Spectroscopic investigations on the charge transfer and hydrogen bonding effect of the potential bioactive agent 4-methyl-benzyl ammonium chloride hemihydrate: A combined experimental and quantum chemical approach

Article abstract of DOI:10.1016/j.cplett.2020.138014

A quaternary ammonium compound, 4-methyl-benzyl ammonium chloride hemihydrate was synthesized and structural and electronic properties were analysed using vibrational spectra and density functional theory calculations. NBO analysis was performed to elucid

Full text of DOI:10.1016/j.cplett.2020.138014

Chemical Physics Letters 761 (2020) 138014
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Research paper
Spectroscopic investigations on the charge transfer and hydrogen bonding
effect of the potential bioactive agent 4-methyl-benzyl ammonium chloride
hemihydrate: A combined experimental and quantum chemical approach
a
a,
*
a
b
b
Jintu Elizabeth Varghese , Lynnette Joseph , D. Sajan , R. Aarthi , C. Ramachandra Raja
a
Centre for Advanced Functional Materials, Department of Physics, Bishop Moore College, Mavelikara, Kerala 690110, India
b
Government Arts College (Autonomous), Kumbakonam 612002, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
A quaternary ammonium compound, 4-methyl-benzyl ammonium chloride hemihydrate was synthesized and
structural and electronic properties were analysed using vibrational spectra and density functional theory cal-
culations. NBO analysis was performed to elucidate the various orbital overlaps resulting in the charge transfer
interaction causing stabilization. The nature and strength of the typical non-covalent interactions were analysed
using Hirshfeld surface analysis, Atoms in Molecules and reduced density gradient analyses. Molecular docking
analysis was carried out to investigate the anti microbial activity which is prominent in systems with increased
extent of hydrogen bonding effect.
DFT
Hirshfeld analysis
AIM analysis
Vibrational spectral analysis
Reduced density gradient analysis
1
. Introduction
Quaternary ammonium compounds (QACs) are probably the best
benzalkonium chloride, cetrimonium chloride/bromide (cetrimide),
cetylpyridinium chloride, alkyl amino alkyl glycines and di-
decyldimethylammonium chloride [9–11].
known surface-active agents with a structure adjoining organic and
inorganic chemistry and possessing unique physicochemical properties
4-methylbenzylammonium chloride hemihydrate (4MBAC), possess
large third order susceptibility values, revealing its application in NLO
devices [12,13]. It is well known that Density functional theory (DFT)
approaches, especially those using hybrid functionals, have evolved to a
powerful and very reliable tool, being routinely used for the determi-
nation of various molecular properties [14–17]. In the present investi-
gation, a first time report on structural, electronic and thermal
properties and a detailed vibrational spectral investigation of 4MBAC
has been performed using the scaled quantum mechanical (SQM) force
field technique based on density functional theory (DFT) calculations.
The intermolecular and intramolecular hydrogen bonding were inves-
tigated using NBO, Hirschfield, AIM, and reduced density gradient an-
alyses. The electronic and photophysical properties were also analysed
all of which has established the strong charge transfer interaction in the
crystal. Molecular Docking analysis was performed to investigate the
effect of the hydrogen bonding effects on the biological activity of
4MBAC.
[
1,2]. Excellent water solubility and low cytotoxicity of QACs make it
useful for a number of industrial and pharmaceutical purposes,
including cleaning and disinfecting surfaces, waste water treatment, and
antimicrobial treatment [3]. In addition, QACs are nowadays widely
used as substrates, reagents, phase-transfer catalysts, ionic liquids,
electrolytes, frameworks, surfactants, herbicides, and antimicrobials
[
1,4]. They have proven their prominent role in modified DNA studies
which reveals that the replacement of a metal ion in DNA with QAC ions,
results in formation of water soluble, hygroscopic materials, the thin
film formed from which, exhibit many interesting optical and opto-
electronic properties, including organic thin film transistor fabrication
application.[5]. They have also recently received considerable attention
owing to their potential application as micellar template in the prepa-
ration of silver nanoparticle, silver nanorods and nanowires [6] and also
as modifier to change the structure of montmorillonite through cations
exchange reaction and to enlarge the interlaminar distance [7,8]. The
main QACs that are common in use are alkyldimethyl-benzyl-
ammonium
chloride,
stearalkonium
chloride,
isothiazolium-
*
Corresponding author.
E-mail address: lynnettejohn@gmail.com (L. Joseph).
https://doi.org/10.1016/j.cplett.2020.138014
Received 4 July 2020; Received in revised form 12 September 2020; Accepted 15 September 2020
Available online 22 September 2020
0
009-2614/© 2020 Elsevier B.V. All rights reserved.

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
2
. Experimental details
basis set with (83,83,52) grid points with an average resolution of
0
.333 Bohr. The structure resulting from the plot of electron density
2
.1. Synthesis and characterisation
surface mapped with electrostatic potential surface depicts the shape,
size, charge density and the site of chemical reactivity of a molecule.
These surfaces were plotted by the computer software GaussView [26].
Electronic transitions and oscillator strengths for 4MBAC molecule were
calculated by time-dependent density functional theory (TD–DFT) using
cc-pVTZ basis set and B3LYP functionals after taking into account the
configuration interaction between singly excited states. The charge
transfer and hyperconjugative interaction in the molecule was studied
by performing Natural Bond Orbital analysis (NBO) implemented in the
Gaussian 09 program.
4
-methylbenzylammonium chloride hemihydrate was prepared by
the slow evaporation method. A solution of 4-methylbenzylamine (2
mmol, 0.242 g) was dissolved in dilute HCl (10 ml, 1 mol) and CaCl (1
2
mmol, 0.147 g) was added. The resulting clear solution was stirred for 3
h and left to stand at room temperature. Colourless single crystals of the
title compound were obtained after 15 days. The X-ray diffraction (XRD)
results reveal that 4MLBACH belongs to the monoclinic crystal system
with the space group C2/c. The observed cell parameters are a =
3
0.5325 (14) Å, b = 4.8966 (2) Å, c = 11.8973 (5) Å, β = 99◦067 and
3
volume = 1756.49 (13) A [12]. The crystallinity and cell parameters
obtained from powder XRD analysis also matched well with the single
crystal XRD values [12]
4. Result and discussion
4.1. Optimized geometry
The geometry optimization of the title compound was performed by
DFT–B3LYP levels with the cc-pVTZ basis set. The optimized structure of
the compound is shown in Fig. 1. The bond lengths, bond angles and
dihedral angles were found to be in agreement with the experimental
data [12] of the title compound (Table S1).
2
.2. Instrumentation
FT-IR spectra of the sample was recorded using the Perkin Elmer FT-
IR spectrometer using KBr pellet technique in the scan range 4000–450
ꢀ
1
ꢀ 1
cm with a resolution of 1 cm and FT-Raman spectrum was recorded
using a BRUKER RFS27 FT-Raman spectrometer. UV–Vis–NIR absorp-
tion spectral data of the sample in water solution was obtained using
JASCO V-760 Spectrophotometer in the region 187–800 nm. The pho-
toluminescence (PL) measurement of the sample was recorded using a
Horiba FLUROMAX-4 Luminescence Spectrometer with xenon lamp as
excitation source.
The calculated values obtained from the DFT method were in good
agreement with the experimental ones. The title compound consists of
2
5 atoms with a methyl benzyl part and an ammonium chloride hemi-
hydrate part. The ammonium chloride part is distorted from the plane
◦
of the benzene ring, the C11
The bond C10
C bond length due to the conjugative effect of the methyl hydrogen
atoms. Similarly conjugative effect has played their role in the elonga-
tion of the bonds C15 18 (1.478) and C10 15 (1.502). N H bond
–
C
10
–
C
15
–
N
18 dihedral angle being 66.9 .
–
C15 has found to be elongated (1.502) from the normal
C
–
3
. Computational details
–
N
–
C
–
lengths are found to be shortened due to strong electrostatic force of
The quantum chemical calculation has been performed using the
+
attraction between positively charged NH cation directly bonded to
Becke-3-Lee–Yang–Parr (B3LYP) supplemented with the standard cc-
pVTZ basis set, using the Gaussian 09 program [18–20]. The self-
consistent field (SCF) equations were iteratively solved to obtain the
optimized geometry corresponding to the minimum on the potential
energy surface. Analytic second derivatives were used to calculate the
harmonic vibrational wavenumbers in order to confirm the convergence
to minima on the potential surface and to evaluate the zero-point
vibrational energies without imposing any molecular symmetry con-
straints. In order to account for the over estimation of vibrational
wavenumbers due to incomplete implementation of electronic correla-
tion and neglect of anharmonicity effects, the force field was scaled
according to the SQM procedure [20], where in the Cartesian repre-
sentation of the force constants were transferred to a non redundant set
of local symmetry coordinates[21], chosen in accordance to the rec-
ommendations of Pulay et al. [22,23]. The scale factor calculation and
characterization of the normal modes using potential energy distribu-
tion (PED) were done with the MOLVIB program 7.0 written by Sundius
negatively charged lone pair of electrons on oxygen and chlorine; with
the N18
with the more electronegative oxygen atom. The H20
Cl25 bonds are found to be elongated, due to the formation of
23… ..Cl25 intra molecular hydrogen bonds.
–
H
21 bond length having the least value due to its association
–
Cl25 and
H
23
–
–
C
15
H
17… .Cl25 and O22
–
H
The C
8
–
C
10
–
C
11 angle at C10 substitution site is found to be slightly
◦
decreased (118.16 ) as a result of the substitution of electron donating
◦
group CH
the Chlorine atom is slightly inclined towards the C11
weak intra molecular C11
12… .Cl25 interaction.
3
. The N18
–
H
20
–
Cl25 bond angle value (177.14 ) shows that
–
H12 revealing the
–
H
4
.2. Vibrational spectral analysis
A comparison of the observed and simulated IR and FT-Raman
spectra of the title compound are presented in Fig. 2 and Fig. 3 respec-
tively. The calculated (DFT) and the observed (IR and Raman) spectra
[
24,25]. For the plots of simulated IR and Raman spectra, pure Lor-
ꢀ 1
entzian band shapes were used with a bandwidth (FWHM) of 10 cm
.
The prediction of Raman intensities was carried out by following the
procedure outlined below. The Raman activities (S ) calculated by
Gaussian 09 and adjusted during scaling procedure with MOLVIB were
converted to relative Raman intensity (I ) using the following relation
i
i
from the basic theory of Raman scattering [26,27]
4
f(
ν
0
ꢀ
ν
i
) S
i
I
i
=
[
(
) ]
(1)
ꢀ hc
ν
i
ν
i
1 ꢀ exp
kT
ꢀ
1
where
ν
0 is the exciting frequency (in cm units),
ν
i is the vibrational
th
wavenumber of the i normal mode, h, c and k are universal constants,
and f is the suitably chosen common scaling factor for all the peak in-
tensities. Iso-electronic molecular electrostatic potential surfaces
Fig. 1. Optimized molecular structure of 4MBAC at the B3LYP/cc-pVTZ level
(
MEPSs) and electron density surfaces were calculated using cc-pVTZ
of theory.
2

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
Table 1
ꢀ 1
Observed and calculated wave numbers (cm ), PED and assignment for 4MBAC
at the B3LYP/cc-pVTZ level of theory in gas phase.
Calculated
Calculated
Intensity*
Experimental
Assignment (%
PED, internal
coordinates
having
wavenumbers
wavenumber
ꢀ
1
ꢀ 1
(
cm
)
(cm
)
Unscaled
Scaled
IR
Raman
IR
Raman
contribution ≥
1
0% are shown)
3
3
3
3
862
516
395.
190
3727
3393
3276
3079
9.21
1.44
0.48
1.44
7.66
ν
OHas (60) +
(40)
NH3asip(78) +
NH3 (21)
OH
(55) +
OHas(41)
ν
OH
s
5.86
ν
ν
S
100.00
62.21
3367 s
ν
s
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
ν
NH3
S
(43) +
NH3asop(37) +
NH3asip(12)
CH (99)
3173
3171
3157
3152.
3062
3060
3046
3042
3007
2996
2.30
1.89
0.75
1.58
0.46
2.17
3.35
4.31
3.35
3.35
1.44
2.39
3055 s
CH
S
CH
S
CH
S
(99)
(99)
(98)
Fig. 2. (a) Simulated IR spectra by B3LYP/cc-pVTZ level of theory and (b)
Experimental FT-IR spectra of 4MBAC in the range 4000–450 cmꢀ 1
3116
CH2as(99)
.
3
105
CH3asop (86) +
CH3asip (12)
CH3asip (84) +
CH3asop (13)
3
076
2968
1.91
2.87
2972 s
2921 s
3071
3028
2302
2963
2922
2221
1.38
3.33
0.18
6.22
5.26
1.44
CH2ST1 (99)
2919w
1630 s
ν
ν
ν
ν
CH3
NH3asop (46) +
NH3
(28) +
NH3asip(15)
S
(95)
s
1
668
659
1609
1601
9.61
1.32
5.74
7.66
1610 s
δ HOH (45) +
δNH3as (34)
1
ν
CC(64) + δ
CCH(20) +
ASYD1 (10)
δNH3as(37) + δ
HOH (34)
1645
1615
1595
1587
1559
1539
17.28
0.18
2.87
1.44
1.44
ν
CC(69) + δ
CCH(11)
38.80
AS1D2N (25) +
τ
H21(19) +
δNH3(14) +
O22(14)
δ CCH(43) +
CC (33) + δNH3
12)
δNH3
τ
1
555
549
1500
1495
1.65
2.30
0.96
1.91
1515
m
ν
S
(
1
S
(33) +
Fig. 3. (a) Simulated Raman spectra of 4MBAC by B3LYP/cc-pVTZ level of
theory (b) Experimental FT-Raman spectra in the range 4000–450 cmꢀ 1
AS1D2N (19) +
.
τ
ν
H20(14)
1
502
498
1449
1445
1435
1401
1.18
2.00
0.86
0.72
0.48
2.39
3.35
3.35
CHSC (90)
1
δCH3as(74)
δCH3as (91)
δ CCH (38) +
are found to be in good agreement with each other except for the
ꢀ 1
1488
broadening of the IR peaks in the region 2900–3700 cm , which hap-
pens due to the hydrogen bonding in the crystal which was neglected in
the gas phase theoretical calculation. Vibrational assignments with
normal co-ordinate analysis of 4MBAC were performed for the wave
number calculated by B3LYP method with cc-pVTZ basis set and the
observed and calculated wave numbers and assignments are listed in
Table 1.
1
452
ν
CC (37) + δCH3as
(11)
1
418
1368
0.15
0.48
1377w
1378
m
S
δ CH3 (89)
1
400
363
1351
1316
14.52
0.29
4.78
6.70
δ CHroc (66)
1
δ CCH(60) + δ
twi
CH (18) +
ν
CC
(
14)
1
342
309
1295
0.70
0.48
ν CC(60) + δ
4
.2.1. Vibrations of CH
Three fundamental
two asymmetric vibrations are usually observed in the region 2850
3
and CH groups
CCH (16) + δ
ν
CH bands corresponding to one symmetric and
3
CHtwi (11)
1
1263
1199
0.94
0.26
1.44
0.48
ν
CC(45) + δ
CHtwi(14)
CC(35) + TRD
29), δ CCH(19)
CC(82)
ꢀ
1
ꢀ 1
ꢀ 1
cm –3000 cm The weak band 2919 cm in IR spectrum and strong
band 2921 cm in Raman spectrum are assigned to the symmetric
stretching vibration [27]. The corresponding theoretical value predicted
ꢀ 1
1242
1210w
1203 s
ν
(
1
1
228
217
1185
1174
0.79
1.16
2.39
6.70
ν
ꢀ 1
ꢀ 1
is 2922 cm with 95% PED value. The strong band at 2972 cm in
Raman spectrum is attributed to asymmetric stretching CH vibration.
δ CCH(67) +
CC(28)
ν
3
This mode is not present in IR spectrum. The asymmetric and symmetric
bending mode of methyl group vibrations usually appears in the region
1154
1114
1102
0.70
0.33
5.74
0.48
δ CCH(40) +
CC(25) +
ν
ꢀ
1
ꢀ 1
δNH3roc (10)
1
410–1550 cm
and 1340–1400 cm
respectively. In the present
1
142
ꢀ 1
study the symmetric methyl bending mode is located at 1377 cm as a
(
continued on next page)
ꢀ 1
weak band in the IR spectrum and at 1378 cm as a medium band in
3

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
Table 1 (continued)
Table 1 (continued)
Calculated
Calculated
Intensity*
Experimental
Assignment (%
PED, internal
coordinates
having
Calculated
Calculated
Intensity*
Experimental
Assignment (%
PED, internal
coordinates
wavenumbers
wavenumber
wavenumbers
wavenumber
ꢀ
1
ꢀ 1
ꢀ 1
ꢀ 1
(
cm
)
(cm
)
(cm
)
(cm
)
having
Unscaled
Scaled
IR
Raman
IR
Raman
Unscaled
Scaled
IR
Raman
IR
Raman
contribution ≥
contribution ≥
10% are shown)
1
0% are shown)
τ
H21(19) +
ν HCl (53) + ν
δNH3roc(16) +
HO (13), NHCB
(11)
τ
O22(14) +
δNH3roc (10)
δNH3roc (20) + δ
CHtwi (16)
172
166
3.27
6.70
HClST1 (20) +
HO (19), ASYT1
(11), HOHWAG
(10)
ν
1
096
067
1057
1030
0.09
0.70
1.44
0.00
1072w
1
ROCK12 (51),
GHCC1 (12),
GCCC1 (12)
103
83
100
80
0.13
0.86
2.39
2.39
τ
τ
τ
O22 (41) +
H21 (18)
1
1
1
9
9
9
8
8
8
044
013
009
76
1007
978
973
942
932
869
841
815
800
0.33
0.09
0.15
1.56
1.80
2.04
0.70
1.93
2.22
0.48
2.39
0.00
0.00
0.48
0.48
3.35
2.87
0.96
TRD (48), ASYD2
H21 (48),
(
26) +
GHCC1 (82) +
C5 (10)
ν
CC (18)
TOH20 (20),
τ
τ
τ
τ
τ
τ
O22 (10)
τ
47
40
37
33
45
39
36
32
0.24
0.55
0.07
0.42
1.44
3.35
16.27
5.26
H21 (51) +
O22 (38)
973 w
δO22roc (59) + ν
CC (19)
O22 (39) +
H21 (37)
GHCC1 (44),
CN (32)
ν
ν
C1 (57) +
66
GHCC1 (47),
CN (36)
GCCC1 (14)
τ
C10 (23) +
(19) + C15 (15)
O22 (14)
τ
C1
01
δ CH wag (53) + δ
Nh3roc (12),
τ
+
τ
71
GHCC1 (62) +
ν
:stretching; γ: scissoring;
bending;
ω
: wagging;
τ: torsion; δ: deformation; β:in plane
τ
C5 (34)
ρ: rocking; Γ: twisting; TR:ring trigonal deformations:symmetric, as:
44
827 m
GHCC1 (42) +
CC (24)
ν
ν
asymmetric; s: strong; m: medium strong; w: weak.
*
29
806 m
GHCC1 (42) +
Normalised to 100.
CC(30) + δCH3as
(
12)
ꢀ
1
Raman spectrum. Also, the strong band at 596 cm in the IR spectrum
7
7
55
43
728
717
0.99
1.44
8.61
PUCK (37),
ꢀ
1
GCCC1 (17)
and a weak band at 638 cm in the Raman spectrum is assigned to the
ꢀ
1
22.48
HOHROC (52) +
asymmetric methyl bending mode [28]. The strong band at 3055 cm
δ HOH(14),
–
of Raman spectrum is assigned to C
H stretching, the DFT predicted
HOHWAG (13)
ꢀ 1
value of which is 3060 cm
.
7
22
696
0.42
7.18
668 s
PUCK (31) +
CC(23) + TRD
13), GCCC1 (10)
ν
(
4.2.2. Vibrations of C
–
C groups
ꢀ
1
6
57
70
634
550
0.11
3.33
0.96
1.91
596 s
638 w
δCH3as(83)
GCCC1 (25),
CXYSC1 (14),
ASYT1 (10)
The medium IR band observed at 1515 cm is assigned to C
–
C
5
549 m
+
stretching mode along with symmetric bending mode of NH
3
group. A
ꢀ
ꢀ
1
weak intensity band at 1210 cm observed in the IR spectrum and a
1
strong intensity band at 1203 cm in Raman spectrum is also assigned
5
4
11
75
493
458
1.54
2.74
0.48
6.70
NHCB (23),
HOHROC (21),
HOHWAG (16)
GCCC1 (20) +
δCH3as (16) +
to C
–
C stretching vibrations which are in good agreement with the
ꢀ 1
calculated value of 1199 cm . The C
–
C stretching vibration is also
457 w
ꢀ 1
ꢀ 1
observed as medium band at 827 cm and 806 cm in IR spectrum.
τO22 (12),
+
HOHWAG (11),
ASYT1 (10),
4.2.3. Vibrations of OH and NH
3
group
OH group vibrations show remarkable shift in the spectra of
hydrogen bonded species as they are sensitive to the environment. OH
HOHROC (10)
4
30
415
12.00
0.96
τO22 (24) +
ꢀ 1
stretching vibrations are usually observed in the region 3500 cm [29].
In the theoretical study, OH asymmetric stretch is predicted at 3727
HOHROC (19),
HOHWAG (13)
ꢀ
1
ꢀ 1
+
δ HOH (12)
cm and symmetric stretch at 3276 cm as mixed mode. Similarly the
NH asymmetric and symmetric stretch wavenumbers are predicted at
4
3
3
19
95
74
405
381
361
0.26
6.54
0.68
5.74
3.83
2.87
τC5 (64) +
3
GHCC1 (31)
CCCB (20) +
HO (12)
ꢀ 1 ꢀ 1
3
3
393 cm and 3079 cm as mixed modes. A strong and broad band at
ν
ꢀ
1
367 cm is observed in the IR spectrum, while the corresponding band
ꢀ 1
CCCB (46),
GCCC1 (10)
is identified theoretically at 3276 cm . The shift in wavenumber elu-
cidates the existence of hydrogen bonds involving NH
3
group and OH
3
29
85
318
275
10.59
0.72
0.96
3.35
ν
HCl (60)
CCCB (44),
12)
ꢀ 1
groups. The strong bands at 1630 cm in the IR spectrum and 1610
2
ν
HCl
ꢀ 1
+
cm in the Raman spectrum are attributed to the NH asymmetric
3
(
ꢀ 1
bending mode and OH bending which is predicted at 1609 cm with
34% and 45% contribution respectively, the red shifting further estab-
lishing the involvement of NH and OH bonds in hydrogen bonding. The
2
53
244
3.51
3.35
ν
HO (22) +
H21 (13),
τ
GCCC1 (12), +
τ
O22 (10)
ꢀ 1
+
weak band at 1072 cm is assigned to NH rocking vibration.
3
2
1
39.
76
231
170
12.15
0.97
6.22
3.35
HOHWAG (63),
HOHROC (19), δ
HOH (12)
4.3. Optical absorption and photoluminescence studies
Optical absorption spectrum of 4MBAC in water and DFT
4

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
computations at the B3LYP/cc-pVTZ level in water environment are
shown in Fig. 4(a). The corresponding tauc plot is shown in Fig. 4(b).
The entire transmission in visible and near infrared region as evident
from the absorption spectrum shows that the crystal is an appropriate
candidate for NLO application [13] as evident from the third order
susceptibility studies with a band gap energy of 5.45 eV. The experi-
mental spectrum shows a broad peak at 212 nm and a comparatively
weak absorption at 260 nm. DFT calculations show that the second
excited state originates from the HOMO (highest occupied molecular
orbital) to LUMO (lowest unoccupied molecular orbital) transition that
corresponds to the λmax absorption band (Table 2) in the UV–Vis spec-
trum corresponding to 212 nm in the experimental spectrum, which
Table 2
Calculated absorption wavelengths, energies and oscillator strengths of 4MBAC
using the TD-DFT method at B3LYP cc-pVTZ level.
Transition
Electronic
Oscillator
strength f
Wavelength
(nm)
energy (eV)
Excited State (S
→ LUMO + 1
1
) HOMO
) HOMO
5.2596
5.8012
6.1678
0.0009
0.1319
0.0015
235.73
213.72
201.02
Excited State (S
2
→
LUMO
Excited State (S
→ LUMO
5
) HOMO-
2
corresponds to n →
experiment is predicted at 235 nm by DFT with oscillator strength
.0009.
The HOMO and LUMO plot of 4MBAC with Density of states (DOS)
π
* transitions. The absorption of 260 nm in the
0
plot is shown in Fig. 5.
In HOMO, the charge density is mainly accumulated all over the
molecule excluding hydrogen atoms, however in the case of the LUMO,
majority of charge density moves to the aromatic -conjugated part. The
π
HOMO and LUMO show that a donor to acceptor charge transfer inter-
action occurs in 4MBAC towards the benzene ring. This agrees well with
experimental observations. The broadening and intensity enhancement
of the UV–Vis absorption spectra clearly shows the charge transfer
interaction.
The conjugated molecules are characterized by a small amount of the
highest occupied molecular orbital–lowest unoccupied molecular
orbital (HOMO–LUMO) separation, which is the result of a significant
degree of intra-molecular charge transfer from the end-capping electron
donor groups to the efficient electron acceptor groups through the
π
-conjugated path. The HOMO and LUMO energies are ꢀ 9.528 eV and
4.502 eV at the DFT level, the energy gap being 5.064 eV. The low
ꢀ
energy gap reflects the biological activity of the molecule as well as
explains the eventual charge transfer interactions taking place within
the molecule.
Fig. 5. Density of States spectra and frontier molecular orbitals of 4MBAC at
the B3LYP/cc-pVTZ level of theory.
The photoluminescence spectrum was recorded for the grown crystal
of 4MBAC with an excitation wavelength of 270 nm in the range
results. A fluorescence peak with comparatively low oscillator strength
3
40–600 nm. The experimental spectrum along with the calculated
(
2
0.002) is predicted at 458.72 nm in S state in calculations corre-
fluorescence emission peak based on the optimized excited state mo-
lecular structure of 4MBAC at the B3LYP/cc-pVTz level of theory is
shown in Fig. 6(a). The emission spectrum in the solid state shows a
broad peak at 443 nm (2.79 eV). The optimized excited state calcula-
tions (Table 3) in gas phase shows that the highest oscillator strength
sponding to the HOMO-1 → LUMO transition, which is not resolved in
the experimental spectrum. The colour of the phosphorescence is
calculated from the emission spectrum using the chromaticity coordi-
nate based on the CIE 1931 (Commisiion International d’ Eclairage)
system and is shown in Fig. 6(b).
3
emission peak is at 444.30 nm (2.7905 eV) in the S state corresponding
The chromaticity coordinates of 4MBAC crystals are (0.135, 0.081),
which is in the blue phospor region of the visible region of
to HOMO-2 → LUMO transition, consistent with the experimental
Fig. 4. (a) Experimental and calculated UV Spectrum of 4MBAC in water solvent (b) Tauc plot of 4MBAC.
5

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
Fig. 6. (a) Experimental Photoluminescence spectra of 4MBAC with an excitation wavelength of 270 nm and calculated fluorescence peak based on optimized
excited state molecular structure at the B3LYP/cc-pVTz level of theory (b) CIE chromaticity diagram of emission.
where the extinction coefficient,
Table 3
Calculated electronic excitation energies and oscillator strengths of 4MBAC
α
λ
using the TD-DFT method at B3LYP cc-pVTZ level.
k =
(5)
4π
Transition
Electronic
Oscillator
strength f
Wavelength
(nm)
The spectra of refractive index and extinction coefficient as a func-
energy (eV)
tion of wavelength are shown in Fig. 7(b).
Excited State (S
2
) HOMO-
2.7028
2.7905
0.0020
0.0159
458.72
444.30
From the spectra it is clear that the refractive index and the extinc-
tion coefficient are strongly dependent on the wavelength especially in
the UV region showing that some interaction takes place between pho-
tons and electrons. The fast decrease of refractive indices with increase
in wavelength and the large value of refractive index indicates that the
grown crystals absorbs in the low wavelength region. The extinction
coefficient is the fraction of electromagnetic energy lost due to scat-
tering and absorption per unit thickness in a particular medium. The
refractive index of grown 4MBAC crystal has been estimated as 1.83 for
visible region.
1
→ LUMO
Excited State (S
2 → LUMO
3
) HOMO
ꢀ
electromagnetic radiation which indicates that 4MBAC crystal can be
used to fabricate blue LED [30]. It is clear that this emission peak is not
related to a direct electronic transition between the valence and con-
duction band since its energy is lower than the band gap energy calcu-
lated for the molecule. Hence it is associated with the radiative
recombination involving trapped electrons and holes occurring between
localised levels situated in the band gap [31]. Furthermore the band gap
and the blue emission of the material indicate that the material can act
as a suitable host material for blue LED applications.
4
.4. Natural bond orbital (NBO) analysis
The NBO analysis is already proven to be an effective tool for
Analysis of the optical transmittance and reflectance spectra can be
chemical interpretation of hyperconjugative interactions and electron
density transfer from the filled lone pair electron. DFT level computation
was used to investigate the various second-order interactions between
the filled orbitals of one subsystem and vacant orbitals of another sub-
system, which is a measure of the delocalization or hyperconjugation.
For each donor (i) and acceptor (j), the hyperconjugative interaction
energy was deduced from the second-order perturbation approach,
used to determine the optical constants such as absorption coefficient
extinction coefficient k and refractive index n. The optical absorption
coefficient ( ) is calculated from the transmittance data using the
α
,
α
following relation:
1
2
.303log( )
T
α
=
(2)
t
2
2
〈
σ
εσ
|F|
σ
〉
= ꢀ n F
ij
where T is the transmittance and t is the thickness of the crystal. The
E(2) = ꢀ n
σ
σ
(6)
*
ꢀ
ε
σ
ΔE
transmittance is given by [32]:
2
ꢀ
α
t
2
2
(
1 ꢀ R) e
where 〈
orbitals,
of the donor
σ
|F|
σ
σ
〉 or Fij is the Fock matrix element between i and j NBO
are the energies of and s the population
* NBO’s and n
orbital [35–37]. NBO analysis has been performed on the
T =
(3)
2
ꢀ 2
α
t
1
ꢀ R e
ε
σ
ε
*
σ
σ
σ
i
σ
where
α
is the absorption coefficient in cm 1 and R is the reflectance.
ꢀ
molecule at the DFT/B3LYP/cc-pVTZ level in order to elucidate the intra
molecular, rehybridization and delocalization of electron density within
the molecule and is summarized in Table 4.
The reflectance spectrum of the grown crystal is shown in Fig. 7(a)
The reflectance R of the material whose refractive index (n) and
extinction coefficient (k) is given by [33,34],
In the phenyl ring, the electron density (ED) at the two conjugated
bond (~1.647e) and
which leads to a stabilization of ~ 94.98 kJ mol . The interactions
π
2
2
2
π
* bond (~0.337e) reveal the strong delocalization,
(
(
n ꢀ 1) + k
ꢀ
1
R =
(4)
σ
2
n + 1) + k
(
C
C
1
–
4
H ) →
σ*(C
5
–
C
13),
σ
(C
1
–
2
H ) →
π*(C
–
C
5 6
),
σ
(C
1
–
H
3
) →
σ*
(
5
–
C ) have interaction energies 20.84, 17.45 and 13.14 KJ/mol
6
6

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
Fig. 7. (a): Reflectance spectrum of 4MBAC (b) Variation of refractive index and extinction coefficient with wavelength (c) Variation of dielectric constant with
wavelength (d) Variation optical and electrical conductivity with wavelength Variation optical and electrical conductivity with wavelength.
respectively, which weakens the C
the high negative charge located on the C
slight positive charge (0.01167 a.u.) located on the C
1
–
C
5
bond. This is also supported by
atom (ꢀ 0.59908 a.u.) and the
atom as compared
Table 4
1
Second order perturbation theory analysis of Fock matrix in NBO basis.
5
Donor NBO (i)
ED/e
Acceptor NBO (j)
ED/e
E(2) kcal/mol
with the negative charges on all the other benzene carbon atoms.
NBO analysis can also be used to understand the hydrogen bonding
π
π
π
π
π
π
π
(C
(C
(C
(C
5
5
8
8
–
–
–
–
C
C
C
C
6
)
1.64775
1.64775
1.68734
1.68734
1.66205
1.66205
1.68734
1.96350
1.82848
1.95136
1.98296
1.97624
1.97624
1.68734
1.97796
1.97796
1.97798
1.97798
1.97784
1.97784
1.98790
1.97592
1.98018
1.95136
1.96350
π
π
π
π
π
π
π
*(C
*(C11
*(C
*(C11
8
–
C
10
13
6)
)
0.36725
0.29365
0.33040
0.29365
0.33040
0.36725
0.33040
0.03678
0.16076
0.04352
0.02452
0.02422
0.02319
0.02524
0.02452
0.02356
0.02356
0.01550
0.02319
0.02422
0.02422
0.33040
0.02356
0.01671
0.01671
94.98
76.90
73.64
82.42
92.13
86.32
1065.87
49.45
229.33
45.73
20.04
18.83
21.25
22.76
21.00
18.16
18.37
17.20
18.95
20.38
20.84
17.45
13.14
1.80
–
C
)
)
6)
interactions. The analyses in 4MBAC predicted a strong n →
teractions due to the lone pairs of the chlorine atom which stabilize the
molecule through n *(N18-H20) interaction with second-order
(Cl25) →
perturbation energy of 229.33 kJ/mol, respectively leading to an
H… .Cl hydrogen bond. From the NBO analysis, the evidence for the
formation of N
H… O hydrogen bonding can be obtained from the
interaction between oxygen lone electron pairs (n) and * (N H) an-
21) being 49.45
π* in-
–
C
–
10)
5
10
)
C
13
–
C
)
)
*(C
*(C8
*(C
–
–
–
C
)
4
π
(C11
13
5
6
(C11–C13
C10)
*(C
8
–
C
10
)
5
C
6
)
N
–
n
n
n
σ
σ
σ
2
(O22
)
σ
σ
σ
σ
σ
σ
σ
σ
σ
σ
σ
σ
σ
σ
*(N18
–
–
–
H
21
20
23
)
)
)
–
4
(Cl25
)
*(N18
*(O22
H
H
σ
–
3
(Cl25
)
(
2)
–
–
–
H16
)
)
)
*(C10
*(C
–
C11
)
tibonding orbitals, the E value of n
2
O
22
→
σ
*(N18 H
–
(C15
(C11
(C11
–
H
12
H12
10
5
–
C
13
)
)
kJ/mol. These supports the existence of strong electrostatic force of
–
C10
+
*(C8
attraction between positively charged NH cation to negatively charged
π
σ
σ
σ
σ
σ
σ
σ
σ
σ
n
n
(C
8
C
)
*(C15
–
N
18
)
)
lone pair of electrons on oxygen and chlorine, which is consistent with
the results obtained from the optimized geometry.
(C
(C
(C
(C
(C
(C
(C
(C
(C
8
8
6
6
6
6
1
1
1
–
–
–
–
–
–
–
–
–
H
H
9
)
*(C10
–
C
11
)
*(C
*(C5
5
–
C
6
)
9
–
–
–
–
–
C )
A weak C
lished from the interaction between n
2) value of 1.80 kJ/mol and that between n
an E(2) value of 1.17 kJ/mol. This is consistent with inclination of the
chlorine atom towards the C11 12 bond as revealed from the opti-
–
H… Cl intramolecular hydrogen bonding can be estab-
(Cl25) to *(C11 12) with an E
(Cl25) to *(C11 12) with
8
C )
6
C
8
)
*(C
*(C
*(C
*(C
1
8
5
5
C
C
C
C
5
)
σ
–
H
3
H
H
H
H
H
7
)
)
)
)
)
10
13
13
)
)
)
–
H
(
2
σ
7
4
2
3
–
H
π*(C
5 6
–
C )
σ
σ
σ
*(C
5
–
C
6
)
mized geometry analysis. The push–pull effect of electron withdrawing
from the chlorine cation and water molecule to the methyl benzyl
ammonium molecule leads to the asymmetry in polarizability which
(Cl25
(Cl25
)
)
*(C11
–
C
C
–
12
12
)
)
3
2
*(C11
1.17
facilities the enhanced activity in 4MBAC. The electron donation from
π
*
(
C
8
–
C
10) to the antibonding acceptor *(C ) with stabilization
π
5
–
C
6
7

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
energy 1065.87 kJ/mol is found to be the most important highest energy
interaction. The large value of the interaction energies (49.45 kJ/mol)
1
π
|∇
ρ
ρ
(r)
(r) |
RDG(r) =
(7)
4/3
2
(3
2)1/3
corresponding to n3 (Cl25) →
interactions within 4MBAC. In this system stabilization occurs by
intermolecular interactions of orbital overlap among and * orbitals,
13), (C 10) and
13) and *(C
10) which results in intermolecular
σ
* (O22 H23) is due to the charge transfer
–
where
point r, can be used to examine weak interactions in real space based on
the electron density. RDG and sign (λ are two important functions
which are together employed in non covalent interactions method (NCI)
38]. Sign (λ is the sign of the second largest eigen value of electron
density Hessian matrix at position r.
gives information about the nature and
ρ
(r) is the electron density and ∇
ρ
(r) is the gradient of
ρ
(r) at the
π
π
8
π
(C
*(C
5
–
5
C
6
) and
π
*(C
(C
(C11
8
–
C
10),
10) and
13) and
π
(C
*(C11
*(C
5
–
C
–
C
6
) and
13),
π
*(C11
–
C
π
–
C
2
)
ρ
π
–
C
6
),
π
8
–
–
C
π
C
π
(C11
–
C
π
5
–
C )
6
as well as
π
C
π
–
8
[
2
)
ρ
charge transfer (ICT). Since ICT makes molecule more polarized, it adds
to the enhanced NLO properties of molecule [36].
The RDG(r) versus sign (λ ρ
2
)
strength of the interactions analysed using RDG surfaces. Strong
4
.5. Hirshfeld surface analysis
attractive interactions such as dipole–dipole or H-bonding interactions
are characterised by large negative values of sign (λ
2
) , while strong
ρ
Hirshfeld surface analysis can be used to quantitatively depict the
repulsion interactions like steric interactions are characterised by large
positive values of sign (λ . Vander Waals interactions are identified
with values close to zero. The plot for 4MBAC as shown in Fig. 10(a)
shows two peaks with sign (λ close to zero, which is due to the Vander
Waal’s attraction produced by the Cl25… .… H12 interaction, as evident
from the Hirshfeld finger print plots. Two sharp spikes in the sign (λ
positive region corresponds to the strong hydrogen/halogen bond
attraction involving O22 21 and Cl25 23 interactions. The major
contribution of the positive sign (λ values come from the benzene
C, H H and C H interactions. Visualisation of the RDG isosurface
intermolecular interactions in 4MBAC [37]. Hirshfeld surface has been
mapped from dnorm, d , d , surface index and curvedness. The transparent
2
)
ρ
i
e
surface as shown in Fig. 8 allows determining the orientation and in-
teractions of the functional group inside the surface. The circular deep
2
)
ρ
red colour indicates the hydrogen bond contacts due to N
H… Cl, O H… Cl and N H… Cl inter molecular interactions and
other spots are due to H H contacts [39].
–
H … O,
2
)
ρ
C
–
–
–
–
–
H
–
H
The 2D finger print plots measure inter and intra molecular contacts
experienced by the molecules in the crystal. The total Hirshfeld surface
area of the molecule is shown in Fig. 9(a).
2
)
ρ
C
–
–
–
for 4MBAC was Multiwfn 3.6 package and was plotted by the VMD
program [41,42]. Color indices from blue to red for stronger attractive
interactions to stronger repulsion interactions, is employed in the visu-
alization of the gradient isosurface. The 3D RDG isosurfaces of 4MBAC at
the B3LYP/cc-pVTZ) level is showed in Fig. 10(b), which clearly depicts
the nature and strength of the interactions present.
The H-H interactions in Fig. 9(b) covered most area in the 2D
fingerprint plots which covers 52.2% of the total Hirshfeld surface while
the C-H interaction as shown in Fig. 9(c) contributes to 16.3% to the
total Hirshfeld surfaces. The sharpest spike in the finger print plots is due
to O… .… H/H...O contacts as seen in Fig. 9(d) with a significant
contribution of 7.3%. This mapping dnorm indicates the O...H contacts of
the water molecule with two ammonium anions and two Chlorine cation
with ionic distances 1.894 Å, 1.820 Å, 1.836 Å and 2.057 Å respectively,
that are within the Van der Waals radii, which shows that the O...H
interactions are prominent in 4MBAC. Two other sharp peaks are
visualised in the 2D finger plots due to the H...Cl/Cl...H contacts with
contribution of 24.0%, which indicates the interaction of the chlorine
cation with the water molecule with ionic distance 2.141 Å, with two
ammonium cations with ionic distances 2.145 Å and with H12 atom with
4
.7. AIM analysis
The electron densities at the bond critical points (BCP) were
analyzed using the atoms in molecules (AIM) approach using the
AIMALL package [41] and the various Bond critical points and ring
critical points were identified as shown in Fig. 11(a).
The theory of atoms in molecule (AIM) measures the strength of the
intermolecular hydrogen bond and is analysed using the values of
distance 2.805 Å. Thus the C11
–
H
12....Cl25 exhibits itself as a weak van
2
electron density (
ρ
), their Laplacians (∇
ρ
(r)
) and total electronic en-
(
r)
der waals interaction [40].
ergy density (H) at the Bond Critical Points (BCPs) [43].
2
The positive values of both Laplacians, ∇
ρ
and H at their critical
(r)
4
.6. Reduced density gradient (RDG) analysis
points as shown in Table S2, for the H12
–
Cl25 interaction with H value=
2
+
0.000996 and ∇
ρ
=+0.020193, clearly implies that an electrostatic
(r)
The Reduced Density gradient given by the equation
type of interaction exists between them, which is in accordance with the
2
results of the Hirshfeld analysis [44]. The values of ∇
critical points H21 Cl25 and H20
interactions are also prominent within the structure. The relief map in
Fig. 11(b) shows the Laplacian electron density in N18 22 plane
which indicates the concentration of charge on and around the water
molecule. The O22 24 bonds show maximum electron density and
charge transfer which exceeds all other regions.
ρ
and H at the
(r)
–
O
22, H23
–
–
Cl25 shows that shared
21
H O
– –
–
H
4
.8. Molecular docking studies
Molecular docking is a very powerful tool to predict the preferred
orientation and interaction of a ligand to a macromolecule when bound
to each other to form a stable complex in three-dimensional space
[
45,46]. The docking procedure identifies the most energetically
favourable conformation of ligands in the binding pocket of a protein
and predicts the affinity between the ligand and protein.
Quaternary ammonium compounds and benzylamines being highly
important building blocks in pharmaceutical and polymer industry, the
anticancer, antibacterial, antifungal, anticoagulant docking activity was
tested for proteins following literature survey. 5 cancer targets CDK2,
CHK2,1HSG,CHK1, 1W6R, 5 antibacterial targets 3FCD, 3M8W, COX2,
Fig. 8. Hirshfeld dnorm surfaces for 4MBAC showing interaction distances.
8

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
Fig. 9. (a)-(f) Relative contributions to the percentage of Hirshfeld surface area for the various intermolecular contacts in 4MBAC. In the plots, each blue dot
represents a 0.01 Å bin of points on the Hirshfeld surface, with coordinates corresponding to the d
i
.
Fig. 10. (a) The scatter diagram of RDG(r) versus sign (λ2)
ρ
(b)coloured iso surface plot for 4MBAC at the B3LYP/cc-pVTZ level.
1
JIL, 1PVG, 3 anti fungal targets 5FX8, 5HS1, 5i87 and 3 anticoagulant
targets 2H9E, 2P3F, 4OKV were identified and the docking score with
MBAC were calculated. The results were compared with the corre-
BAC. The binding scores of 4MBAC are also found to be more negative
with the coagulant proteins 2H9E and 4OKV. The docking depiction of
4MBAC to the 5HS1 antifungal protein and 4OKV anti coagulant protein
along with the key bond distances to residues are shown in Fig. 12. It is
observed that the Cys470, Pro462, Pro379, His378, Thr318, Phe463 and
Ala476 residues in 5HS1 form important interactions with the ligand
and theTyr95, Trp103, Gly104 and Gln105 residues in 4OKV forms the
prominent interactions with the ligand. Thus molecular docking analysis
points to the existence of comparatively high antifungal and anticoag-
ulant activity of 4MBAC.
4
sponding docking scores of Benzalkonium Chloride (BAC), a quaternary
ammonium compound widely used in pharmaceutical industry owing to
its antimicrobial activity. The results and are shown in Table 5.
The docking scores indicate that the anticancer, antifungal, antico-
agulant and antibacterial activity of 4MBAC is comparable with that of
the docking scores of Benzalkonium Chloride. The docking score of
4
MBAC with 5HS1 fungal protein is found to be more negative than that
of the corresponding value of BAC, which shows the enhanced anti-
fungal activity of 4MBAC. Docking analysis further shows that 4MBAC is
more actively bound to the 3FCD and 1JIL bacterial protein sites than
9

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
Fig. 11. (a) Molecular graph of 4MBAC with green dots representing position of BCP and red dots representing position of RCP (b) Relief map of N H O molecular
– –
plane at the B3LYP/cc-pVTZ level of theory.
Table 5
Docking scores of 4MBAC and BAC.
Cancer Protein
Docking Score with
Fungal Protein
Docking Score with
Bacterial Protein
Docking Score with
Coagulant protein
Docking Score with
4
MBAC
BAC
4MBAC
BAC
4MBAC
BAC
4MBAC
BAC
CDK2
CHK2
ꢀ 5.8
ꢀ 6.0
ꢀ 5.5
ꢀ 5.6
ꢀ 6.5
ꢀ 6.3
ꢀ 6.2
ꢀ 6.5
ꢀ 6.1
ꢀ 6.9
5FX8
5HS1
5i87
ꢀ 6.4
ꢀ 6.8
ꢀ 5.0
ꢀ 7.4
ꢀ 6.6
ꢀ 5.3
3FCD
3M8W
COX2
1JIL
ꢀ 5.2
ꢀ 4.9
ꢀ 6.6
ꢀ 5.7
ꢀ 6.1
ꢀ 5.0
ꢀ 5.2
ꢀ 7.2
ꢀ 5.1
ꢀ 6.6
2H9E
2P3F
4OKV
ꢀ 4.4
ꢀ 5.2
ꢀ 4.7
ꢀ 4.2
ꢀ 5.6
ꢀ 4.2
1
HSG
CHK1
W6R
1
1PVG
and experimental techniques to elucidate the structure property rela-
tionship within the crystal. The comparison of the theoretical and
experimental IR and Raman spectra of the compound provided evidence
about the increased extent of inter and inter-molecular hydrogen bonds
in the molecule. Photoluminescence studies revealed a prominent
emission in the blue phosphor region with an excitation of 270 nm,
revealing its applicability in photonic devices. NBO analysis clearly
brought out the evidence of the various intra molecular interactions
within the molecule quantitatively. The HOMO and LUMO show that a
donor to acceptor charge transfer interaction occurs in 4MBAC towards
the benzene ring from both the methyl group and the ammonium
chloride hemihydrate part. The electron densities at the bond critical
points (BCP) were analyzed using the atoms in molecules (AIM)
approach and the nature of the interactions were identified, from which
the possibility of the enhanced biological activity of the compound was
identified. The molecular docking studies further established the
enhanced anti fungal activity of 4MBAC.
CRediT authorship contribution statement
Jintu Elizabeth Varghese: Conceptualization, Investigation, Project
administration, Writing – original draft. Lynnette Joseph: Supervision,
validation, Writing - review & editing. D. Sajan: Resources, Writing -
review & editing. R. Aarthi: Resources, Validation. C. Ramachandra
Raja: Resources, Validation.
Declaration of Competing Interest
Fig. 12. Docking depiction with key bond distance of 4MBAC to the (a) 5HS1
protein site (b) 4OKV protein site.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
5
. Conclusion
An analysis on the molecular, vibrational and electronic properties of
MBAC was performed with the aid of various quantum chemical tools
4
1
0

J.E. Varghese et al.
Chemical Physics Letters 761 (2020) 138014
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