Evans hole and non linear optical activity in Bis(melaminium) sulphate dihydrate: A vibrational spectral study

Article abstract of DOI:10.1016/j.saa.2015.06.089

Bis(melaminium) sulphate dihydrate (BMSD), an interesting melaminium derivative for nonlinear optical activity, has been subjected to vibrational spectral analysis using FT IR and FT Raman spectra. The analysis has been aided by the Potential Energy Distribution (PED) of vibrational spectral bands, derived using density functional theory (DFT) at B3LYP/6-31G(d) level. The geometry is found to correlate well with the XRD structure and the band profiles for certain vibrations in the finger print region have been theoretically explained using Evans hole. The detailed Natural Bond Orbital (NBO) analysis of the hydrogen bonding in BMSD has also been carried out to understand the correlation between the stabilization energy of hyperconjugation of the lone pair of donor with the σ? orbital of hydrogen-acceptor bond and the strength of hydrogen bond. The theoretical calculation shows that BMSD has NLO efficiency, 2.66 times that of urea. The frontier molecular orbital analysis points to a charge transfer, which contributes to NLO activity, through N-H...O intermolecular hydrogen bonding between the melaminium ring and the sulphate. The molecular electrostatic potential (MEP) mapping has also been performed for the detailed analysis of the mutual interactions between melaminium ring and sulphate ion.

Full text of DOI:10.1016/j.saa.2015.06.089

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Evans hole and non linear optical activity in Bis(melaminium) sulphate
dihydrate: A vibrational spectral study
V.R. Suresh Kumar a, J. Binoy b, S. Dawn Dharma Roy a, M.K. Marchewka c, V.S. Jayakumar a,
⇑
a Department of Physics, N M Christian College, Marthandam, Tamil Nadu, India
b Department of Physics, Government College, Nedumangad, Thiruvananthapuram, Kerala, India
c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okolna 2, 50-422 Wroclaw, Poland
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
The crystalline network and the optimized geometry of Bis(melaminium) sulphate dihydrate.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Spectral analysis of BMSD has been
performed by FT IR and FT Raman
techniques.
Molecular geometry and vibrational
spectra have been computed by
B3LYP method.
Evans hole analysis has been used to
explain the band profile of IR
spectrum.
NBO analysis has been carried out to
assess the strength of hydrogen
bonding.
The HOMO LUMO and MEP plots have
been generated to explain NLO
activity.
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(melaminium) sulphate dihydrate (BMSD), an interesting melaminium derivative for nonlinear optical
activity, has been subjected to vibrational spectral analysis using FT IR and FT Raman spectra. The anal-
ysis has been aided by the Potential Energy Distribution (PED) of vibrational spectral bands, derived using
density functional theory (DFT) at B3LYP/6-31G(d) level. The geometry is found to correlate well with the
XRD structure and the band profiles for certain vibrations in the finger print region have been theoreti-
cally explained using Evans hole. The detailed Natural Bond Orbital (NBO) analysis of the hydrogen bond-
ing in BMSD has also been carried out to understand the correlation between the stabilization energy of
hyperconjugation of the lone pair of donor with the r⁄ orbital of hydrogen-acceptor bond and the
strength of hydrogen bond. The theoretical calculation shows that BMSD has NLO efficiency, 2.66 times
that of urea. The frontier molecular orbital analysis points to a charge transfer, which contributes to
NLO activity, through N–H. . .O intermolecular hydrogen bonding between the melaminium ring and
the sulphate. The molecular electrostatic potential (MEP) mapping has also been performed for the
detailed analysis of the mutual interactions between melaminium ring and sulphate ion.
Received 9 December 2014
Received in revised form 12 June 2015
Accepted 16 June 2015
Available online 24 June 2015
Keywords:
FT-IR and FT-Raman spectra
DFT
HOMO–LUMO
NLO
Evans hole
MESP mapping
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
Melaminium derivatives are the object of extensive chemical
research for their role in living organism [1], self-assembly process
in supra molecular architectures [2] and nonlinear optics [3]. A few
⇑
Corresponding author.
E-mail address: vsjkumar@gmail.com (V.S. Jayakumar).
hydrogen bonded complexes of melaminium, possessing second
http://dx.doi.org/10.1016/j.saa.2015.06.089
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
293
harmonic generation (SHG) activity, have been reported [4]. The
recent toxicological and pharmacological investigations explore
nephrotoxicity, crystal formation in kidney and renal toxicity
induced by melamine or its derivatives [5,6]. Polyamide 6, one of
the principal thermoplastics, used in the plastic engineering and
in textile industries is prepared from Melaminium derivative [7].
The colour change induced by triple hydrogen-bonding enables
on-site and real-time detection of melamine in raw milk and in
infant formula, even at a concentration of 2.5 ppb without the
aid of any advanced instruments [8]. The diverse applications of
melamine and its derivatives offer tremendous scope for exploring
structural and bonding features of BMSD and their role in its NLO
activity, with the aid of Fourier Transform Infrared and Raman
spectroscopy.
The present study focuses on geometry optimization and vibra-
tional spectral analysis of Bis(melaminium) sulphate dehydrate
(BMSD), a compound that potentially exhibits non-linear optical
properties, using FT IR and FT Raman spectra aided by density
functional theoretical computations at B3LYP/6-31G(d) level, along
with the analysis of Evans hole and MEP mapping. NLO activity has
been theoretically investigated using the computed hyperpolariz-
ability tensor, supported by NBO and frontier molecular orbital
analysis.
2. Experimental
2.1. Preparation
The starting compound Melamine (99%, Aldrich) and sulphuric
acid (Merck) has been used for the present preparation.
Melamine was dissolved in a 20% solution of H2SO4 at molar ratio
2:1, and the resulting solution was evaporated slowly. After several
days, colourless crystals of the title salt appeared [9].
Fig. 2. (a) XRD showing formula unit with intermolecular hydrogen bonding. (b)
The molecular arrangement in the unit cell showing the hydrogen bonding
interactions.
2.2. Spectroscopic measurements
The vibrational measurements were carried out at room tem-
perature. The polycrystalline powders of BMSD were achieved by
grinding in agate mortar with pestle. Samples, as suspensions in
Nujol, were put between KBr windows. Infrared spectrum has been
recorded with a Bruker IFS-88 spectrometer in the region 4000–
80 cmꢁ1 with a resolution of 2 cmꢁ1. The Fourier Transform
Raman (FT Raman) spectrum of powder sample of BMSD was taken
with FRA-106 attachment to the Bruker IFS-88 spectrometer
equipped with Ge detector cooled to liquid nitrogen temperature.
The signal to noise ratio was established by 32 scans, weak apodi-
sation. Nd3+:YAG air-cooled diode pumped laser, 1064 nm of
power 200 mW was used as an exciting source. The scattered light
was collected at the angle of 180° in the region 3600–80 cmꢁ1, res-
olution 2 cmꢁ1, 256 scans.
3. Computational
The computed results reported in this paper are obtained with
the Gaussian ‘03 program package [9]. The geometry optimization
was carried out by employing hybrid Becke’s three-parameter Lee–
Yang–Parr correlation functional (B3LYP), at 6-31G(d) level of the-
ory, which can provide reliable results for energy data. The IR and
Raman spectra were simulated from the optimized geometry of
Fig. 1. Optimized geometry of BMSD at B3LYP 6-31 G(d) level.

294
V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
BMSD, and the computed wavenumbers are scaled using 0.9613 for
the correlation with experimental band positions [10]. The vibra-
tional Potential Energy Distribution (PED) has been calculated from
the simulated vibrational spectra using the software VEDA [11].
PED can provide the contribution of stretching, bending and defor-
mation motions of the functional groups to each vibrational mode,
in terms of (3N-6) linearly independent, internal coordinates,
which allows unambiguous vibrational assignment. The HOMO–
LUMO analysis has been carried out in BMSD to explain the charge
transfer within the molecule. In order to elucidate the intermolec-
ular hydrogen bonding, intermolecular charge transfer, rehy-
bridization and delocalization of electron density, NBO analysis
has been carried out on BMSD at B3LYP/6-31G(d) level. Also the
Molecular Electrostatic Potential surfaces (MEPs) were calculated
using the same basic set.
interconnection through SO24ꢁ and H2O molecule. The XRD data
show that the internal C–N–C angle at protonated nitrogen atom
is significantly greater than other internal C–N–C angles of the ring.
For the two melaminium rings, the C–N–C angles (Table 1) are
119.8° and 119.7° at protonating position, while the corresponding
angles at non-protonated nitrogen are 115.5° and 115.9° for the
first ring and 115.9° and 115.7° for the other ring. This ring distor-
tion arises from the steric effect of nitrogen lone pair, consistent
with VSEPR theory [12]. This is supported by DFT computation
which gives the C–N–C angles at protonating positions equal to
119.5° and 118.7° while the values are 115.5°, 115.4°, 115.7° and
115.9° at non-protonating positions.
The experimental data shows that protonating hydrogen is
almost linear with the ring, while for DFT predicted geometry a sig-
nificant deviation from planarity is observed, which is obvious
from the torsion angle of H–N–C–N, whose values are ꢁ159.8°
and 165.7°. The deviation from planarity is evidently due to the
intermolecular N–H. . .O interactions of the ring with sulphate
group. The DFT predicted hydrogen bonding parameters for proto-
4. Results and discussion
nating hydrogen are N–H = 1.06 Å, H. . .O = 1.59 Å and
H. . .O = 174.8°, while the XRD data shows that this hydrogen bond
is weakened, with N–H = 0.873 Å, H. . .O = 1.9 Å and
\N-H. . .O = 168.5°. Also in the crystalline network, one of the pro-
tonating hydrogen engage in intermolecular N–H. . .O interaction
with water molecule whose parameters are N–H = 0.89 Å,
\ N–
4.1. Geometry
Density Functional Theoretical (DFT) computation shows that
the two melaminium ions of BMSD (Fig. 1) are inclined to each
other by around 60°. The XRD data [12] reveal that the two mela-
minium groups are almost in the same plane (Fig. 2), because of the
Table 1
The geometric parameters of BMSD computed at B3LYP/6-31G(d) level.
Bond
Bond length (Å)
XRD
Bond angle
Angle (°)
Torsion angle
Angle (°)
DFT
XRD
DFT
XRD
DFT
C1–N2
N2–C3
C3–N4
N4–C5
C5–N6
C1–N7
N7–H8
N7–H9
C5–N10
1.34
1.32
1.37
1.35
1.33
1.32
0.85
0.87
1.32
0.85
0.83
0.87
1.31
0.85
0.86
1.91
1.48
1.48
1.46
1.45
1.99
0.88
0.89
1.31
1.34
1.34
1.35
1.33
1.36
0.89
1.31
0.88
0.85
1.33
0.88
0.85
1.90
0.84
1.01
2.02
0.88
0.83
1.35
1.33
1.37
1.37
1.33
1.35
1.01
1.01
1.33
1.01
1.03
1.06
1.34
1.04
1.01
3.15
1.52
1.49
1.54
1.52
1.91
1.03
1.01
1.34
1.33
1.35
1.35
1.33
1.37
1.06
1.34
1.01
1.03
1.35
1.01
1.01
1.87
0.99
0.98
1.93
0.98
0.97
C3–N2–C1
N4–C3–N2
C5–N4–C3
N6–C5–N4
N7–C1–N2
H8–N7–C1
H9–N7–C1
N10–C5–N4
H11–N10–C5
H12–N10–C5
H13–N4–C5
115.7
121.4
119.7
121.3
117.3
120.3
120.2
118.4
120.1
119.9
118.1
121.7
118.0
118.5
168.5
116.9
110.3
108.4
108.9
135.8
137.9
119.5
120.6
120.7
115.5
126.3
115.9
121.1
118.5
120.2
118.3
121.5
116.9
119.7
118.4
119.3
174.5
109.5
96.3
114.9
121.8
119.1
121.4
116.1
119.5
119.4
116.9
120.9
122.7
120.0
121.3
119.8
116.3
133.7
54.4
112.3
106.6
109.1
117.3
154.6
118.3
116.1
122.5
114.7
127.0
114.6
121.3
120.6
122.5
115.7
116.2
116.5
119.0
119.0
107.6
150.3
98.9
C1–N2–C3–N4
N2–C3–N4–C5
C3–N4–C5–N6
N7–C1–N2–C3
H8–N7–C1–N2
N2–C1–N7–H9
C3–N4–C5–N10
N4–C5–N10–H11
N4–C5–N10–H12
N2–C3–N4–H13
ꢁ1.9
ꢁ1.4
0.9
ꢁ4.7
3.6
4.6
ꢁ176.3
ꢁ177.5
ꢁ2.9
0.1
176.8
ꢁ178.9
ꢁ177.9
4.5
175.2
177.8
2.0
179.7
ꢁ115.2
101.5
164.4
81.1
ꢁ75.8
111.7
ꢁ169.5
19.4
ꢁ176.2
ꢁ175.7
178.9
8.5
N
N
10–H11
10–H12
170.0
ꢁ179.9
ꢁ18.1
ꢁ175.6
100.9
ꢁ139.5
ꢁ160.7
80.5
C1–N2–C3–N14
N4–H13
C3–N14
N14–C3–N2
N4–C3–N14–H15
N4–C3–N14–H16
O17–H13–N4–C3
S18–O17–H13–N4
O19–S18–O17–H13
O20–S18–O17–H13
O21–S18–O17–H13
H22–O19–S18–O20
N23–H22–O19–S18
H24–N23–H22–O19
C25–N23–H22–O19
H24–N23–C25–N26
N23–C25–N26–C27
C25–N26–C27–N28
N26–C27–N28–C29
C27–N28–C29–N30
N28–C29–N30–H31
C27–N28–C29–N32
N28–C29–N32–H33
N28–C29–N32–H34
C25–N26–C27–N35
N26–C27–N35–H36
N26–C27–N35–H37
H38–O17–S18–O20
O39–H38–O17–S18
H40–O39–H38–O17
H41–O39–H38–O17
O42–H41–O39–H38
H43–O42–H41–O39
H15–N14–C3
H16–N14–C3
O17–H13–N4
S18–O17–H13
O19–S18–O17
O20–S18–O17
O21–S18–O17
H22–O19–S18
N23–H22–O19
H24–N23–H22
C25–N23–H22
N26–C25–N23
C27–N26–C25
N28–C27–N26
C29–N28–C27
N30–C29–N28
H31–N30–C29
N32–C29–N28
H33–N32–C29
H34–N32–C29
N35–C27–N26
H36–N35–C27
H37–N35–C27
H38–O17–S18
O39–H38–O17
H40–O39–H38
H41–O39–H38
O42–H41–O39
H43–O42–H41
N14–H15
N14–H16
H13–O17
O17–S18
S18–O19
S18–O20
S18–O21
ꢁ35.9
14.1
79.3
118.
–25.5
6.9
H22–O19
H22–N23
N23–H24
N23–C25
ꢁ149.7
7.5
ꢁ179.5
ꢁ1.2
ꢁ175.9
C
N
C
N
C
N
C
25–N26
26–C27
27–N28
28–C29
29–N30
30–H31
29–N32
9.1
ꢁ0.9
ꢁ9.2
2.0
ꢁ4.5
ꢁ179.6
ꢁ179.1
2.3
ꢁ179.1
177.1
5.4
179.5
ꢁ144.3
ꢁ148.3
ꢁ46.6
51.4
ꢁ168.6
175.9
ꢁ6.8
ꢁ152.4
ꢁ173.5
ꢁ6.6
N
N
32–H33
32–H34
C
27–N35
ꢁ171.1
133.4
ꢁ3.6
N35–H36
N35–H37
O17–H38
H38–O39
O39–H40
O39–H41
H41–O42
O42–H43
16.1
ꢁ152.4
105.9
103.9
130.1
158.6
103.7
ꢁ60.8
169.2
108.6
ꢁ163.7

V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
295
H. . .O = 1.89 Å and \ N–H. . .O = 177°, which is absent in DFT pre-
dicted geometry of BMSD formula unit. Hence the deviation from
planarity of protonating hydrogen of melaminium ring in the DFT
predicted geometry, compared to XRD structure, is justified.
Deviation from normal tetrahedral structure can be observed
for sulphate, as predicted by both XRD and DFT, which is evident
from the unequal length of S–O bonds and unequal O–S–O angles
resulting from the intermolecular interactions with melaminium
groups. DFT provides 1.52 Å, 1.49 Å, 1.54 Å and 1.51 Å respectively
for bond distances S18–O17, S18–O19, S18–O20 and S18–O21 and the
bond angles are equal to 112.3°, 108.6°, 108.0° and 109.1° for
vibration of primary amino group attached to the non-aromatic
ring. The band position indicates the extent of aromaticity [13]
and here non-aromaticity in the ring is evident from the unequal
C–N bond lengths, in addition to the lower wave number of vibra-
tions compared to the vibrations of aromatic primary amines. The
medium width and intensity is observed for the band and the
broadness can be attributed to the mixing with O–H stretches.
The DFT computation predicts primary amino group symmetric
stretching at 3483 cmꢁ1 in both IR and Raman. The absence of sym-
metric stretching can be regarded as the mixing of possible band
within the broad profile in that region, which is the case in
Raman also and a weak shoulder is observed in broad profile in
Raman. The characteristic NH stretching frequency of the sec-
ondary amino group [14] attached to the non-aromatic ring is
around 3300 cmꢁ1 and in BMSD, this vibration produces bands at
lower frequencies. The IR wavenumbers at 3166 cmꢁ1 and
3104 cmꢁ1 are found in good agreement with the computed wave
numbers at 3167 cmꢁ1 and 3101 cmꢁ1 respectively where the for-
mer band is found to possess a medium intense Raman counterpart
at 3149 cmꢁ1. The deviation of band positions from the character-
istic wavenumbers can be attributed to the N–H. . .O hydrogen
bonding interaction, evident from DFT computed results which
correlates with the XRD data. Hence the shift of wave numbers
to the lower wave number region in both experimental and com-
putational spectrum is justified.
O
17–S18–O19, O19–S18–O20, O20–S18–O21 and O21–S18–O17 respec-
tively. The deviation can be attributed to the intermolecular hydro-
gen bonding in the network, leading to deviation from resonance
structure as well.
4.2. Vibrational analysis
The vibrational analysis (Table 2) is based on the FT IR (Fig. 3a)
and FT Raman (Fig. 4a) spectra of BMSD and the PED and eigenvec-
tor distribution of the computed vibrational spectra
(Figs. 3b and 4b) have been used to assign vibrational bands
unambiguously.
4.2.1. Vibrations of amino group
The strong broad band in IR at 3345 cmꢁ1 corresponds to the
asymmetric stretching vibration of NH2, which is the characteristic
The shift to higher frequencies for bending motions of NH2
group in melamine molecules due to intermolecular interactions
Table 2
Vibrational assignment of BMSD.
Computed spectrum
B3LYP/6-31G(d)
Experimental spectrum
IR
PED (%)
Raman
(cmꢁ1
mscaled (cmꢁ1
)
Relative Intensity
m
(cmꢁ1
)
Relative Intensity
m
)
Relative Intensity
IR
Raman
3514
3483
3167
3091
m
m
s
w
vs
vs
m
3345
vsbr
m
m
m
m
m
m
asNH2(51)msOH2(30)
sNH2(100)
NH(95)
NH(91)
NH (90)
NH (89)
3318
3149
–
–
–
w brsh
m
–
–
–
3166
3104
2845
2685
1718
1685
1670
vwsh
s br
w sh
W
w sh
vs
vs
1709
1683
1674
1635
1617
1606
1512
1504
1403
1322
1189
1174
s
m
s
w
s
m
s
m
m
vw
w
vw
w
w
w
w
w
w
w
w
1711
1680
w
w
ds HOH (77)
ds OH2 (60) ds NH2 (15)
ds NH2 (28)ds OH2 (16)
ds NH2 (61)
mNC (12)
vs
1632
w
1613
1566
1513
1481
1404
1332
1182
1169
1146
w sh
w
s
w sh
m
w sh
vwsh
vwsh
s
Ring mode 3a (21)ds NH2 (35)
NC (17)ds NH2 (59)
Ring mode 3b (64)
Ring mode 3b(43)ds NH2 (14)
d CNH (28) NC (31)
1570
1515
1490
1415
1336
1187
m
w
vwsh
vw
w
m
q NH2 (26)
w
m
vw
vw
vw
Ring mode 5(36)ds HNC (11)
Ring mode 6a (29)d HNC (28)
Ring mode 6b (31)6b HNC (26)
w
1135
W
1123
1090
1028
979
911
782
726
685
627
580
425
409
187
129
s
m
s
m
m
s
m
w
s
m
w
m
w
m
w
w
w
w
m
vw
m
vw
w
w
w
w
vw
vw
m
SO (65)
1090
1035
978
910
779
721
688
619
572
455
408
182
125
m
vwsh
vs
w sh
vs
m
w sh
s
m
w sh
s
m
Ring mode 8 (23)m SO (17)q NH2 (14)
m
m
m
SO (54)
978
775
s
sSO24ꢁ (33)
q NH2(12)
SO (11)3b (25)
vw
Ring mode 12 (60)
Ring mode 12 (66)
Ring mode 12 (85)
lib OH2 (25)c NH (28)
ds OSO (51)
688
625
573
442
379
185
120
vvs
w
s
w
m
q
OSO (60)def SO4 (14)
ds OSO (39)def SO4 (30)
m
m
w sh
w sh
O. . .H (24)
O. . .H (27)
m
x OH2 (15)
m – medium, s – strong, vs – very strong, wsh – weak shoulder, vw – very weak, vwsh – very weak shoulder, vvs – very very strong, sbr – strong branch, vsbr – very strong
branch, wbrsh – weak branch shoulder, – stretching, as – asymmetric stretching, s – symmetric stretching, – out plane bending, d – inplane bending, ds – scissoring,
rocking, – wagging, lib – libration, def – deformation.
m
m
m
c
q –
x

296
V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
through amino group has been reported in melamine derivatives
[15]. The NH2 bending motion at 1659 cmꢁ1 observed in the
Raman spectrum of melaminium is found to be shifted to
1680 cmꢁ1 in BMSD, showing a strong hydrogen bonding through
NH2. Similarly, the intense bands observed at 1683 cmꢁ1 and
1674 cmꢁ1 has been respectively shifted to 1685 cmꢁ1 and
of melamine is found to lower the frequency for ring modes [18],
compared to the corresponding value for melamine alone, owing
to the intense ionic and donor–acceptor type interactions within
the crystalline environment. This makes the triazine ring more
rigid. The ring mode at 1174 cmꢁ1 reported as the characteristic
band in IR spectrum of melaminium derivatives [19] has also been
observed in BMSD with wavenumbers 1169 cmꢁ1 and 1146 cmꢁ1
corresponding to two melaminium cations in the formula unit of
the crystal indicating that two melaminium ions are crystallo-
graphically non-equivalent. The most intense band of triazine ring
in FT Raman spectrum is observed at 688 cmꢁ1 which is the char-
acteristic band observed in all melaminium complexes, [15] origi-
nating from the symmetric vibration mode 12, (Table 3) of triazine
ring.
1670 cmꢁ1
. ,
The DFT computed frequencies at 1596 cmꢁ1
1568 cmꢁ1 and 1567 cmꢁ1 for amino group bending in melami-
nium [16] is found respectively at 1635 cmꢁ1, 1617 cmꢁ1 and
1606 cmꢁ1 in the computed spectrum of BMSD. The bands at
1635 cmꢁ1 and 1606 cmꢁ1 are found to be affected by N–H. . .O
interaction between amino group of melaminium ring and the oxy-
gen atom of sulphate. The mode at 1617 cmꢁ1 is affected by N–
H. . .O interaction involving the protonating hydrogen, in addition
to the above interaction.
The NH out of plane bending vibrations in pure melamine crys-
tals have been reported at 676 cmꢁ1 and 615 cmꢁ1 in IR and for the
latter mode, there is intense Raman band at same wavenumber
owing to NH2 bend mixed with ring breath [15]. For the present
compound, NH out of plane bending mode can be observed in IR
as strong band at 619 cmꢁ1 and a weak band at 625 cmꢁ1 in
Raman.
4.2.3. Sulphate
The bands for sulphate group can be found in two regions of
vibrational spectrum, that is 1100–900 cmꢁ1 and 600–400 cmꢁ1
,
the former arises due to symmetric/asymmetric stretching motion
of sulphate group and the latter is due to the sulphate deformation
[20]. The medium intense band at 1090 cmꢁ1 in IR is due to SO
stretching motion mixed with the melaminium ring vibration. A
highly intense band has been observed at 978 cmꢁ1 in both IR
and Raman and the vibrational PED shows that the band is due
to SO24ꢁ symmetric stretching vibration [21]. In Raman, the split-
ting of this non-degenerate mode into a doublet have been
reported [22] for sulphate derivatives and the absence of the
removal of degeneracy of the stretching mode, indicates that the
angular distortion is greater than the linear distortion [22]. On
comparison with other sulphate derivatives, the sulphate
4.2.2. Ring mode
The ring modes arise from the vibrations of central triazine ring
and the assignments are based on normal coordinate analysis and
the mode numbers suggested by Hannaet et al. [17]. In BMSD crys-
talline network, spectral changes have been noticed for vibrations
of melaminium ring, compared with the vibrational behaviour of
the melamine crystal. The analysis indicates that the complexation
Fig. 3. IR spectra of BMSD (a) experimental. (b) Simulated at B3LYP 6-31G(d) level.

V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
297
stretching frequency for the above mode is higher and can be
attributed to the intermolecular hydrogen bonding of sulphate
with neighbouring melaminium units. The deformation modes of
sulphate groups, corresponding to OSO scissoring, OSO rocking
and SO42ꢁ deformation can be observed at 580 cmꢁ1, 425 cmꢁ1
and 409 cmꢁ1 respectively and their band positions and intensities
are found to be correlating with that of computed spectrum.
window in its position due to Fermi interaction [23]. Such trans-
mission window/hole, termed Evans hole, is an effective tool in
interpreting the shape of hydroxyl stretching profile. In melami-
nium derivatives the coupling between stretching vibrations of
C–N bonds and deformation of N–H. . .N intermolecular interaction
leads to Evans hole [15]. The above interaction is found to create
Evans holes at 1613 cmꢁ1, 1481 cmꢁ1, 741 cmꢁ1 and 598 cmꢁ1 in
BMSD. The shape of above mentioned bands for both melamine
and BMSD can be well explained by theoretically analyzing Evans
holes.
Evans holes originate from the interaction between certain
vibrational energy levels appreciable in a fraction of the mole-
cules, where one of the energy levels involved is sensitive to
intermolecular interaction [26]. Such a restricted perturbation
between energy levels ꢀ10 and ꢀ02 having corresponding Eigen
functions w01 and w02 respectively is found to create mixed energy
levels given by
4.2.4. The vibrations of water molecules
The two water molecules lying in the elementary unit of BMSD
crystal has been involved as a donor in hydrogen bond with sul-
phate ion which simultaneously affects the vibrational spectral
behaviour of both sulphate group and water molecule. The stretch-
ing band of water molecule is not observed due to mixing with the
vibrations of primary amino group, where the broadness and
strength of the bands at 3345 cmꢁ1 in IR and 3318 cmꢁ1 in
Raman might be contributed by OH stretches also. The inplane
bending vibrations of water molecule can be found at 1718 cmꢁ1
,
ð
ꢀ10
þ
ꢀ02Þ þ
D
1685 cmꢁ1 and 1670 cmꢁ1 in IR with appreciable strength and
the corresponding Raman bands are dormant or weak. The band
positions and intensities are found to be correlating with that of
computed spectrum. The librations of water molecule can be found
in the expected region, as an intense band at 619 cmꢁ1 in IR and as
a weak band at 625 cmꢁ1 in Raman.
ꢀ1
ꢀ2
¼
ð1Þ
2
ð
ꢀ10
þ
ꢀ02Þ þ
D
¼
ð2Þ
ð3Þ
2
w1 ¼ xw0 þ yw0
1
2
4.3. Evans hole
w2 ¼ xw0 ꢁ yw0
ð4Þ
1
2
A sharp and weak absorption band in IR falling within the
energy range of strong and broad band may create a transmission
The perturbation energy separation
separation d as
D
is related to unperturbed
Fig. 4. Raman spectra of BMSD (a) experimental. (b) Theoretical at B3LYP 6-31G (d) level.

298
V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
Table 3
Table 3 (continued)
The computational vibrational modes of triazine at B3LYP/6-31G(d) level.
Sl. no
21
Mode no
14b
Frequency (cmꢁ1)
339
Eigen vector distribution
Sl. no
1
Mode no
1
Frequency (cmꢁ1)
3075
Eigen vector distribution
2
3
4
5
6
2a
2b
3a
3b
4a
2974
2974
1550
1550
1397
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
D
¼
d2 þ 4w212
ð5Þ
W12 is the perturbation energy. The intensity ratio is given by
I1
I2
D
D
ꢁ d
ꢁ d
¼
ð6Þ
The observed band position and the position of the hole are
found for BMSD and observed separation has been computed from
the intensity ratio (Table 4). The unperturbed separation and inter-
action energy are calculated and unperturbed m1 and m2u
þ
m2g are
deducted from Fermi interaction formula [24] and are tabulated.
It has been reported that the close agreement between unper-
turbed band position m2u
essential criteria for identifying Evans hole [25]. Such a close agree-
þ
m2g and position of the hole (
mh) is the
7
4b
5
1397
1361
1158
1158
1150
1113
997
ment can be observed for BMSD. The unperturbed position has
been computed as 1626 cmꢁ1
,
1488 cmꢁ1
,
739 cmꢁ1 and
1481 cmꢁ1
,
603 cmꢁ1 corresponding to the holes 1613 cmꢁ1
,
8
741 cmꢁ1 and 598 cmꢁ1 respectively, confirming the existence of
Evans hole. This explains the presence of shoulder appearing in
the bands having peaks 1670 cmꢁ1, 1513 cmꢁ1, 721 cmꢁ1 and
619 cmꢁ1, in BMSD.
9
6a
6b
7
4.4. Natural Bond Orbital (NBO) analysis
10
11
12
13
The NBO analysis has been carried out to investigate hydrogen
bonding and delocalization of electron density from occupied
Lewis type (donor) NBOs to properly unoccupied non Lewis type
(acceptor). NBOs can provide valuable information on charge
transfer or conjugative interactions within the molecule [26]. The
strength of interaction between electron donors and acceptors
can be quantitatively expressed using the stabilization energy
E(2) associated with i (donor) - > j (acceptor) delocalization [27].
8
9a
2
ꢁqiðFijÞ
Eð2Þ
¼
ð7Þ
Ej ꢁ Ei
14
15
16
17
18
19
20
9b
997
975
906
730
666
666
339
qi is the donor orbital occupancy, Ei, Ej are the diagonal elements,
representing the orbital energies and Fij is the off diagonal element
of NBO Fock matrix, computed using second order perturbation
theory.
10
In NBO analysis, intermolecular N–H. . .O hydrogen bonding has
been manifested as the electron transfer from the lone pair of oxy-
gen to antibonding orbital of N–H, with appreciable stabilization
energy [28]. In BMSD, the stabilization energy of electron transfer
process, Lp (O21) ? r⁄ (N4–H13) is 33.46 kcal/mole and that of elec-
tron transfer Lp (O20) ? r⁄ (N14–H13) is 14.03 kcal/mol. Their
bonding parameters are H13. . .O12 = 1.64 Å and H15. . .O20 = 1.79 Å
respectively. A direct correlation is found between the stabilization
energy of electron transfer E(2) and strength of hydrogen bond,
which is dependent on bonding parameters (Fig. 5).
Thus NBO analysis is an effective method for investigating
intermolecular hydrogen interactions in the molecule/crystalline
network. In BMSD, the intermolecular hydrogen bonding of sul-
phate anion with the melaminium rings, the stabilization energy
is found to increase with decrease of H. . .A distance. For the pro-
cess, Lp (O17) ? r⁄ (N32–H34), stabilization energy has been com-
puted as 9.95 kcal/mol, while that for Lp (O20) ? r⁄ (N30–H31) is
34.92 kcal/mol. These H. . .A distances are 1.91 Å and 1.61 Å
11
12
13a
13b
14a

V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
299
Table 4
Vibrational data on Evans hole in melamine and Bis(melaminium) sulphate dihydrate.
Details of data
Melamine
BMSD
Melamine
BMSD
Melamine
BMSD
Melamine
BMSD
mh
m1
m2u
m2u
m1
m2u
D
(cmꢁ1
)
)
1606
1588
1572
1601
1598
1595
18
15
5
0.09
1613
1670
1570
1602
1657
1626
57
31
24
0.290
1489
1448
1520
1521
1648
1635
41
23
17
0.27
1481
1513
1415
1403
1506
1488
32
19
13
0.263
742
776
766
659
767
751
34
31
7
0.048
741
721
775
777
723
739
20
10
9
0.325
598
552
558
562
562
588
46
36
14
0.12
598
619
573
580
614
603
21
16
7
0.143
(Observed)
(Observed)
(cmꢁ1
+
+
m2g
(cmꢁ1) Raman
(cmꢁ1) (DFT)
(Observed)
(Calculated)
m2g
(cmꢁ1
)
(Unperturbed)
+
m2g
(cmꢁ1
)
(Unperturbed)
(cmꢁ1
)
d (cmꢁ1
)
W12 (cmꢁ1
I1
I2
)
D
– perturbed separation, d – unperturbed separation, I1 – observed intensity of weak band, I2 – observed intensity of strong band.
W12 – perturbation energy, mu2 m2g – unperturbed band position, mh – position of the hole.
þ
melaminium ring, the stabilization energy between adjacent C–N
double and single bonds are found larger, responsible for hyper-
conjugative interaction within the ring. This has been formed to
create a resonance structure for the ring, evident from almost
equal bond lengths for all C–N bonds of the ring, apart from the
distorted ring structure in many melaminium derivatives [16].
4.5. Molecular electrostatic potential mapping
The reactive sites for the investigated molecule are predicted
with the MEP at B3LYP/6-31G(d) level. The electrophilic attack is
represented by the negative (red and yellow) region and nucle-
ophilic attack is represented by positive (blue) region [29] as
shown in (Fig. 6). The most negative V(r) value is associated with
oxygen atoms of sulphate ions. Alternatively a maximum value
of positive region of V(r) found on the amino group of melaminium,
indicates that this site is probably involved in nucleophilic process,
which explains the electrostatic interaction between sulphate and
melaminium ions responsible for the formation of crystalline net-
work. Uniform negative potential can be observed for all four oxy-
gen atoms of sulphate ion which shows the resonance structure of
Fig. 5. Variation of stabilization energy with hydrogen-acceptor distance of
hydrogen bonding in BMSD.
SO24ꢁ
.
respectively, showing a direct dependence between strength of
hydrogen bonding and extent of hyperconjugation.
4.6. Frontier molecular orbitals
For SO24ꢁ, the stabilization energies of each four oxygen lone
pair with neighbour S–O antibonding orbital, whose values are
12.3, 12.9, 13.7 and 14.5 kcal/mol respectively for the processes
Lp (O17) ? r⁄ (S18–O20), Lp (O19) ? r⁄ (S18–O21), Lp (O20) ? r⁄
(S18–O21) and Lp (O21) ? r⁄ (S18–O17), is found appreciable and
are of same range. This shows almost equal strength of four sul-
phur–oxygen bonds, in the resonance structure of SO24ꢁ. In the
Frontier molecular orbitals provide insight into structural and
physical properties of molecules such as conjugation, aromaticity
and lone pairs [30]. Significant degree of intramolecular charge
transfer from electron donor group to electron acceptor group is
characterized by small HOMO–LUMO separations [31]. HOMO
and LUMO are directly related to ionization potential and electron
affinity respectively and the energy gap represents the stability of
Fig. 6. Molecular electrostatic potential mapping of BMSD. Colour gradient with most electropositive blue and most electronegative red is shown. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)

300
V.R. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 292–301
HOMO (-0.221 au)
LUMO (-0.032 au)
Fig. 7. HOMO–LUMO analysis of simulated BMSD at B3LYP 6-31 G(d) level.
Table 5
The table showing hyper polarisability tensors of BMSD and calculation of NLO activity at B3LYP 6-31 G(d) level.
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
byyz
bxzz
byzz
bzzz
b(esu)
b(compound)/b(urea)
ꢁ157.85
183.71
ꢁ27.47
ꢁ308.3
185.93
43.6
ꢁ9.78
149.59
89.71
ꢁ210.18
5.22 ꢂ 10ꢁ31
2.66
the molecule. For BMSD, HOMO and LUMO energies has been cal-
culated as, HOMO = ꢁ0.221 au, LUMO = 0.032 au, energy
gap = 0.253 au (6.88 eV). The calculated energy gap of HOMO–
LUMO explains the ultimate charge transfer interaction within
the molecule.
The positive and negative phases are represented schematically
in red and green colours respectively (Fig. 7). The plots reveal that
region of HOMO level spreads over the sulphate ion and LUMO is
found to spread over melaminium cation. This reveals that the
charge transfer takes place through hydrogen bonds connecting
sulphate and melaminium.
obvious from the lower wavenumber of NH2 stretching frequen-
cies, compared to the wavenumbers of asymmetric stretches of
aromatic primary amines and also from the unequal C–N bond dis-
tances. The intense ionic and donor accepter type of interactions
within the crystalline environment is found to make the triazine
ring more rigid, raising the frequencies of ring modes in BMSD,
compared to those in melamine. The analysis of Evans hole has
been identified as an effective method to explain the band profiles
of certain modes in the finger print region and here the presence of
shoulder appearing in the bands having peaks 1670 cmꢁ1
,
1513 cmꢁ1, 721 cmꢁ1 and 619 cmꢁ1, in BMSD has been found to
arise due to Evans hole, as explained theoretically. The NBO analy-
sis reveals that there is a direct correlation between the strength of
hydrogen bonding or hydrogen-acceptor bond distance and stabi-
lization energy of hyperconjugative interaction between the oxy-
gen lone pair and the antibonding orbital of NH bond. Hence
NBO analysis can be effectively used to assess the strength of
hydrogen bonding. The HOMO–LUMO analysis has been used to
study the charge transfer taking place through hydrogen bonds
connecting sulphate and melaminium ion. MEP mapping have
been performed to analyze the electrostatic interaction between
sulphate and melaminium ions responsible for the formation of
crystalline network.
4.7. Nonlinear optical activity
The first order hyperpolarizability tensor of BMSD have been
computed at B3LYP/6-31G(d) level, represented by 3 ꢂ 3 ꢂ 3
matrix (Table 5) and the twenty seven components of the 3D
matrix is reduced to 10 components due the Kleinman symmetry
[32]. The calculated values of btot for the title compound is
3.198 ꢂ 10ꢁ30 esu, 2.66 times that of urea. Thus the title compound
can be a potential applicant in the development of NLO materials
[33–35]. This phenomenon can be understood with the context
of molecular orbital picture. The HOMO–LUMO energy gap of the
title compound was calculated as 5.14 eV. As can be seen from
the btot value for this compound, there is an inverse relationship
between first hyperpolarizability and HOMO–LUMO gap, allowing
the molecular orbitals to overlap to have a proper electronic com-
munication, which is a marker of the intra-molecular charge trans-
fer from the electron donating group system to the electron
accepting group [36,37].
Acknowledgement
The author J Binoy is grateful to UGC, Govt of India for the finan-
cial support (Sanction Number: MRP(S)-0284/12-13/KLKE047/UG
C-SWRO), for carrying out this work.
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