4ACl11
Cl10AC3AC4ACl11
Cl10AC3AC4AC5
C2AC3AC4AC5
Cl11AC4AC5AC6
C5AC6AC1AC2
C5AC6AC1AN12
N12AC1AC2AC3
N13AN12AC1AC2
N13AN12AC1AC6
C1AN12AN13AC14
N12AN13AC14AC15
N12AN13AC14AC16
C14AC15AC17AC21
C15AC17AC21ACl24
C15AC17AC21AC19
178.92
0.067
ꢁ179.17
ꢁ0.321
179.15
1.815
171.93
ꢁ174.57
ꢁ140.12
49.41
178.85
0.080
ꢁ179.11
ꢁ0.341
179.11
1.954
172.30
ꢁ174.98
ꢁ138.93
50.38
179.24
ꢁ0.555
178.55
ꢁ1.650
178.85
ꢁ3.63
179.25
ꢁ0.595
178.45
ꢁ1.705
178.85
ꢁ3.83
ꢁ174.51
172.00
50.18
ꢁ175.06
172.63
50.80
ꢁ139.23
ꢁ138.22
10.09
49.41
9.69
50.39
10.03
9.56
48.80
50.03
ꢁ140.12
ꢁ138.93
ꢁ140.70
ꢁ139.22
0.844
0.859
0.858
0.845
179.14
ꢁ1.595
179.11
ꢁ1.668
179.16
ꢁ1.614
179.00
ꢁ1.749
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
5
Table 1 (continued)
Parameter
Cis-Ia
Cis-IIa
Expb,c cis
Expc,d cis
6-31G*
6-311++G**
6-31G*
6-311++G**
Cl24AC21AC19ACl23
Cl24AC21AC19AC16
C17AC21AC19ACl23
C17AC21AC19AC16
C21AC19AC16AC14
C19AC16AC14AN13
Cl23AC19AC16AC14
0.067
0.080
0.076
0.191
178.92
ꢁ179.17
ꢁ0.321
2.98
178.85
ꢁ179.11
ꢁ0.341
3.15
178.90
ꢁ179.13
ꢁ0.308
2.98
178.99
ꢁ179.03
ꢁ0.238
3.12
ꢁ174.57
ꢁ174.98
ꢁ174.61
ꢁ175.10
ꢁ178.14
ꢁ178.04
ꢁ178.17
ꢁ178.05
a
This work.
From Ref. [10].
From Ref. [42].
From Ref. [5].
b
c
d
Table 2
Calculated geometrical parameters for the trans conformer of TCAB.
Parameter
Trans-Ia
Trans-IIa
Expb Trans
6-31G*
6-311++G**
6-31G*
6-311++G**
Bond length (Å)
C1AC2
C2AC3
C3AC4
C4AC5
1.403
1.388
1.408
1.397
1.390
1.400
1.400
1.390
1.397
1.408
1.388
1.403
1.415
1.415
1.260
1.748
1.743
1.748
1.743
1.401
1.386
1.405
1.394
1.388
1.398
1.398
1.388
1.394
1.405
1.386
1.401
1.415
1.415
1.252
1.748
1.743
1.748
1.743
1.403
1.388
1.408
1.397
1.390
1.400
1.405
1.386
1.402
1.402
1.393
1.399
1.416
1.416
1.261
1.748
1.743
1.746
1.743
1.401
1.386
1.405
1.394
1.388
1.398
1.403
1.383
1.399
1.398
1.391
1.396
1.416
1.415
1.252
1.748
1.743
1.745
1.742
1.385
1.379
1.389
1.368
1.390
1.384
1.385
1.379
1.389
1.368
1.390
1.384
1.433
1.433
1.243
1.735
1.735
1.735
1.735
C5AC6
C6AC1
C14AC15
C15AC17
C17AC21
C21AC19
C19AC16
C16AC14
C1AN12
C14AN13
N12AN13
C3ACl10
C4ACl11
C19ACl23
C21ACl24
RMSD
0.0038
0.0033
Bond angle (°)
C1AC2AC3
C2AC3AC4
C2AC3ACl10
C4AC3ACl10
C3AC4ACl11
C5AC4ACl11
C3AC4AC5
119.98
120.07
118.79
121.14
121.40
118.82
119.78
120.15
120.15
119.87
124.32
115.80
114.82
114.83
115.80
120.15
120.15
118.82
121.40
119.78
121.14
118.79
120.08
119.98
124.32
119.87
119.92
120.13
118.77
121.10
121.37
118.86
119.76
120.18
120.10
120.00
124.20
115.90
115.41
115.41
115.90
120.10
120.18
118.86
121.37
119.76
121.10
118.77
120.13
119.92
124.20
119.91
119.98
120.06
118.79
121.14
121.41
118.81
119.78
120.14
120.17
119.86
124.36
115.78
114.78
114.61
125.07
119.6
120.68
118.70
121.52
119.78
121.58
118.84
119.58
120.50
115.07
119,87
119.91
120.13
118.76
121.10
121.38
118.86
119.76
120.17
120.11
119.90
124.21
115.88
115.34
115.23
124.95
119.53
120.72
118.71
121.52
119.77
121.57
118.81
119.62
120.42
115.15
119,90
119.6
120.3
119.9
120.4
119.6
120.3
C4AC5AC6
C5AC6AC1
C6AC1AC2
N12AC1AC2
N12AC1AC6
C1AN12AN13
N12AN13AC14
N13AC14AC15
C14AC15AC17
C15AC17AC21
C17AC21ACl24
C19AC21ACl24
C17AC21AC19
C21AC19ACl23
C16AC19ACl23
C21AC19AC16
C19AC16AC14
C16AC14AN13
C16AC14AC15
115.5
113.6
113.6
119.6
120.3
119.9
120.4
119.6
115.5
120,3
RMSD
0.14
0.18
Dihedral angles (°)
(continued on next page)
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

6
M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
Table 2 (continued)
Parameter
Trans-Ia
Trans-IIa
Expb Trans
6-31G*
6-311++G**
6-31G*
6-311++G**
C1AC2AC3ACl10
C1AC2AC3AC4
ꢁ180
–
ꢁ180
–
180
–
C2AC3AC4ACl11
Cl10AC3AC4ACl11
Cl10AC3AC4AC5
180
–
180
180
–
180
–
ꢁ180
–
180
–
C2AC3AC4AC5
Cl11AC4AC5AC6
C5AC6AC1AC2
180
–
180
–
ꢁ180
–
C5AC6AC1AN12
N12AC1AC2AC3
180
180
0.036
ꢁ180
180
179.99
–
–
180
–
0.005
ꢁ180
ꢁ180
0.004
–
180
180
0.008
ꢁ179.99
ꢁ179.99
180
–
ꢁ180
180
–
ꢁ180
179.99
–
N13AN12AC1AC2
N13AN12AC1AC6
C1AN12AN13AC14
N12AN13AC14AC15
N12AN13AC14AC16
C14AC15AC17AC21
C15AC17AC21ACl24
C15AC17AC21AC19
Cl24AC21AC19ACl23
Cl24AC21AC19AC16
C17AC21AC19ACl23
C17AC21AC19AC16
C21AC19AC16AC14
C19AC16AC14AN13
Cl23AC19AC16AC14
180
–
180
–
ꢁ180
ꢁ180
–
–
–
–
ꢁ180
ꢁ180
–
ꢁ180
ꢁ180
–
–
–
180
180
ꢁ180
ꢁ180
180
a
This work.
From Ref [10].
b
section, they have photochemical properties of industrial interest
as light-triggered switches [8,9]; hence the two pairs of TCAB con-
formers were studied by means of NBO calculations [16]. Tables S7
shows the second order perturbation energies E(2) (donor ? accep-
tor) corresponding to the most important delocalization for all the
conformers of TCAB. For both forms, four contributions of the sta-
together with the N12 and N13 atoms are strongly conjugated by
the electron density LP ? p* and LP ? r* donations where the LP
transitions are p orbitals of the C, N12 and N13 atoms. These mech-
anisms explain the existence of a relatively high occupancy of p*
orbitals and of the p*
? p
* transitions. On the other hand, probably
the predicted p* * transitions due to the shorter lifetime of the
? p
bilization energies are observed by using both basis sets,
D
ET
p
,
?p*
D
ETLP? ETLP? and ET
,
D
D
of which the n ? r* and n ? p*
r
*
p
*
p*?p*
contributions favors to the two cis forms while the two remaining
favors to the trans forms. This analysis justify clearly the stabilities
of the two trans forms because the planarity of the CAN@NAC
dihedral angles produce a higher total electron delocalization
and, as consequence favors the
p ?
p* type interactions and the
energetical stabilities of the two trans conformations. Table S7
shows that the C atoms of the two rings in all the conformers
Fig. 2. (a) Comparisons between the experimental infrared spectra of 3,30,4,40tet-
rachloroazobenzene with the corresponding theoretical at B3LYP/6-31G* for: (b) Cis
I conformer, (c) Cis II conformer, (d) Trans I conformer and, (e) Trans II conformer.
Fig. 3. (a) Comparisons between the experimental Raman spectra of 3,30,4,40tetra-
chloroazobenzene with the corresponding theoretical at B3LYP/6-31G* for: (b) Cis I
conformer, (c) Cis II conformer, (d) Trans I conformer and, (e) Trans II conformer.
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
7
Table 3
Observed and calculated wavenumbers (cmꢁ1) and assignment for TCAB conformers.
Experimentala
Cis I
Cis II
Trans I
Trans II
IR solid
Raman
SQMb
Assignmenta
SQMb
Assignmenta
SQMb
Assignmenta
SQMb
Assignmenta
3118 vw
3100
3100
3097
3097
3087
3087
1587
1577
1555
1540
1537
1455
1455
1380
1377
1277
1269
1248
1248
1152
1142
1134
1130
1106
1106
1024
1023
950
950
906
894
862
840
831
825
791
691
686
675
669
615
605
604
543
m
m
m
m
m
m
m
m
m
m
m
C6AH9
3105
3100
3097
3097
3087
3083
1588
1577
1552
1545
1538
1456
1455
1380
1374
1278
1269
1252
1248
1152
1138
1134
1128
1109
1106
1025
1023
955
949
904
888
871
844
829
827
784
693
682
679
673
616
613
601
541
m
m
m
m
m
m
m
m
m
m
m
C2AH7
3113
3113
3099
3099
3087
3087
1586
1581
1565
1556
1484
1453
1448
1383
1383
1289
1288
1258
1246
1193
1153
1126
1125
1103
1102
1023
1022
963
962
960
927
925
887
837
833
763
695
695
682
674
644
607
569
561
m
m
m
m
m
m
m
m
m
m
m
C2AH7
C16AH20
C5AH8
C17AH22
C6AH9
C15AH18
C15AC17
C5AC6
3114
3110
3102
3100
3088
3087
1587
1583
1564
1552
1487
1458
1449
1383
1378
1290
1287
1256
1246
1196
1155
1129
1125
1104
1102
1023
1022
977
962
948
926
909
887
839
834
777
695
686
681
679
644
606
566
565
m
m
m
m
m
m
m
m
m
m
m
C2AH7
C15AH18
C16AH20
C2AH7
C5AH8
C17AH22
C5AC6
C15AC17
C1AC2;
C6AC1;
C15AH18
C16AH20
C5AH8
C17AH22
C6AH9
N12AN13
C5AC6
C6AC1
C15AH18
C16AH20
C5AH8
C17AH22
C6AH9
C15AC17
C5AC6
C6AC1
3089 w
3082 sh
3062 w
1578 vw
1578 vw
1558 m
1558 m
1466 vs
1456 sh
1456 sh
1382 s
1373 sh
1287 w
1270 vw
1254 w
1254 w
1209 s
3071 vw
1577 m
1577 m
1568 sh
m
m
C16AC14
C14AC15
C6AC1;
C16AC14
N12AN13
mC14AC15
C1AC2;
C14AC15
C16AC14
N12AN13
1474 m
1445 s
1445 s
1378 m
N12AN13
b(C17AH22)
b(C5AH8)
b(C17AH22)
b(C5AH8)
m
m
m
m
b(C2AH7)
b(C16AH20)
b(C15AH18)
b(C17AH22)
b(C5AH8)
b(C17AH22)
b(C5AH8)
m
m
m
m
C19AC16
C2AC3
C3AC4
C4AC5; mC17AC21
C19AC16
C2AC3
C4AC5;
C3AC4
m
m
m
m
m
C2AC3
m
m
m
m
m
C2AC3
C19AC16
C21AC19
C17AC21
C4AC5
C19AC16
C21AC19
C3AC4
1290 vw
1268 vw
1252 w
mC16AC14
b(C2AH7)
b(C16AH20)
b(C6AH9)
C4AC5
b(C16AH20)
b(C16AH20)
m
m
b(C15AH18)
b(C6AH9)
mC17AC21
b(C2AH7)
bR1 (A2)
bR1 (A1)
c
c
mC1AC2
c
c
1213 vw
1169 vs
1138 vw
1124 w
1105 sh
m
m
N13AC14
C1AN12
N13AC14
C1AN12
1169 vw
1132 m
1120 s
b(C15AH18)
m
C15AC17
mC1AN12
mN13AC14
mC21AC19
mC4ACl11
b(C6AH9)
b(C15AH18)
b(C6AH9)
m
m
m
N13AC14
C3AC4
C21AC19
1077 vw
mC3AC4
b(C2AH7)
bR1 (A2)
bR1 (A1)
c
c
1028 s
1028 s
958 vw
939 vw
905 m
894 s
1029 vw
bR1 (A1)
bR1 (A2)
c
c
c
c
bR1 (A1)
bR1 (A2)
c
c
c
c
958 vw
939 vw
C15AH18
C6AH9
C16AH20
C2AH7
C6AH9
C15AH18
C6AH9
C15AH18
C6AH9
C17AH22
C16AH20
C2AH7
C1AN12
C5AH8
C5AH8,
mC1AC2
c
c
C2AH7
C16AH20
C2AH7
C16AH20
886 sh
d(C14AN13)
C5AH8
C3ACl10
C17AH22
dN12AC1
R1(A1)
m
c
m
c
c
c
mC3ACl10
mC14AC15
c
c
823 s
c
C15AH18
c
c
C17AH22
C5AH8
C17AH22
C5AH8
814 w
794 w
705 m
690 m
671 vw
671 vw
656 w
606 w
c
C15AH18
795 vw
693 vw
d(C14AN13)
R1 (A1)
bR2 (A1)
bR3 (A1)
bR2 (A1)
R1 (A1)
bR3 (A1)
R1 (A2)
bR3 (A2)
C4ACl11
(N12AC1)
d(C14AN13)
(N13AC14)
s
s
s
bR3 (A1)
bR3 (A2)
bR3 (A1)
bR3 (A2)
s
c
m
m
c
s
m
s
s
m
b(C3ACl10)
s
bR2 (A1)
bR2 (A2)
c
c
s
s
R1 (A1)
R1 (A2)
673 vw
s
sR1 (A2)
R1 (A2)
C19ACl23
C4ACl11
C21ACl24
C3ACl10
R2 (A2)
C19ACl23
R3 (A1)
R3 (A2)
C3ACl10
bR3 (A2)
C4ACl11
c
C3ACl10
m
m
c
m
s
s
C21ACl24
N@N
N@N,
c
c
(N12AC1)
(N13AC14)
561 vw
544 vw
493 vw
458 vw
442 vw
430 vw
415 vw
393 vw
391 vw
374 m
548 vw
d(C14AN13)
C21ACl24
b(C3ACl10)
c
542
456
450
440
435
404
379
359
331
293
286
213
203
198
182
164
c
(N12AC1)
525
457
443
437
435
415
395
352
339
292
285
215
202
198
189
166
511
451
445
444
438
392
385
374
345
308
218
213
200
200
180
156
m
517
454
445
439
434
422
373
356
337
305
221
213
201
194
181
146
m
m
s
s
m
C21ACl24
C19ACl23
R3 (A2)
R3 (A1)
C3ACl10
462 vw
452 vw
mC19ACl23
sR3 (A2)
sR3 (A1)
sR3 (A2)
sR3 (A2)
mC19ACl23
sR3(A1)
436 vw
388 w
c
C4ACl11
b(C14AN13)
bR2(A1)
c
c
b(C19ACl23)
bR2 (A2)
b(C3ACl10)
C4ACl11
bR2 (A2)
C21ACl24
b(C19ACl23)
C19ACl23
372 vw
365 vw
335 vw
296 vw
290 vw
217 vw
b(C3ACl10)
bR2 (A1)
bR2 (A2)
c
c
b(C14AN13)
b(C21ACl24)
b(C4ACl11)
b(C19ACl23)
N@N
c
C4ACl11
C21ACl24
c
C21ACl24
C21ACl24
C4ACl11
C21ACl24
c
b(C14AN13)
b(C21ACl24)
b(C4ACl11)
b(C19ACl23)
c
C19ACl23
b(C21ACl24)
C19ACl23
C3ACl10
b(C21ACl24)
b(C4ACl11)
180 vw
m
c
c
C3ACl10
b(C1AN12)
C2AC1AN12AN13
b(C14AN13)
170 vw
137 vw
120 vw
109 vw
c
C19ACl23
R2 (A1)
b(C1AN12)
R2 (A2)
(N13AC14)
c
(N13AC14)
N@N
b(C1AN12)
R2 (A1)
(N12AC1)
sC2AC1AN12AN13
s
135
113
84
35
s
133
103
83
31
s
133
121
64
45
sR2(A2)
142
125
67
44
b(C1AN12)
R2 (A1)
dN12AC1
s
s
R2 (A2)
R2 (A1)
s
c
s
c
s
dN12ACl
22
20
s
C2AC1AN12AN13
24
20
s
s
C2AC1AN12AN13
C16AC14AN13AN12
30
15
s
N@N
30
13
s
s
N@N
sC16AC14AN13AN12
sC16AC14AN13AN12
C16AC14AN13AN12
Abbreviations: m, stretching; b, deformation in the plane; c, deformation out of plane; wag, wagging; s, torsion; bR, deformation ring sR, torsion ring; q, rocking; sw, twisting;
d, deformation; Butt, butterfly; a, antisymmetric; s, symmetric; A1, Ring 1; A2, Ring 2.
a
This work.
b
From scaled quantum mechanics force field at B3LYP/6-31G*.
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

8
M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
excited species in solution cannot be detected in this study since
electronic transitions are more rapid than molecular vibrations or
possibly because the absorption maximum is outside the spectral
range of 240–640 nm, as observed in Fig. S3.
AIM analysis
The electronic distribution on the atoms corresponding to all
the conformers of TCAB were analysed by means of Bader’s charge
electron density topological analysis [17]. The calculated electron
density, (
q
) and the Laplacian values,
r
2q(r) in the ring critical
points (RCPs) for the four structures are shown in Table S8. The
results show for the two trans forms the higher topological prop-
erties values by using 6-31G* basis set while when increase the
size of the basis set at 6-311++G** increase only the (
q
) and
r
2q(r) values for the two cis forms, as observed in Table S6. The
analysis shows a slightly dependence of the topological properties
with the size of the basis set, stabilizing in general to the trans
conformers of TCAB. Then, the topological properties in part
explain the different stabilities between the cis and trans struc-
tures of TCAB.
Fig. 4. Experimental in ethanol solution (upper) and theoretical (bottom) ultravi-
olet–visible spectrum of 3,30,4,40tetrachloroazobenzene.
Vibrational analysis
of TCAB, the CACl in-plane deformation modes were predicted at
lower wavenumbers in relation to the corresponding out-of-plane
deformation modes, hence they were assigned in agreement with
the calculations and, as observed in Table 3.
This analysis was performed taking into account the energeti-
cal stabilities of the two pairs of conformers of TACB and the
small difference energies among them. This way, the vibration
normal modes corresponding to the two pairs of conformers were
considered in the study. The comparison of the calculated infra-
red and Raman spectra for TCAB by using B3LYP/6-31G* theory
level with the corresponding experimental ones demonstrate a
good correlation, as observed respectively in Figs. 2 and 3. Note
that the strong bands between 1500 and 1000 cmꢁ1 in the exper-
imental IR spectrum are justified by the presence of both trans
forms while probably in this region these bands overlap those
bands attributed to the cis forms. On the other hand, the bands
observed in the higher and lower wavenumbers regions in the
Raman spectrum are probably attributed to the two cis forms
and, this way, those bands justify the existence of both forms in
the solid phase. The two pairs of conformations of TCAB have
C1 symmetries and 66 normal vibration modes where all the
vibrations are IR and Raman active. The assignment of the exper-
imental bands to the expected normal vibration modes were
made on the PED P10% contributions, by using of symmetry
coordinates and taking into account the assignments for similar
compounds [10,29–32,37,39]. Table 3, Tables S9 and S10 show
the experimental and calculated frequencies, potential energy
distribution based on the 6-31G* basis set, and assignment for
the two pairs of conformers of TCAB. We considered the B3LYP/
6-31G* calculations because the used scale factors are defined
for this basis set. The SQM force fields for this compound can
be obtained upon request.
Skeletal modes. In accordance with the lower calculated N12@N13
distances for the cis I and II forms (1.249 and 1.243 Å, respectively)
the frequencies for the corresponding N@N stretching modes are
predicted by calculations at higher wavenumbers than the trans
stretching modes (1.261 and 1.252 Å), hence, they were assigned
to the Raman bands at 1577 and 1474 cmꢁ1, where the latter band
is observed as intense in the IR spectrum at 1466 cmꢁ1. Those
stretching modes for the trans forms are also assigned at
1466 cmꢁ1
, because both modes are predicted at 1484 and
1487 cmꢁ1. The NAC stretching modes for the trans and cis forms
of TCAB are easily assigned to the strong IR bands at 1209 and
1120 cmꢁ1, respectively. The ring stretching modes for both forms
of TCAB are assigned to the IR and Raman bands respectively at
1558 and 1577 cmꢁ1, as indicated in Table 3. The remaining CAC
stretching modes are assigned as predicted by SQM calculations.
On the other hand, the three phenyl ring deformations and torsions
are assigned in the expected regions, as observed in molecules con-
taining similar rings [10,29–32,36,37,43,44] and as indicated in
Table 3.
Force field
Table 4 shows the calculated forces constants for the two pairs
of conformers of TCAB. The values were calculated by means of the
SQM methodology [14] with the Molvib program [38]. Note that in
all the forces constants there are a certain dependence of the
values with the size of the basis set diminishing slightly when
increase the basis set of 6-31G* at 6-311++G**. The analyses show
that the f(N@N) force constants for the cis forms of TCAB have
higher values than the corresponding to the trans forms, in
accordance with the lower calculated N12@N13 distances which,
in the cis I and II forms are respectively 1.249 and 1.243 Å while
in the trans I and II forms the values are approximately 1.261
and 1.252 Å. On the contrary, the higher C1AN12 and C14AN13
distances in both cis conformers justify the differences in their
Assignments
CAH modes. The IR bands between 3118 and 3062 cmꢁ1 (Fig. 2 and
Table 3) can be assigned to the CH stretchings modes of both rings,
as predicted by calculations. Taking into account the assignments
for compounds containing CH groups [10,29–32,36,37,43,44] the
IR bands between 1466 and 1169 cmꢁ1 are assigned to the CH
in-plane deformation modes for the two pairs of conformers while
the corresponding out-of-plane deformation modes are associated
to the IR bands between 958 and 814 cmꢁ1
.
CCl modes. The CACl stretching modes are predicted in the
expected region [43,44] and, for this, they can be clearly assigned
to the bands between 606 and 442 cmꢁ1. For cis and trans forms
f(mCAN) force constant values being highest in the trans forms, as
can be seen in Table 4. On the other hand, the different
C19ACl23 and C21ACl24 distances values for all the conformers
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
9
Table 4
Comparison of scaled internal force constants for TCAB conformers.
Force constant
Cis TCAB
Cis I
Trans TCAB
Trans I
Cis II
Trans II
6-31G*
6-311++G**
6-31G*
6-311++G**
6-31G*
6-311++G**
6-31G*
6-311++G**
f(CAN)
f(NAN)
f(CAC)ring
f(CACl)
f(CAH)
4.001
10.296
6.436
3.456
5.253
3.903
10.195
6.320
3.400
5.169
4.016
10.293
6.444
3.455
5.254
3.908
10.191
6.329
3.399
5.169
4.958
9.583
6.447
3.430
5.261
4.847
9.558
6.325
3.374
5.186
4.948
9.583
6.446
3.443
5.273
4.838
9.558
6.324
3.388
5.188
Units are mdyn Åꢁ1 for stretching and stretching/stretching interaction and mdyn Å radꢁ2 for angle deformations.
of TCAB justify the values of the corresponding f(CACl) force
constants. Here, the lower values in the f(N@N) and f(CAC)ring force
constants for the trans forms are probably related to the strong
Conclusions
We have synthesized and characterized TCAB by infrared,
Raman, multidimensional nuclear magnetic resonance (NMR) and
ultraviolet–visible spectroscopies. The assignments of the 66 nor-
mal modes of vibration corresponding to both cis and trans isomers
of the compound are reported. The theoretical structures of two cis
and trans forms of TCAB were determined by using the 6-31G* and
6-311++G** basis sets. The molecular electrostatic potentials,
atomic charges and bond orders support the high stability of the
trans forms together with the NBO studies, which suggest that
p
? p delocalizations observed by NBO analysis due to the planar-
ity of those two forms.
HOMO–LUMO energy gap
The presence of lone pairs on the two N atoms and of four elec-
tronegative Cl atoms conifer to TCAB interesting structural and
electronic properties, as reveled by above studies. To study the role
of those two atoms on the reactivity of TCAB the frontier molecular
HOMO and LUMO orbitals [45] for all the conformers were calcu-
lated at different levels of theory and compared among them, as
observed in Table S11. The results clearly show that: (i) both fron-
tier orbitals are localized on the rings, (ii) the energies gap follows
the trend: cis I > trans II > trans I > cis II and (iii) the large gap
energy makes TCAB kinetically inert because present a good stabil-
ity and a high chemical hardness.
the electronic transitions of the two cis forms are n ?
p
* transitions
while for the trans forms are n ? p* and p* p* transitions. Thus,
?
the topological parameters and the main delocalization energies
favor to the trans forms. The calculated highest occupied molecular
orbital (HOMO)–lowest unoccupied molecular orbital (LUMO)
energy gaps for the four conformations of TCAB follow the trend:
cis I > trans II > trans I > cis II. The SQM/B3LYP/6-31G* force field
for the two pairs of conformers of TCAB were obtained together
with the more important force constants. This study shows that
the cis and trans isomers exhibit different structural and vibra-
tional properties and absorption bands. Additionally, the size of
the basis set reveals the importance of it in all the studied
properties.
Electronic spectrum
A
comparison between the electronic spectrum of TCAB
registered in ethanol solution with the corresponding calculated
for the two cis and trans forms at B3LYP/6-31G* level of theory
can be seen in Fig. 4. The graphical show that the cis and trans
isomers exhibit different absorption bands. Thus, the theoretical
calculations by using both basis sets predict for the cis forms four
bands located at 490 nm (2.5324 eV) with an oscillator strength
f = 0.0607, at 278 nm (4.4614 eV) with f = 0.1283, at 200 nm
(6.2282 eV) with f = 0.3658 and at 154 nm (8.0212 eV) with
f = 0.1345, while for the trans forms the theoretical calculations
by using 6-31G* basis set predict four bands while when the other
basis set is used only three bands are predicted. In the first case,
the bands are located at 357 nm (3.4738 eV) with an oscillator
strength f = 0.8978, at 239 nm (5.1812 eV) with f = 0.1527, at
188 nm (6.5686 eV) with f = 0.2437 and, at 149 nm (8.3075 eV)
with f = 0.0840. On the other hand, the measured experimental
show only two bands, an intense and broad centered in approxi-
mately 335 nm and other band at 445 nm. The deconvolution of
the first experimental band produces three bands at 293, 335
and 361 nm that are shifted in relation to those calculated using
TD/B3LYP calculations. These bathochromic shifts found for trans
molecules, as compared with the cis forms, were also observed
by Chen and Chieh [10] in the azobenzene compounds and they
have attributed that variation to a better conjugated effect for
the trans forms in reference to the cis forms. The four experimen-
tally bands observed were assigned to the chromophores present
in the molecule, being higher the n ? p* transitions for the cis
Acknowledgements
This work was supported with grants from CIUNT (Consejo de
Investigaciones, Universidad Nacional de Tucumán) and CONICET
(Consejo Nacional de Investigaciones Científicas y Técnicas, R.
Argentina). The authors thank to Dr. V. Daier (IQUIR) for the
UV–Visible spectrum and to Prof. T. Sundius for the Molvib
program.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.07.032.
References
[1] (a) D.D. Kaufman, J.R. Plimmer, J. Iwan, U.I. Klingebiel, J. Agric. Food Chem. 20
(4) (1972) 916–919;
(b) R. Bartha, D. Pramer, Science 156 (1967) 1617–1618.
[2] B.P. Singh, A. Nyska, G.E. Kissling, W. Lieuallen, S.L. Johansson, D.E. Malarkey,
M.J. Hooth, Toxicol. Pathol. 38 (2010) 372–381.
[3] J.K. Lee, K.S. Kyung, J. Agric. Food Chem. 43 (1995) 519–523.
[4] C.J. Brown, Acta Crystallogr. 21 (1966) 146–152.
[5] A. Mostad, C.H.R. Rømming, Acta Chem. Scand. 25 (1971) 3561–3568.
[6] H. Hope, D. Victor, Acta Crystallogr., B 25 (1969) 1849–1853.
[7] J.A. Bouwstra, A. Schouten, J. Kroon, Acta Crystallogr., C 39 (1983) 1121–1123.
[8] A. Cembran, F. Bernardi, M. Garavelli, L. Gagliardi, G. Orlando, J. Am. Chem. Soc.
126 (2004) 3234–3243.
forms and the n ? p
* transitions for the trans forms, in accordance
with reported data for azobenzene [10] and with the results of the
NBO calculations performed here.
[9] N. Kurita, T. Ikegami, Y. Ishikawa, Chem. Phys. Lett. 360 (2002) 349–354.
[10] P.C. Chen, Y.C. Chieh, J. Mol. Struct. (THEOCHEM) 624 (2003) 191–200.
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032

10
M.V. Castillo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
[11] A.J. Wilczyn´ ska-Piliszek, S. Piliszek, J. Falandysz, J. Environ. Sci. Health B 47 (2)
(2012) 111–128.
[26] J. Zinczuk, A.E. Ledesma, S.A. Brandán, O.E. Piro, J.J. López-González, J. Phys.
Org. Chem. 22 (2009) 1166–1177.
[12] M. Ribagorda, E. Merino, An. Quím. 105 (4) (2009) 290–299.
[13] B.H. Besler, K.M. Merz Jr., P.A. Kollman, J. Comput. Chem. 11 (1990) 431–439.
[14] (a) G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093–3100;
(b) G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 14572.
[27] A.E. Ledesma, J. Zinczuk, J.J. López González, A. Ben Altabef, S.A. Brandán, J.
Mol. Struct. 322 (2009) 924–926.
[28] S.A. Brandán, E. Erog˘lu, A.E. Ledesma, O. Oltulu, O.B. Yalçınkaya, J. Mol. Struct.
993 (2011) 225–231.
[15] A.E. Reed, L.A. Curtis, F. Weinhold, Chem. Rev. 88 (6) (1988) 899–926.
[16] E.D. Glendening, J.K. Badenhoop, A.D. Reed, J.E. Carpenter, F. Weinhold, NBO
3.1, Theoretical Chemistry Institute, University of Wisconsin, Madison (WI),
1996.
[29] C.D. Contreras, M. Montejo, J.J. López González, J. Zinczuk, S.A. Brandán, J.
Raman Spectrosc. 42 (1) (2011) 108–116.
[30] A.B. Brizuela, A.B. Raschi, M.V. Castillo, L. Davies, E. Romano, S.A. Brandán, J.
Mol. Struct. 1074 (2014) 144–156.
[17] R.F.W. Bader, Atoms in Molecules, a Quantum Theory, Oxford University Press,
Oxford, 1990 (ISBN: 0198558651).
[31] P. Leyton, J. Brunet, V. Silva, C. Paipa, M.V. Castillo, S.A. Brandán, Spectrochim.
Acta, Part A 88 (2012) 162–170.
[18] F. Biegler-Köning, J. Schönbohm, D. Bayles, J. Comput. Chem. 22 (2001) 545–
559.
[32] E. Lizarraga, E. Romano, R. Rudyk, C.A.N. Catalán, S.A. Brandán, Spectrochim.
Acta, Part A 97 (2012) 202–208.
[19] S.M. Mehta, M.V. Vakilwala, J. Am. Chem. Soc. 74 (1952) 563–564.
[20] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652.
[33] E. Romano, N.A.J. Soria, R. Rudyk, S.A. Brandán, J. Asia Spectrosc. 17 (2013) 1–
28.
[21] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789.
[22] A.B. Nielsen, A.J. Holder, Gauss View 5.0, User’s Reference, GAUSSIAN Inc.,
Pittsburgh (PA), 2009.
[34] E. Romano, N.A.J. Soria, R. Rudyk, S.A. Brandán, J. Mol. Simul. (2012) 1–6.
[35] M.V. Castillo, E. Romano, A.B. Raschi, A. Yurquina, S.A. Brandán, Comput. Theor.
Chem. 995 (2012) 43–48.
[23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann,
O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 09, Revision A.02, Gaussian Inc., Pittsburgh (PA), 2009.
[24] A. Brizuela, E. Romano, A. Yurquina, S. Locatelli, S.A. Brandán, Spectrochim.
Acta, Part A 95 (2012) 399–406.
[36] P. Leyton, C. Paipa, A. Berrios, A. Zárate, M.V. Castillo, S.A. Brandán, J. Mol.
Struct. 1031 (2013) 110–118.
[37] G.R. Argañaraz, E. Romano, J. Zinczuk, S.A. Brandán, J. Chem. Chem. Eng. 5 (89)
(2011) 747–758.
[38] T. Sundius, J. Mol. Struct. 218 (1990) 321–326.
[39] J.R. Sambrano, A.R. de Souza, J.J. Queralt, M. Oliva, J. Andrés, Chem. Phys. 264
(2001) 333–340.
[40] S.A. Brandán, G. Benzal, J.V. García-Ramos, J.C. Otero, A. Ben Altabef, Vib.
Spectrosc. 46 (2008) 89–99.
[41] A.E. Ledesma, S.A. Brandán, J. Zinczuk, O.E. Piro, J. López González, A. Ben
Altabef, J. Phys. Org. Chem. 21 (12) (2008) 1086–1097.
[42] M.G. Ruba Priya, T. Srinivasan, K. Girija, N. R Chandran, D. Velmurugan, Acta
Crystallogr., Sect. E 67 (2011) o2310.
[43] E. Romano, M.V. Castillo, J.L. Pergomet, J. Zinczuk, S.A. Brandán, J. Mol. Struct.
1018 (2012) 149–155.
[44] E. Romano, M.V. Castillo, J.L. Pergomet, J. Zinczuk, S.A. Brandán, Open J. Synth.,
Theory, Appl. 2 (2013) 8–22.
[45] R.G. Parr, R.G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512–7516.
[25] A.B. Brizuela, A.B. Raschi, M.V. Castillo, P. Leyton, E. Romano, S.A. Brandán,
Comput. Theor. Chem. 1008 (2013) 52–60.
Please cite this article in press as: M.V. Castillo et al., A complete vibrational study on a potential environmental toxicant agent, the 3,30,4,40-tetrachlo-
roazobenzene combining the FTIR, FTRaman, UV–Visible and NMR spectroscopies with DFT calculations, Spectrochimica Acta Part A: Molecular and Bio-
molecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.07.032