Structural and vibrational spectral studies on hydrogen bonded salts: 4-chloroanilinium maleate and nitrate

Article abstract of DOI:10.1007/s12039-015-0914-y

In the present study, proton transfer from nitric and maleic acids to amine group (4-chloroaniline) led to hydrogen bonded crystals of 4-chloroanilinium maleate (4CAM) and 4-chloroanilinium nitrate (4CAN) which are investigated by the experimental and the

Full text of DOI:10.1007/s12039-015-0914-y

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0914-y
Structural and vibrational spectral studies on hydrogen bonded salts:
4-chloroanilinium maleate and nitrate
R ANITHAa, M GUNASEKARANa, S SURESH KUMARb and S ATHIMOOLAMb,∗
aDepartment of Physics, Regional Centre, Anna University Tirunelveli Region, Tirunelveli 627 007, India
bDepartment of Physics, University College of Engineering, Nagercoil, Anna University,
Nagercoil 629 004, India
e-mail: athi81s@yahoo.co.in
MS received 25 October 2014; revised 17 May 2015; accepted 19 May 2015
Abstract. In the present study, proton transfer from nitric and maleic acids to amine group (4-chloroaniline)
led to hydrogen bonded crystals of 4-chloroanilinium maleate (4CAM) and 4-chloroanilinium nitrate (4CAN)
which are investigated by the experimental and theoretical approaches. The molecular structures of these two
compounds were optimized with the Density Functional Theory (DFT) using B3LYP function and the Hartree-
Fock (HF) level with a6-311++G(d,p) basis set. Geometrical parameters of the molecules were also ana-
lyzed along with their intermolecular hydrogen bond, which tailors the ions. These analyses show that present
molecules are stabilized through the N–H· · · O and O–H· · · O hydrogen bonds. The vibrational modes were
computed by quantum chemical methods. Further, these modes are investigated by FT-IR and FT-Raman spec-
troscopy in the range of 4000–400 cm−1. The optimized molecular geometry and computed vibrational spectra
are compared with experimental results, which show significant agreement. The natural bond orbital (NBO)
analysis was carried out to interpret hyperconjucative interaction and intramolecular charge transfer (ICT).
This analysis gives the precise insight into the nature of H-bond interactions. The chemical hardness, elec-
tronegativity and chemical potential of the molecules were determined by HOMO–LUMO plot. The frontier
molecular orbitals have small band gap value, which signify the possible biological/pharmaceutical activity of
the compounds.
Keywords. FT-IR spectrum; FT-Raman spectrum; Vibrational analysis; quantum chemical analysis; HOMO–
LUMO; NBO.
1. Introduction
identify hydrogen bond–enriched assemblies by means
of efficient organic hydrogen bonding synthons.
One of the most important reactions in chemistry and
biochemistry is proton transfer between donor and
acceptor.1 These interactions are playing an important
role in various chemical and biological processes such
as stabilizing the biomolecular structures,2 controlling
the speed of the enzymatic reactions3 and constructing
supramolecular structures.4 In most of the cases, the ap-
plications are ensued due to the intra- and intermole-
cular interactions particularly with the hydrogen bonds,
which have more importance in the areas of molecular
recognition, crystal engineering research and supramo-
lecular chemistry. Understanding of these non-covalent
interactions has proved to be the most valuable because
of its strength and directional properties.5 The strength
and directionality of hydrogen bonding interactions are
responsible for solid-state formation and other physical
properties of the system.6 Also, we continuously seek to
Substituted anilines are known to be candidates for
supramolecular synthons as they have both donor and
acceptor sites, viz., nitrogen and chlorine atoms in the
present case. The present work was attempted with
nitrate and maleate salts of 4-chloroanilinium crystal.
Though the single-crystal XRD clearly predicts the mo-
lecular structure and packing tendency of the molecule
in the solid crystalline state, the spectral measurement
gives an account of the strength of the intermolecular
interactions by means of shifting of wavenumbers and
the intensity of the peaks. Further, the molecular geom-
etry obtained from the crystallographic data is a well-
suited input for the quantum chemical calculations. By
which it is possible to optimize the molecule in dif-
ferent environments and the corresponding vibrational
modes can be calculated. The hyperconjugative interac-
tions and band gap values can be inferred from natu-
ral bond orbitals (NBOs) and frontier molecular orbital
(FMO) studies. Hence, a combined crystallographic,
spectroscopic and quantum chemical calculations were
∗For correspondence

R Anitha et al
attempted on 4-chloroanilinium maleate (4CAM) and refined isotropically. Graphical molecular illustrations
4-chloroanilinium nitrate (4CAN), and the results are were done with ORTEP3 for Windows9 and Mercury10
summarized.
packages.
Though good-quality crystals of 4CAN were obtai-
ned, the attempts of single crystal X-ray diffraction was
not fruitful due to the hygroscopic nature of the crystal.
2. Experimental
2.1 Crystal Growth
2.4 Vibrational Spectroscopic Measurements
Crystals of 4CAM and 4CAN were synthesized from
aqueous mixtures containing 4-chloroaniline with ma-
leic and nitric acid in the 1:1 stoichiometric ratio at
room temperature by slow evaporation method, respec-
tively. After a week, light yellow colour crystals of
4CAM and 4CAN were obtained. The qualities of the
crystals were improved by recrystallization.
A Jasco spectrometer FT-IR, model 410 under a resolu-
tion of 4 cm−1 and with a scanning speed of 2 mm/sec
was used for IR spectral measurements. The samples
were prepared using pellet technique and the spectrum
was recorded over the range 4000–400 cm−1. The FT-
Raman spectrum was recorded in the frequency range
of 4000–50 cm−1 using a BRUKER RFS 27 FT-Raman
Microscope module with the resolution of 2 cm−1. The
Nd: YAG Laser source was operated at 1064 nm for the
excitation.
2.2 Density Measurement
The density of the grown crystals were measured by
sink and swim method (flotation technique) using a liq-
uid mixture of carbon tetrachloride and xylene. Initially,
carbon tetrachloride (5 mL) was taken in a test tube and
2.5 Computational details
a good-quality three-dimensional crystal was placed on The geometrical optimization for 4CAM and 4CAN
it. Due to high density of the liquid, the crystal began molecules was carried out theoretically by 6-311++
to float. Then, drops of xylene were added drop by drop G(d,p) level on an Intel Core i5/3.20 GHz computer
with continuous agitation to get uniform density over using Gaussian 09W11 program package without any
the liquid. When the density of the crystal matches that constraint.12 Initial geometry was taken from the single-
of liquid mixture, the crystal levitates to the middle of crystal X-ray studies and it was minimized (optimized)
the test tube. Then, the density of the liquid was found by Hartree−Fock (HF) method using the 6-311++G
with specific gravity bottle from the concept of rela- (d,p) basis set. The molecular geometries were also
tive density. Thus, the densities of the crystals are found optimized by the Density Functional Theory (DFT)
to be 1.495 mg.m−3 for 4CAM and 1.465 mg.m−3 for using the Becke’s three-parameters exchange functional
(B3)13 in combination with the Lee–Yang–Parr correla-
4CAN.
tion functional (LYP).14 It is accepted as a cost-effective
approach for the computation of molecular structure,
vibrational frequencies and energies of optimized struc-
2.3 Single-Crystal XRD Studies
The entire crystallographic calculations of 4CAM were tures. The optimized structural parameters were used in
performed done using single-crystal X-ray diffraction the vibrational frequency calculations at the same level
by Bruker SMART APEX CCD area detector diffracto- to characterize all stationary points as minima. Then
meter (graphite- monochromated, MoKα = 0.71073 Å). vibrationally averaged nuclear positions of the structure
A good-quality single-crystal was selected from the were used for harmonic vibrational frequency calcula-
grown crystals and used for the single crystal X-ray tions resulting in IR and Raman frequencies. Finally,
diffraction studies. Consequently, the full data collec- the calculated normal mode vibrational frequencies
tion of the crystal was carried out. The cell refine- provide thermodynamic properties through the princi-
ment and data reduction were done with XCAD4 ple of statistical mechanics. By combining the analy-
software.7 The structure was solved and refined with sis preformed using the GAUSSVIEW program15 with
the SHELXL97 program.8 All the H atoms were dis- symmetry considerations, vibrational frequency assign-
cernible in the difference electron density maps. Never- ments were made with a high degree of accuracy. There
theless, the aryl H atoms were constrained and refined is always some ambiguity in defining internal coordi-
in the riding atom approximation: C−H = 0.93,Å nation. However, the defined coordinate from the com-
and Uiso(H) = 1.2Ueq(C). The other H atoms, which plete set matches quite well with the motions observed
are involved in the N−H· · · O and O–H. . .O hydrogen using the GAUSSVIEW program. Also, the NBO cal-
bonds, were located from electronic density map and culations were carried out using HF and DFT methods

Structural and spectral studies on two hydrogen bonded salts
with the 6-311++G (d,p) basis set to get more detailed
optimized C–N bond distances are 1.418 and 1.426Å in
HF and DFT levels, respectively, for 4CAN and 1.415
and 1.425 Å by the HF and DFT levels, respectively,
for 4CAM (table 1). In theoretical calculations the C–N
bond distances of the molecules are observed to be
smaller than the single crystal XRD study which is
due to the effect of N–H· · · O intermolecular hydrogen
bonds. Even though the hydrogen atom seems to have
deviated from the N atom of the cation, there must be some
attraction between cation and anion which is confirmed
by the Mulliken charge distribution analysis. The bond
geometry between N, H and O atoms which connect
the ion pairs, are shown scheme 1. From the Mulliken
charge distribution analysis of the two complexes, N
atom is more electronegative than the O atom. These N
information about the chemical bonds and hydrogen
bonding interaction within the ions. The FMOs have
been computed and analyzed by HF and DFT method
with the 6-311++G (d,p) basis set.
3. Results and Discussions
3.1 Molecular Geometry
The solid-state molecular and crystal structures of 4CAM
are illustrated from the single-crystal XRD studies. The
asymmetric part of the 4CAM contains a 4-chloroani-
linium cation and a maleate anion shown in figure 1. As
4CAN is prepared with 4-chloroaniline and nitric acid,
the crystals are expected to possess 4-chloroanilinium atoms of the cations are attracting the proton from acids
cation and a nitrate anion in the asymmetric part. The and it forms the N–H· · · O intermolecular hydrogen
optimized structures of both the molecular assemblies bond. The N–H· · · O angle is 180◦ and 179◦ at the HF
are shown in figure 2. In both the compounds, the trans- and DFT/B3LYP methods, respectively, for 4CAN and
fer of proton from the inorganic and organic acid leads 172◦ in both methods for 4CAM, whereas the experi-
to protonation on the NH2 sites of the 4-chloroaniline mental counterpart is only 158 (2)◦. Hence, this inter-
and it forms the 4-chloroanilinium cation. This proto- molecular hydrogen bonding effect is perceived to be
nation on the N site of the cation is confirmed from dominant in the DFT/B3LYP and HF methods than in
the elongated C−N bond distance [1.456 (3) Å] in the experimental methods.
4CAM.16 The protonated bond distance is compared
In 4CAM, the planes of the cation and maleate anion
with the theoretically computed bond distances. The are inclined at an angle 15.6 (1)◦ in the single crystal
Figure 1. The molecular structure of the 4CAM with the numbering scheme for the atoms and 50% probability displacement
ellipsoids.

R Anitha et al
Figure 2. Optimized molecular geometry and atomic numbering scheme by HF (a) DFT; (b) methods for
4CAM and HF; (c) DFT; (d) for 4CAN.
XRD, these planar angles are calculated at 73.6◦ and whereas it is the strongest among the computed vibra-
73.9◦ in HF and DFT methods respectively. These devi- tional peaks in the DFT/B3LYP level.
ations occur due to the reorientation of N–H· · · O inter-
The optimized C–C bond lengths of phenyl ring
molecular hydrogen bond. The C–O bond distances of are in the falling range 1.381−1.392 Å, according in
the carboxylate group from the maleate anion are 1.182– the theoretical calculations, whereas the single-crystal
1.302 Å at the HF level and 1.209–1.330 Å by the XRD reveals the C–C bond lengths in the range of
B3LYP method, whereas in experimental methods it is 1.368–1.376 Å. On comparison, it is observed that most
1.234 (2)–1.267 (2) Å (table 2). The planarity of the of the optimized bond lengths are slightly deviated from
maleate anion is confirmed by the torsion angles.
the experimental values. Because theoretical calcula-
In 4CAN, the nitrate group exists in a planar orien- tions were carried out for the molecules in the free
tation with the O–N–O angles varying from 115.5 to gaseous phase and the experimental results correspond-
127.6◦ in HF level and from 115.1 to 128◦ in the DFT/ ing to the molecules are in the solid crystalline state.
B3LYP method. The average bond lengths varied at
1.189–1.318 and 1.222–1.379 Å in the HF and DFT/
B3LYP methods respectively (table 3). One of the N–O
bond distances (1.318 Å in HF and 1.379 Å in B3LYP)
in the nitrate anion increases more than the literature
value (∼1.240 Å). This elongation is due to the N–
H· · · O hydrogen bond between the cation and anion.
It shows the possible charge transfer between donor N
and acceptor O atoms. This hydrogen bonding effect is
less in the HF level, which is revealed in the computed
spectra. The –NH2 stretching mode of NH+3 group
is observed to be low-intense peak in the HF level,
3.2 Mulliken Charge Analysis
In general, Mulliken atomic charge calculation has an
important application of quantum chemical calculations
to molecular systems. It plays a vital role in the packing
of crystals in the solid state by means of intermolecular
interaction and it has significant influence on dipole mo-
ment, polarizability, electronic structure and vibrational
modes.17 The Mulliken charge analyses of molecules
4CAM and 4CAN are calculated at the HF and DFT/

Structural and spectral studies on two hydrogen bonded salts
Table 1. Comparison of optimized molecular geometrical parameters of 4-
chloroanilinium cation.
Atom
4CAN
4CAM
Connected
HF
DFT
HF
DFT
Experimental#
Bond length (Å)
C1-C2
C1-C6
C1-H7
C2-C3
C2-CL11
C3-C4
C3-H8
C4-C5
C4-H9
C5-C6
C5-N16
C6-H10
N16-H17
N16-H19
1.381
1.384
1.074
1.381
1.745
1.383
1.074
1.388
1.076
1.387
1.418
1.076
1.001
1.000
1.391
1.392
1.082
1.391
1.758
1.392
1.082
1.397
1.085
1.397
1.426
1.085
1.016
1.015
1.381
1.384
1.074
1.381
1.745
1.384
1.074
1.388
1.076
1.388
1.415
1.076
0.999
1.000
1.391
1.392
1.082
1.391
1.758
1.392
1.082
1.397
1.085
1.397
1.425
1.085
1.015
1.016
1.370 (2)
1.373 (2)
0.930
1.368 (2)
1.728 (2)
1.373 (2)
0.930
1.363 (19)
0.930
1.363 (19)
1.456 (3)
0.930
0.820 (4)
0.940 (2)
Bond Angle (◦)
C2-C1-C6
C2-C1-H7
C6-C1-H7
C1-C2-C3
C1-C2-CL11
C3-C2-CL11
C2-C3-C4
C2-C3-H8
C4-C3-H8
C3-C4-C5
C3-C4-H9
C5-C4-H9
C4-C5-C6
C4-C5-N16
C6-C5-N16
C1-C6-C5
C1-C6-H10
C5-C6-H10
C5-N16-H17
C5-N16-H19
H18-N16-H19
Torsion Angle (◦ )
C6-C1-C2-C3
C6-C1-C2-CL11
H7-C1-C2-C3
H7-C1-C2-CL11
C2-C1-C6-C5
C2-C1-C6-H10
H7-C1-C6-C5
H7-C1-C6-H10
C1-C2-C3-C4
C1-C2-C3-H8
119.7
120.1
120.2
120.4
119.8
119.8
119.8
120.1
120.1
120.4
119.6
120.0
119.3
120.1
120.5
120.5
119.4
120.1
112.1
112.3
113.3
119.5
120.2
120.3
120.7
119.6
119.7
119.7
120.2
120.3
120.4
119.6
120.0
119.3
120.2
120.4
120.5
119.4
120.1
112.6
112.5
110.4
119.8
120.1
119.6
120.4
119.8
119.8
119.7
120.1
120.1
119.2
119.4
120.1
119.2
120.3
120.5
119.8
120.1
120.1
112.5
112.4
109.2
119.6
120.2
119.7
120.6
119.7
119.7
120.3
119.5
120.3
119.3
125.5
119.4
119.3
120.3
120.4
119.6
120.2
120.3
112.5
112.7
109.1
119.4 (15)
120.3 (11)
120.3 (11)
121.1 (2)
119.5 (11)
119.5 (11)
119.4 (15)
120.3 (11)
120.3 (11)
121.5 (2)
120.4 (11)
120.4 (11)
121.5 (2)
119.2 (3)
119.2 (3)
119.4 (15)
120.3 (11)
120.3 (11)
109.0 (3)
112.9 (15)
107.0 (2)
−0.1
179.8
179.9
−0.1
−0.1
179.7
179.8
−0.3
0.1
−0.0
180.0
180.0
−0.01
−0.2
179.6
179.8
−0.4
0.1
0
−0.1
179.8
180.0
0
1.1 (4)
−178.4 (14)
178.9
179.9
180.0
−0.1
−0.2
179.7
179.8
−0.4
0.1
−1.6
0
−0.3 (4)
−179.7 (5)
179.7 (5)
0.3 (3)
−0.5 (3)
−178.9
−178.4
1.6
180
179.8
−0.2
0
−179.8 −180.0
−180
180
CL11-C2-C3-C4 −179.8 −179.8 −179.9 −179.9
CL11-C2-C3-H8
C2-C3-C4-C5
C2-C3-C4-H9
H8-C3-C4-C5
H8-C3-C4-H9
C3-C4-C5-C6
C3-C4-C5-N16
0.2
0.1
0.1
0.1
0.1
0.2
0.1
0.1
−0.3
−179.4 −180.0 −179.7 −179.8
179.7
−180.0 −180.0 −179.8 −179.8
179.7
0.4
0.5
0.4
−0.3
0.3
−0.4
0.4
−0.4
−0.4
−0.5
−177.6 −177.1 −177.5 −177.1
−178.7

R Anitha et al
Table 1. (continued)
Atom
4CAN
4CAM
Connected
HF
DFT
HF
DFT
Experimental#
H9-C4-C5-C6
H9-C4-C5-N16
C4-C5-C6-C1
C4-C5-C6-H10
C4-C5-N16-H17
179.2
2.0
0.4
179.5
2.7
0.3
179.5
2.4
0.4
179.6
2.8
0.3
179.5
1.3
−0.5 (3)
−179.5
−29.9
−151.9
151.9
−179.5 −179.5 −179.5 −179.8
−32.0 −29.5 −29.7 −31.6
C4-C5-N16-H19 −154.9 −153.1 −153.4 −155.6
C6-C5-N16-H17
C6-C5-N16-H19
150.8
28.0
153.7
30.1
153.3
29.5
151.7
27.7
29.9
#Ref16
4CAN
179 °
180 °
-0.067 e
-0.095 e
O-
+0.469 e
+0.491 e
O-
N+
N+
-0.474 e
H
H
-0.485 e
1.73 Å
1.01Å
1.87 Å
0.97Å
HF
DFT
4CAM
172 °
172 °
-0.308 e
-0.378 e
O-
158 (2) °
N+
O-
+ 0.577 e
+0.573 e
O-
-0.418 e N+
H
N+
H
-0.412 e
H
1.75 Å
1.008 Å
0.94 (2) Å
1.87 (2) Å
1.94 Å
0.96Å
Experimental
HF
DFT/B3LYP
Scheme 1. Atomic charges and molecular geometry parameters calculated from experimental and
theoretical calculations of the N–H· · · O hydrogen bond.
Table 2. Comparison of optimized molecular geometrical
parameters of nitrate anion.
charge distribution of 4-chloroanilinium cation shows
that the five carbon atoms attached to hydrogen atoms
(C1 C3, C4, C5 and C6) are negative. Out of five car-
bon atoms in the 4-chloroanilinium cation, the carbon
C5 atom has more negativity than the others, because
the carbon (C5) atom is attached to NH+3 group. The
elongation of C−N bond distance due to the protonation
is reiterated with the electronegative repulsion between
these two atoms (C5 and N16). Interestingly, another
carbon (C2) atom from the cation is attached in the
chlorine (Cl11) atom and both of them have electropos-
itive charge. The characteristic feature of Cl atom is
electropositive and it does not participate in any hydro-
gen bond. In the present case, both the methods predict
that the Cl atom of the cation (Cl11) is positive (0.277
e in HF and 0.340 e in DFT) for 4CAM and (0.275
e in HF and 0.335 e in DFT) for 4CAN. This may be
accorded with the formation of hydrophobic region in
the crystal with the presence of the Cl atoms and the
absence of N−H· · · Cl hydrogen bond.
Nitrate Anion
Atom Connected
HF
DFT
Bond length (Å)
N15-O12
N15-O13
N15-O14
O14-H18
1.189
1.168
1.318
0.971
1.222
1.200
1.379
1.014
Bond Angle (◦ )
N15-O14-H18
O12-N15-O13
O12-N15-O14
O13-N15-O14
Torsion Angle (◦)
O14-O13-N15-O12
H18-O14-N15-O12
H18-O14-N15-O13
107.6
127.6
116.9
115.5
106.1
128.0
116.9
115.1
−179.9
−0.4
−180.0
−0.1
179.6
179.9
B3LYP levels for the molecule under study which are
given in table 4 (a), 4 (b) and 4 (c) and the correspond-
ing population analysis graph is shown in figure 3. The
In the 4CAM anion, all the oxygen atoms have elec-
tronegative charges. Out of four oxygen atoms, O19

Structural and spectral studies on two hydrogen bonded salts
Table 3. Comparison of optimized molecular geometrical
parameters of maleate anion.
Table 4. (a) Atomic charges for optimized geometry of 4-
chloroanilinium cation.
Maleate Anion
HF DFT
4-chloroanilinium cation
4CAN
4CAM
Atom Connected
Experimental
Atom
HF
DFT
HF
DFT
Bond Length (Å)
C1
C2
C3
C4
C5
C6
H7
H8
−0.497
−0.507
−0.345
−0.397
C16-C17
C16-O19
C16-O20
C17-H18
C17-C23
O19-H21
H21-O25
C22-C23
C22-O25
C22-O26
C23-H24
1.488
1.199
1.302
1.074
1.326
1.716
0.954
1.514
1.307
1.182
1.075
1.481
1.476 (2)
1.267 (18)
1.234 (19)
0.93
1.325 (3)
1.207
0.468
0.606
0.482
0.623
1.230
1.321
1.084
1.343
1.635
0.990
1.509
1.33
−0.292
−0.404
−0.132
−0.398
−0.327
0.186
−0.465
−0.588
−0.595
0.180
−0.533
−0.493
−0.459
0.062
−0.109
−0.494
−0.370
0.234
0.236
0.189
0.185
0.148
0.151
0.930
0.237
0.205
0.186
0.159
0.233
0.187
1.476 (2)
1.267 (18)
1.234 (19)
0.93
H9
H10
Cl11
N16
H17
H19
0.190
0.275
0.151
0.335
0.202
0.277
1.209
1.085
0.340
Bond Angle (◦)
−0.485
−0.474
−0.412
−0.418
0.292
0.292
0.283
0.269
0.313
C17-C16-O19
C17-C16-O20
O19-C16-O20
C16-C17-H18
C16-C17-C23
H18-C17-C23
C16-O19-H21
H9-O20-C16
C23-C22-O25
C23-C22-O26
O25-C22-O26
C17-C23-C22
C17-C23-H24
C22-C23-H24
H21-O25-C22
125.5
111.6
122.9
113.4
128.9
117.7
111.4
110.7
120.7
117.2
122.1
134.2
116.6
109.2
113.4
112.1
120.5 (14)
117.8 (14)
121.7 (14)
114.9
130.3 (8)
114.9
115.4
111.2
120.5 (14)
117.8 (14)
121.7 (14)
130.3 (8)
114.9
0.287
0.275
0.310
122.4
113.4
128.7
117.8
110.3
109.4
169.6
120.3
117.8
121.9
133.8
116.7
109.6
111.8
Table 4. (b) Atomic charges for optimized geometry of
nitrate anion.
Atom
HF
DFT
O12
O13
O14
N15
H18
−0.117
−0.060
−0.095
−0.159
0.491
−0.047
0.009
−0.067
−0.311
0.469
114.9
115.4
Torsion Angle (◦ )
O19-C16-C17-H18
O19-C16-C17-C23
O20-C16-C17-H18
O20-C16-C17-C23
C17-C16-O19-H21
O20-C16-O19-H21
C16-C17-C23-C22
C16-C17-C23-H24
H18-C17-C23-C22
H18-C17-C23-H24
O25-C22-C23-C17
O25-C22-C23-H24
O26-C22-C23-C17
O26-C22-C23-H24
C23-C22-O25-H21
O26-C22-O25-H21
−179.2
−179.9
179.9
0.04 (18)
0.2
0.8
0.2
0.1
0.8
Table 4. (c) Atomic charges for optimized geometry of
maleate anion.
−179.2
−0.8
179.2
0.2
−179.9
−179.8 (9)
0.4
−0.2
179.6
0.1
180
−179.7
−0.4
−0.3
179.8
179.7
−0.2
0.1
179.6
0.0
Atom
HF
DFT
180
180
H18
C23
C24
H25
O22
O20
H21
C12
C26
H27
O14
O13
0.573
−0.188
0.265
0.577
−0.310
0.333
−179.8
−180.0
0
0
−0.7
179.5
179.3
−0.6
0.2
0.04 (18)
179.9
179.8
−0.2
0.287
0.248
−0.435
−0.378
0.421
−0.332
−0.308
0.360
−0.070
−0.174
0.231
0.2
0.049
−179.9
−179.9
−179.6
−0.134
0.269
−0.387
−0.327
−0.274
(–0.435 e in HF and 0.332 e in DFT) and O25 (–0.387
e in HF and –0.274 e in DFT) have more electronega-
tivity than the others. These two oxygen atoms behave
as a donor and acceptor in the O–H· · · O intramolecu-
lar hydrogen bond and these intramolecular hydrogen
bonds are also observed in single-crystal XRD.
−0.261
DFT) because it is surrounded by two electronegative
atoms, viz., nitrogen (N16) and oxygen (O14). This hy-
drogen atom plays a key role in the solid state through
the N–H· · · O intermolecular hydrogen bond.
The hydrogen atom in the 4CAN anion has more
electropositive charges (0.491e in HF and 0.469 e in

R Anitha et al
Figure 3. The atomic charges plots by HF (a) and DFT; (b) methods for 4CAM and HF; (c) DFT; (d) methods for 4CAN.
3.3 Vibrational Analysis
quantum chemical calculations and compared with their
experimental vibrational spectra. The optimized struc-
tural parameters are used in the calculation of vibra-
tional frequencies to characterize all stationary points
as minimum. The calculated vibrational wave numbers,
measured by FT-IR and FT-Raman band positions with
their corresponding assignments for the molecules 4CAM
and 4CAN, are tabulated in tables 5 and 6, respectively.
The FT-IR and FT-Raman spectra of the compounds are
shown in figures 4 and 5, respectively.
Vibrational spectroscopy can give valuable informa-
tion about the hydrogen bonding forces and strength
of the intermolecular bonds in addition to symmetry of
the individual species. The effect of hydrogen bond is
very important in crystal environment. The strong, nor-
mal and weak hydrogen bonds cause red shifting of
stretching mode vibrations and blue shifting of defor-
mation mode vibrations with different wave numbers
depending on their strength. Normally, the vibrational
shifts of the stretching modes are greater than that of
the deformation modes. This indicates that the linear
distortion is much greater than the angular distortion.
The molecular structures of 4CAM and 4CAN have
various functional groups such as –NH+3 , -CH, -C-N,
-C-C-, -OH, -C=O, disubstituted benzene ring, etc.
The vibrational bands of these groups are expected
to change in their intensity and position due to their
environments.18 Complete vibrational analyses of the
72 fundamental modes of 4CAM and 51 fundamental
modes of 4CAN are attempted and predicted by the
3.3a N–H Vibrations: In general, the N–H stretch-
ing vibrations from the primary amines occur in the
region 3600–3300 cm−1.19,20 In the present case, 4-
choloanilinium cation has more intense peak calculated
at 3588 cm−1 and 3504 cm−1 in B3LYP level to assign
the NH2 asymmetric and symmetric stretching modes,
respectively, for the molecules 4CAN. Similarly, the
asymmetric and symmetric –NH2 stretching occurred
as the less-intense peak at 3586 and 3499 cm−1 for the
molecule 4CAM. In 4CAN, the medium band observed

Structural and spectral studies on two hydrogen bonded salts
Table 5. Experimental, HF and B3LYP levels computed vibrational frequencies (cm−1) obtained for 4-CAN.
Experimental
HF/6-311++G(d,p)
aIIR bI Raman νcal
1.9
B3LYP/6-311++G(d,p)
Mode Number FT-IR Raman νcal
aIIR
1.9
bI Raman
Assignment
Lattice Vibration
1
2
3
4
5
6
7
8
15
1.2
5.2
18
2
5.7
4.5
1
1.2
0.1
2.4
0.4
0.3
2
30
37
82
91
1.3
0.1
0.3
29
37
79
1.3
0.2
0.2
0.7
6.9
37.8
0.8
5
3.5
3.4
0.1
3.2
33.5
61.6
0.4
0.5
4
Lattice Vibration
Lattice Vibration
Lattice Vibration
Lattice Vibration
ω NH2 + γ C-H
ω NH2 + γ C-H
ρ NH2 + β C-H
ω NH2 + γ C-H
ρ NH2 + β C-H
β (C-H + NH2)
γ C-H
3.8
0.5
1.5
2
99
120
199
275
363
393
411
454
477
564
690
696
733
7.7
14.5
0.8
2
9.3
3.1
0.1
4.6
40.6
39.3
0.3
5.1
7.1
15.2
14.9
4.1
39.6
1.2
0.4
1.3
0.5
0.6
0.6
10.2
0.2
0.2
0.5
1.8
6.1
2.9
0.3
118
216
258
337
377
384
420
465
515
644
649
653
688
718
787
813
822
829
904 292.5
945 160.5
953
968
1026
1032 197.5
1104
1107
1156
1201
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
7.8
0.1
0.2
0.5
2.3
6.3
4.1
4.8
0.7
0.3
3.8
1.4
17.9
18.1
11.3
0.5
0.1
13.1
12.1
29.8
1.7
1.1
5.1
22.2
2.2
25.4
0.5
0.1
5
t NH2
γ C-H
β (C-H + NH2)
β C-H
δ O-N-O
777
ν N=O +δ O-N-O
ω NH2 + γ C-H
γ O-N-O +ω NH2 + γ C-H
ω NH2 + γ C-H
γ C-H
733 w
734 w 799
859
2.4
6.9
10.4
45.1
2.9
19.5
11.4
18.4
0.3
0.5
0.5
13.8
0.6
0.1
0.9
14.4
0.9
892
821 m 802 m 917
922
951
1024 399.3
1064
1088
1102
γ C-H
81.4
ω NH2 + γ O-H
t CH-CH +ν N=O
t CH-CH
16.3
0.3
36.9
95.8
3.7
5.8
46.1
6.7
9.2
1.2
19.3
t CH-CH
1014 w
1042 w 1047 m 1155
1095 m 1094 w 1160
1113 w
1149 w 1175 m 1197
β C-H +γ O-H
ω NH2 + β O-H
β C-H
ρ NH2 + β C-H
ρ NH2 + β C-H
β C-H
50.4
7.7
6.5
1192
0.5
20.7
3.2
1202 w 1209 w 1289
6.5
1317
1354
1449
0.4
79.6
0.6
1.2
16.2
0.7
6.2
0.6
10.9
1.3
1261 118.8
1319 4.1
1328 279.5
1350
1458
1497
ν C-N +β C-H
β C-H
ν N=O
1567 233.1
1573
1611 569.6
1542 m 1535 w 1657 126.6
1.2
3.3
395
ρ NH2 +β C-H
ρ NH2 +β C-H
β O-H
4
1489 m
1525 111.2
1
3
β C-H
1601 m 1772
1785
2.1
12.1
79.8
4.6
1628
1641
1661
1734
2.6
8.7
63.1
280
ρ NH2 + ν C-C +β C-H
δ NH2 + ν C-C +β C-H
δ NH2
23.2
24.4
2.9
54.7
14.6
3.5
364.1
82
67.6
38.1
177.3
165.6
46.4
1806
1752 m
1928 626.4
ν N=O +β O-H
3078 w 3324
11.5
5.5
3.4
0.5
71.2
60.4
48.7
142.8
158.5
113.1
41.2
2907 2497.3
ν N-H· · · O
3332
3362
3364
3167
3172
3203
3204
3504
3588
8.6
5.5
1.5
0.1
24.3
24.9
ν C-H
ν C-H
νas C-H
νsC-H
νs NH2
3595 1529.6
3742
3826
36.5
26.4
νas NH2
w – weak; vs – very strong; m – medium;
ν, Stretching; νs, sym.stretching; νas, asym.stretching; β – in-plane bending; γ – out-of-plane bending; ω – Wagging;
t – twisting; δ – Scissoring; ρ – Rocking;

R Anitha et al
Table 6. Experimental, HF and B3LYP levels computed vibrational frequencies (cm−1) obtained for 4-CAM.
Experimental
HF/6-311++G(d,p)
aIIR bI Raman νcal
1.8
B3LYP/6-311++G(d,p)
Mode Number FT-IR Raman νcal
aIIR
1.8
bI Raman
Assignment
Lattice Vibration
1
2
3
4
5
6
7
8
8
1.9
2.8
3.3
1.6
1.2
0.1
0.6
2
6
2.5
3.8
3.3
3.4
1
0.1
0.7
1.6
4.4
0.6
0.5
2.8
1.9
0.5
2
24
27
30
58
76
3.6
2.3
0.9
4.1
6.7
1.2
4.7
5.1
26
30
35
64
78
2.7
3.2
0.1
4.4
7.4
1.3
5.2
Lattice Vibration
Lattice Vibration
Lattice Vibration
Lattice Vibration
β (C-H + O-H)
β (C-H + O-H)
γ C-H +β (C-H + O-H)
ω NH2
87
90
107
192
256 19.6
260 35.6
297
103
181
250
276
9
2.2
0.5
0.4
1.9
5.2
0.8
0.5
9.3
4.6
0.2
0.5
0.6
1.7
0.9
1.6
2.4
6.2
0.8
0.4
0.2
12.7
8.4
16.3
8.5
0.8
1.2
1.6
13.5
2.4
0.3
0.1
1.2
7.2
0.8
0.5
21.1
3.3
0.7
13.3
17.7
3.4
18.7
0.8
5.2
3.7
0.3
3.7
1.2
5
13
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
31
0.3
ρ NH2
ρ NH2 + β O-H
304 11.7
3.8
t CH-CH
331
363
389
411
433
455
475
567 40.9
649 10.2
653
3
1.9
10
3.1
4.8
0.2
1.2
299 12.3
β (C-H + O-H)
ω NH2 + γ C-H
ρ NH2 + β C-H
ρ NH2 + β C-H
ρ NH2 + β O-H
t CH-CH
t NH2
ω CH-CH
ω CH-CH
β (C-H + O-H) +ω CH-CH
β (C-H + O-H) +ω CH-CH
β (O-H + C- NH2)
Ring deformation
ω NH2 + γ C-H
γ (C-H + O-H)
β (C-H + O-H)
ω NH2 + γ C-H
ω CH-CH
338
377
384
409 22.9
420
474
517 30.4
593 1.6
601 11.2
634 21.2
646 51.4
651
719
784 17.5
788 21.2
815 55.8
823
830 11.5
834 101.5
850
875
905 339
954 13.1
956 36.7
970
1028 23.8
1031 0.2
1086 122.3
1104 50.4
1110 22.3
1156
1201
1219 90.7
1262 174.1
1288 327.3
1317 40.3
4.7
6
4.4
6.3
6.5
0.1
0.6
0.7
0.7
2.9
1.8
1.7
5.8
0.9
1.9
0.5
3.3
2.8
20.3
1.1
9.4
0.8
25.2
1.6
3
0.5
2
489 m
569 m
694 s
2.8
636 s
671 51.1
691
697
43
0.3
0.9
8.6
782 10.8
786 114.8
847 44.5
867 29.8
881
900
913
922
964
815 w 806 s
2.4
15
2.3
0.3
4.3
ω NH2 + γ C-H
γ O-H
1.4
25
β (C-H + O-H)
ω CH-CH
877 m
904 w
53
973 155.2
1025 354
1028 33.8
ω NH2 + γ C-H
t CH-CH
ρ CH-CH +β O-H
t CH-CH
ρ CH-CH
t CH-CH (Maleate)
ρ NH2 + γ O-H
β C-H
ρ NH2 +β C-H
ρ NH2+β C-H
δ CH-CH
β C-H (Maleate)
ν C-N +β C-H +ω NH2
ν C-O+β (C-H+ O-H) (Maleate)
β ( C-H + O-H) (Maleate)
β C-H
ρ NH2 + β C-H
β ( C-H + O-H) (Maleate)
β ( C-H + O-H)
β ( C-H + O-H) +ρ NH2
β (O-H + C-H) (Maleate)
β C-H
1065
999 w 1013 w 1089
6.6
0.3
2.6
0.2
2.1
3.1
16.2
31.6
5
1101 31.5
1089 w 1094 m 1139
0.1
4.1
6.4
1160
1191
1176 w 1183 w 1197 46.3
1213 w 1208 w 1289
1316
7.8
6.6
1
7.6
3
5.5
12.6
33.3
16.7
21.8
1
0.5
18.7
0.9
0.5
9.3
1
1242 w 1326 85.8
1357 109.1
1395 407.1
1432 44.9
1319
1350
1416 52.1
1457 94.4
1459 111.9
1492 84.8
1525 149.3
5.5
1.8
1394 w 1394 s 1449
0.1
1511 12.8
1558 297.3
1573 13.1
1590 147.1
1657 153.4
1460 m
1579 m
1616 m 1771
2.9
12
87.7
1628
3.3
20.2
74.2
5.7
64.3
13
C-H sextant +ρ NH2
β C-H +δ NH2
δ NH2
1785
1807
28.1 1641
23.2 1663

Structural and spectral studies on two hydrogen bonded salts
Table 6. (continued)
Experimental
Mode Number FT-IR Raman
HF/6-311++G(d,p)
B3LYP/6-311++G(d,p)
νcal
aIIR
bI Raman
νcal
aIIR
bI Raman
Assignment
60
61
62
63
64
65
66
67
68
69
70
71
72
1685 m 1850
192.6
405.4
497.3
11.7
6.3
0.1
3.9
159.1
39.1
95.2
71.1
65.3
42.5
51.8
101.9
141.7
317.1
88.8
43.4
35.4
1680
1706
1786
117.6
323.1
319.1
201.6
45
111.4
628.1
82
71.7
41.9
131.6
63.6
155.1
72.8
172.9
52
ν C=C +β O-H
ν C=O +β (C-H +O-H)
ν C=O +β (C-H +O-H)
ν N-H· · · O
1918
2003
3041 w 3043 s 3324
2971 3159.4
3082 s 3332
3343
3167
3172
3173
3190
3203
3204
3290
3499
3586
8.6
5.9
0.3
1.5
1.5
ν C-H
νas CH-CH
3362
3362
3363
3725 1297.9
3748
3831
3872
νas CH-CH (Maleate)
νs CH-CH (Maleate)
νs C-H
ν C-H
ν O-H..O
νs NH2
νas NH2
1
0.4
0.2
358.9
30.4
582.4
810.6
30.5
26.8
w – weak; vs – very strong; m – medium;
ν, Stretching; νs, sym.stretching; νas, asym.stretching; β – in-plane bending; γ – out-of- plane bending; ω – Wagging; t –
twisting;
δ− Scissoring; ρ – Rocking
at 3078 cm−1 in Raman spectrum is assigned to the N– as a very strong band at 3324 and 2907 cm−1 in HF and
H stretching. This stretching vibration was calculated B3LYP level. In 4CAM, the same stretching vibration is
Figure 4. Comparative representation of FT-IR spectra of Figure 5. Comparative representation of FT-Raman spec-
4CAM and 4CAN.
tra of 4CAM and 4CAN.

R Anitha et al
observed as a weak band at 3041 cm−1 in IR spectrum
and strong band at 3043 cm−1 in Raman spectrum. This
corresponding vibration is calculated at 3324 and 2971
cm−1 in HF and B3LYP methods, respectively. These
calculated stretching vibrations are occurred as a very
strong peak in theoretical methods due to the N–H· · · O
hydrogen bond. But, in the case of experimental IR
spectra, the N–H· · · O hydrogen bond trace is observed
as a very broad spectrum around 3200–2200 cm−1 for
both the molecules. The occurrences of sharp and broad
stretching vibrations are not observed in the experimen-
tal results. In 4CAM, the strong peak observed at 3082
cm−1 in Raman spectrum is assigned to the C–H stret-
ching vibration; this stretching vibration is computed
at 3332 cm−1 in the HF level and at 3167 cm−1 in the
B3LYP level.
The substituted benzene ring has six C–C stretch-
ing vibrations. These vibrations mainly involve ‘quad-
rant stretching’ of the phenyl C–C bonds. But there is a
small interaction with C–H in-plane bending vibration.
peaks of the spectrum in the theoretical and experi- There are two quadrant-stretching components in sub-
mental methods are due to the fact that the theoret- stituted benzenes are expected to appear in the regions
ical calculations are carried out in isolated gas state 1620–1585 and 1590–1565 cm−1, respectively. The
whereas the crystalline state is dominated with the 1600 cm−1 doublet region is not frequency sensitive to
excessive hydrogen bonding interactions. As the hydro- the nature of substitution at ortho, meta and para posi-
gen atom is placed between two heavier electronegative tions. For these types of substitutions, the quadrant
atoms (donor and acceptor) the resulted vibration is ob- stretching vibrations are infrared inactive because all
served at a very strong intensity peak for the N–H· · · O atoms from the ring are moving to the opposite direc-
band in theoretical computation. It confirms the reloca- tions. The C–C stretching vibrations mixed with NH2
tion of proton from the acids to amino group.
deformation modes and observed in the modes of 41
and 42 for 4CAN molecule and 57 for 4CAM molecule.
In 4CAM, the weak band at 1242 cm−1 in Raman spec-
trum and IR inactive is assigned to the C−N stretch-
ing mode. These modes are mixed with C–H in-plane
deformation modes. The same vibration is computed
at 1326 and 1262 cm−1 in the HF and B3LYP meth-
ods respectively. It is matched well with the 4CAN
molecule.
In general, free O–H stretching vibrations appeared
around 3600 cm−1.22 In the present case, the intense
band is computed at 3748 cm−1 and 3290 cm−1 in the
HF and DFT/B3LYP methods respectively (table 5).
These results show that the DFT/B3LYP method gets
downshifted to the literature value which is due to
the strong O–H· · · O intramolecular interaction in the
maleate anion. It confirms the migration of the proton
transfer between the two O atoms of the anion. The O–
H...O stretching vibration in the intramolecular hydro-
gen bonds is crucial in the crystal packing. The medium
The scissoring vibration of the amine group appears
at 1630–1610 cm−1. As one of the H atoms of the amine
group is strongly involved in the N–H· · · O intermole-
cular interactions between ion pairs, the NH+3 group
vibration is reduced to –NH2molecular vibration. In mo-
lecule 4CAN, NH2of NH+3 scissoring vibration was cal-
culated at 1661 and 1641 cm−1 in DFT/B3LYP method.
The vibration coincides with 4CAM at 1663 cm−1. In
both the molecules, the HF level is more deviated from
the DFT level. The vibrational mode mentioned above
is not observed in the experimental spectra. In 4CAN,
the medium band at 1601 cm−1 in Raman spectrum
is assigned to the NH2 rocking vibration and the corres-
ponding vibration is predicted at 1772 and 1628 cm−1
in HF and B3LYP method, respectively. In lower wave-
number regions, most of the bands are observed in the
scissoring, rocking and wagging vibrations. It is in good
agreement with the experimental results.
3.3b C–H, C–C and C–N vibrations: The aromatic band observed at 1685 cm−1 in Raman spectrum is
C–H stretching vibrations are observed in the region assigned to O–H in-plane bending vibration. The theo-
3100–3000 cm−1.21 This is the characteristic region for retically computed frequency for O–H in-plane bending
identification of the C–H stretching vibrations. Also, vibration by DFT/B3LYP method matches well with the
the bands are not affected appreciably by the nature of experimental results. The medium band at 1460 cm−1
the substituents in this region. The asymmetric C–H in IR spectrum is assigned to the O–H in-plane bending
stretching is calculated at 3595 and 3204 cm−1 in HF vibration mixed with C–H in-plane bending vibration.
and DFT/B3LYP methods, respectively for 4CAN. In The O–H outof-plane bending vibration is observed at
4CAM, the same vibration is observed at 3725 and 3204 the weak band at 1089 cm−1 in the IR spectrum and
cm−1 in HF and DFT/B3LYP methods respectively. the medium band at 1094 cm−1 in the Raman spectrum;
The C–H symmetric stretching vibration is predicted at this vibration is mixed with NH2 rocking vibration. In
3364 cm−1 in HF level and 3203 cm−1 in DFT/B3LYP lower wavenumber regions most of the bands assigned
method for 4CAN and 3363 cm−1 in HF and 3203 cm−1 to the O–H in-plane bending vibrations are mixed with
in DFT/B3LYP methods for 4CAM. These asymmetric the C–H in-plane bending vibration.

Structural and spectral studies on two hydrogen bonded salts
3.3c O–H Vibrations of Maleate Anion:
where qi is the donor orbital occupancy, εi and εj
are the orbital energies and F(i, j) is an off- diagonal
NBO Fock matrix element. NBO analysis provides an
efficient method for studying intra- and intermolecular
bonding and interaction among bonds and also provides
a convenient basis for investigating charge transfer or
conjugative interaction in molecular systems. Some
electron donor orbital, acceptor orbital and the interact-
ing stabilization energy resulting from the second-order
micro-disturbance theory are reported.28,29
3.3d Vibrations of the Nitrate Anion: In the free state
the nitrate group has D3h symmetry and its normal
modes of vibrations are Aꢁ1 (ν1), A2ꢁꢁ(ν2) and Eꢁ(ν3 and
ν4). In which, ν1 is Raman active, ν2is IR active, ν3
and ν4are both IR and Raman active. In the free state
of the NO−3 anion, the symmetric stretching vibration
occurs at 1049 cm−1 the symmetric bending vibration
appears at 530 cm−1 and ν3 and ν4 (asymmetric stretch-
ing and bending modes) occur at 1355 and 690 cm−1,
respectively.23–25
NBO analyses of both the molecules have been car-
ried out by the B3LYP/HF method with 6-311++
G(d,p) basis set, in order to explain the intramolecu-
lar, re-hybridization and delocalization of electron den-
sity within the molecules. In the NBO analysis of the
hydrogen bonded systems, the most significant element
is the charge transfer between the lone pairs of the pro-
ton acceptor and the anti-bonds of the proton donor. In
case of the present molecular structures, intramolecu-
lar charge transfer leads to C–C bond in the aromatic
ring. As a result of this intramolecular charge transfer,
the aromatic ring of the molecules is stabilized, which
is due to the N–H· · · O intermolecular hydrogen bonds.
The interaction between bonding and anti-bonding of
C–C bond has large stabilization energy due to the hy-
perconjugative interactions of π (C–C) → π* (C–C).
The intermolecular hyperconjugative interactions π
(C1–C2) → π* (C3–C4) lead to stabilization of 39.47
kJ/mol in HF and 19.45 kJ/mol in DFT method for
4CAN and 38.46 kJ/mol in HF and 18.52 kJ/mol in DFT
method for 4CAM. This interaction enhances further
conjugation with anti-bonding orbital of π*(C5–C6),
which leads to strong delocalization energies for both
the molecules. The same kind of interaction was identi-
fied as π (C3–C4), which is presented in tables TS1 and
TS2 (supplementary material) for both the molecules.
The lone pair chlorine atom in both molecules has the
highest occupancies and the stabilization energies also
increase. But in the present case, the chlorine atom
does not participate in any intermolecular hydrogen
bonds because the chlorine atom has an electroposi-
tive charge. Much of the delocalization takes place in
the aromatic ring, which is due to the proton transfer
between the cation and anion. This larger delocalization
of the 4-chloroaninline cation stimulates the bioactivity
nature.
The medium band at 1752 cm−1 in IR spectrum is
assigned to the N=O stretching vibration. The same
vibration is computed at 1928 and 1449 cm−1 in HF and
1734 and 1328 cm−1 in DFT/B3LYP methods respec-
tively (table 6). The weak band at 1042 cm−1 in IR spec-
trum and medium band at 1047 cm−1 in Raman spec-
trum is assigned to the N–O in-plane bending vibra-
tion. The corresponding vibration is computed at 1155
and 1032 cm−1 in the HF and DFT/B3LYP methods
respectively. The out-of-plane deformation is observed
at 859 (HF) and 787 cm−1 (DFT/B3LYP) method.
These deformation modes are mixed with the NH2 wag-
ging vibration. These results show migration of pro-
ton transfer between the cation and nitrate group of the
anion. This out-of-plane deformation is not present in
the experimental results.
3.4 NBO Analysis
In the NBO analysis, the electronic structures are
explained in terms of a set of occupied Lewis and a set
of non-Lewis localized orbitals.26 Also, delocalization
effects can be identified by the presence of off-diagonal
elements of the Fock matrix in the NBO basis. The
NBO method gives information about interactions in
both filled and virtual orbital spaces that could enhance
the analysis of intra- and intermolecular interactions.
The delocalization effects of the present molecules
can be identified by the diagonal elements of the Fock
matrix in the NBO basis. The strengths of these delocal-
ization interactions, E(2), are estimated by second-order
perturbation theory. The second-order Fock matrix was
calculated to evaluate the donor–acceptor interac-
tions.27 These interactions result in loss of occupancy
from the localized NBO of the idealized Lewis structure
into an empty non-Lewis orbital. For each donor (i) and
acceptor (j), the stabilization energy E(2) associated
with the delocalization i → j is estimated as
3.5 HOMO and LUMO analysis
In general, the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) are called as frontier molecular orbitals. These
orbitals give information about the chemical hardness,
F(i, j)2
E(2) = qi
(1)
(εj − εi)

R Anitha et al
electro-negativity and chemical potential of the is an important feature for stability of the structures.
molecule.30 The HOMO and LUMO energies of the The calculated energy values of LUMO and HOMO
molecules 4CAM and 4CAN were calculated by are 0.027 a.u. and -0.328 a.u. for 4CAM at the HF
the B3LYP/HF methods with basis set of 6-311++ level and -0.109 a.u. and -0.250 a.u. by the DFT
G(d,p) as listed in table TS3 (supplementary material). method, -0.028 a.u. and - 0.330 a.u. at the HF level,
The corresponding energy level diagrams are shown -0.079 a.u. and -0.250 a.u. in DFT level for 4CAN. The
in figures 6 and 7, respectively. From these figures, frontier molecular orbital energy gap value of 4CAM
it is observed that the energy levels 320 and 429 molecule is 0.355 a.u. at the HF and 0.141 a.u. at the
in 4CAM and 4CAN are in the energy ranges of DFT level and the same energy gap of 4CAN is 0.358
-104.843 a.u. to 219.354 a.u. and -101.563 a.u. to a.u./ 0.171 a.u. in the HF/B3LYP method. The results
215.760 a.u. in HF and DFT method, respectively, for show that the molecules are having a small frontier
both molecules. The energy difference between HOMO energy gap. This small energy gap is associated with
and LUMO analyses is called as energy gap, which a high chemical reactivity, because a huge amount of
Figure 6. Energy level diagram of molecular orbits of 4CAM by (a) HF and; (b) B3LYP levels.
Figure 7. Energy level diagram of molecular orbits of 4CAN by (a) HF and; (b) B3LYP levels.

Structural and spectral studies on two hydrogen bonded salts
4. Parker L L, Houk A R and Jensen J H 2006 J. Am. Chem.
charge transfer occurs within the molecule. The narrow
energy gap between HOMO and LUMO supports the
possible bioactivity of the molecule.
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4. Conclusions
The experimental and theoretical investigations reveal
the proton transfer between the 4-chloroanilinium
cation and maleate and nitrate anions. This proton
transfer provides the support for the molecular assem-
blies dominated by the strong inter and intramolecu-
lar N–H· · · O and O–H· · · O hydrogen bonds. The opti-
mized molecular structure, vibrational frequencies and
NBO analysis of 4-chloroanilinium salts were found
out by quantum chemical calculations at the HF and
DFT/B3LYP methods invoking 6-311++G(d,p) basis
sets. The shifting of vibrational bands due to the inter-
molecular hydrogen bonds was analyzed with experi-
mental and theoretically computed vibrational spectra.
Natural bond orbital analysis indicates the strong inter-
molecular interactions and in an agreement with the
experimental intermolecular hydrogen bonding results.
Natural bond orbital analysis of the molecule confirms
that the charge transfer caused by π electron cloud
movement from donor to acceptor must be responsible
for the bioactivity. The value of the energy gap between
the HOMO and LUMO gived the information about
chemical softness, chemical hardness, electronegativity
and chemical potential of the molecules.
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Supplementary Information
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Gunasekaran M 2012 Acta Cryst. E 68 o959
X-ray crystallography data of the compound, 4CAM, is
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Acknowledgement
The authors SA and SSK thank the Department of Sci-
ence and Technology, SERB for the financial support
of this work in the form of Fasttrack Research Project
scheme.
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