Synthesis, growth and spectral studies of S-benzyl isothiouronium nitrate by density functional methods

Article abstract of DOI:10.1016/j.saa.2013.01.080

S-benzyl isothiouronium nitrate (SBTN), was synthesized and characterized by X-ray diffraction, FTIR, UV-Vis and NMR spectra. The Centro-symmetric single crystal of S-benzyl isothiouronium nitrate (SBTN), which crystallizes in monoclinic crystal system with space group P2₁/C, exhibits second order non-linear optical (NLO) susceptibility, due to intermolecular charge transfer. S-benzyl isothiouronium ion forms well defined charge transfer (CT) salt with anion nitrate through N-H?O and C-H?O hydrogen bonds. It is to identify the direction of specific N-H?O hydrogen bond between the -NH₂ group and O^- in the anion and also sacking in the solid state responsible for NLO activity in this crystal. The SHG technique confirms the non-linear optical property of the grown crystals. Density functional theory (DFT) calculation has been carried out to study the nature of hydrogen involved in the SBTN crystal. The bond lengths and bond angles of the structure of SBTN crystal calculated using B3LYP method with 6-311+(2d,2p) basis set. These calculations are compared with experimental values to provide deep insight into its electronic structure and property of grown crystal.

Full text of DOI:10.1016/j.saa.2013.01.080

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, growth and spectral studies of S-benzyl isothiouronium nitrate by
density functional methods
a
b,
⇑
c
d
P. Hemalatha , S. Kumaresan , V. Veeravazhuthi , S. Gunasekaran
a
Department of Physics with Computer Applications, L.R.G. Government Arts College for Women, Tirupur, Tamil Nadu 641 604, India
PG & Research Department of Physics, Arignar Anna Government Arts College, Cheyyar, Tamil Nadu 604 410, India
Department of Physics, P.S.G. College of Arts and Science, Coimbatore, Tamil Nadu 641 014, India
b
c
d
PG & Research Department of Physics, Pachaiyappas College, Chennai, Tamil Nadu 600 030, India
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
"
S-benzyl isothiouronium nitrate, a
new NLO crystal was grown by slow
evaporation method.
"
"
STBN crystallized in monoclinic
system with space group P2 /C.
1
SBTN exhibits second order NLO
susceptibility, due to intermolecular
charge transfer.
"
"
UV–Vis spectral study reveals that
SBTN crystals are transparent in the
wavelength region 200–1000 nm.
The TGA study supports the crystal is
thermally stable up to 196 °C. SHG
efficiency of SBTN is 0.45 times of
urea.
a r t i c l e i n f o
a b s t r a c t
Article history:
S-benzyl isothiouronium nitrate (SBTN), was synthesized and characterized by X-ray diffraction, FTIR,
UV–Vis and NMR spectra. The Centro-symmetric single crystal of S-benzyl isothiouronium nitrate (SBTN),
which crystallizes in monoclinic crystal system with space group P2 /C, exhibits second order non-linear
1
Received 3 November 2012
Received in revised form 30 January 2013
Accepted 31 January 2013
Available online 20 February 2013
optical (NLO) susceptibility, due to intermolecular charge transfer. S-benzyl isothiouronium ion forms
well defined charge transfer (CT) salt with anion nitrate through N–HÁ Á ÁO and C–HÁ Á ÁO hydrogen bonds.
À
It is to identify the direction of specific N–HÁ Á ÁO hydrogen bond between the –NH
2
group and O in the
Keywords:
anion and also sacking in the solid state responsible for NLO activity in this crystal. The SHG technique
confirms the non-linear optical property of the grown crystals. Density functional theory (DFT) calcula-
tion has been carried out to study the nature of hydrogen involved in the SBTN crystal. The bond lengths
and bond angles of the structure of SBTN crystal calculated using B3LYP method with 6-311+(2d,2p) basis
set. These calculations are compared with experimental values to provide deep insight into its electronic
structure and property of grown crystal.
S-benzyl isothiouronium nitrate
Non-linear optical material
Single X-ray diffraction
FTIR
UV
NMR DFT calculation
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
communication [2], and optical data storage technology [3]. These
applications have unique and often competing material require-
New non-linear optical (NLO) frequency conversion materials
can have a significant impact on laser technology [1], optical
ments and illustrate that no single nonlinear material will be
suitable for all uses. Organic compounds are often formed by weak
van der Waals bonds and hydrogen bonds and hence possess high
degree of delocalization. Thus they are expected to be optically
more nonlinear than inorganic compounds. Some of the
⇑
Corresponding author. Tel.: +91 4427268219.
E-mail address: yeskay72@yahoo.com (S. Kumaresan).
1
386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.01.080

2
P. Hemalatha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
advantages of organic materials include ease of varied synthesis,
scope for altering the properties by functional group substitutions,
inherently high nonlinearity, high damage resistance, etc. The pro-
totype organic nonlinear optical (NLO) material contains one or
more delocalized bonds, typically a ring structure like benzene.
Owing to the high polar nature of the molecules, organic NLO
materials often tend to crystallize as long needles or thin platelets.
The high optical thresholds and much versatility in the molecular
design of organic materials provide many more opportunities for
improving the second harmonic generation (SHG) response [4].
SHG is one of the most important aspects to NLO research since
it leads itself to a myriad of applications: from telecommunications
to visible light lasers, through to surface-enhanced Raman spec-
troscopy [5–7]. The level of SHG response of a given material is
inherently dependent upon its structural attributes. On a molecu-
lar scale, the extent of charge transfer (CT) across the NLO chromo-
phore determines the level of SHG output [8–12]: the greater the
CT, the larger the SHG output. The presence of strong intermolec-
ular interactions, such as hydrogen bonds can extend this level of
CT into the supramolecular realm, owing to their electrostatic
and directed nature, thereby enhancing the SHG response. The
structural characteristic of S-benzyl isothiouronium nitrate (SBTN),
on the molecular scale, reveals that they are: extremely p-conju-
gated, good planarity and strong electron donor and acceptor
groups at opposite ends of the molecule. Density functional theory
oughly with triple distilled water. The salt was repeatedly recrys-
tallised from 0.2 M HNO to get the material of very high purity.
The melting point of the SBTN was determined to be 190 °C. The
3
formation of SBTN has been represented by the following scheme:
Crystal growth
A clear saturated solution of SBTN was prepared using 0.2 N
HNO and the solution was filtered through a quantitative what-
3
mann filter paper to eliminate the suspended impurities. The fil-
tered solution was kept at a room temperature and optimally
closed with a perforated mica paper for controlled evaporation.
Care was taken to minimize mechanical shock and temperature
fluctuations. A well grown transparent and high quality optical sin-
gle crystal of SBTN was harvested in a span of 15 days. The as
grown crystal of title material is shown in Fig. 1.
(
DFT) is a cost-effective ab initio quantum chemistry procedure for
studying physical, chemical and electronic properties of the mole-
cules. DFT calculations have shown promising conformity with
experimental results [13–15]. Therefore, in this present investiga-
tion DFT is used to study the geometry, complete spectral studies,
vibrational assignment, electronic transitions and nature of hydro-
gen bonding in the SBTN crystal have been studied and reported for
the first time.
Computational details
Density functional theory has been used to investigate the elec-
tronic structure of the SBTN crystal. The geometry optimization of
the SBTN crystal was carried out using density functional method
with B3LYP exchange–correlation functional [16–18] at 6-
311G+(2d,2p) basis set. The vibrational frequencies computed
using the analytical second derivatives at B3LYP/6-311G+(2d,2p)
[Becke 3-parameters (Exchange), Lee, Yang and Parr (Correlation)
level of theory for SBTN. There is no imaginary frequency, which
implies that the optimization is at global minima. The geometry,
electronic charge densities have been calculated using atoms-in
Experimental
Material synthesis
S-benzylisothiouronium chloride (SBTC) was synthesized by
mixing 5 g of thiourea (99% AR, Merck) and 5 ml of benzyl chloride
(
99%, AR, Merck) in ethanol. The contents were refluxed for half an
hour and on cooling a white crystalline precipitate of S-benzyliso-
thiouronium chloride salt was obtained. The synthesized material
was washed several times with doubly distilled water to remove
the impurities. The sample was repeatedly recrystallized from 0.2
M HCl to enhance the degree of purity. Equimolar solutions of S-
benzyl isothiouranium chloride (SBTC) and potassium nitrate were
prepared separately in minimum amount of water and mixed to-
gether. The mixture was refluxed for half an hour and cooled to
room temperature. The precipitate was filtered and washed thor-
Table 1
Crystal data and structure refinement for SBTN.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C
8
H
11
N
2
SÁNO
3
229.26 gm
293 (2) K
0.71073 A
Monoclinic
P2
1
/c
Unit cell dimensions
a = 5.8569 (4) Å, b = 7.5931 (5) Å
c = 23.9488 (16) Å
b = 93.3°
a = c = 90;
Volume
Z
Density
Absorption coefficient
F(0 0 0)
1063.28 (12) Å3
4
3
1.376 kg/m
0.30 mm
480
À1
Theta range for data collection
Index ranges
1.99–28.01°
À6 = h 6 6; À9 = k 6 9; À28 = l 6 28
Reflections collected
Independent reflections
Completeness to theta = 25.00°
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
11,620
2492 [R(int) = 0.021]
99.9%
Full-matrix least square on F2
2337/0/110
1.0332
R1 = 0.02884, wR2 = 0.0838
R1 = 0.0309, wR2 = 0.0862
0.0200(16)
1.08 e Å and À0.79 e ÅÀ3
À3
Fig. 1. Photograph of as grown SBTN single crystals.

P. Hemalatha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
3
Table 4
Molecular geometry of SBTN crystal selected bond lengths (Å), bond angles
Ã
determined by X-ray diffraction and DFT calculations.
Ã
Bond lengths (Å)
X-ray data ( )
B3LYP/6-311G+(2d,2p)
C(1)–N(2)
C(1)–N(1)
C(2)–H2A
C(2)–H2B
C(4)–H(4)
C(8)–H(8)
C(1)–S(1)
S(2)–C(3)
N(1)–H1A
N(1)–H1B
N(1)–H1C
N(1)–H1D
N(3)–O(2)
N(3)–O(3)
N(3)–O(1)
1.3038
1.3073
0.9700
0.9700
0.9300
0.9300
1.654
1.375
1.331
1.102
1.008
1.008
1.107
1.783
1.783
0.996
0.994
1.000
1.009
1.204
1.224
1.264
1.665
0.8600
0.8600
0.8600
0.8600
1.2088
1.2345
1.2702
Fig. 2. ORTEP diagram of structure of SBTN.
Table 2
Hydrogen bonding geometry (Å).
Bond angles (°)
C(1)–N(1)–N(2)
C(1)–N(2)–S(1)
C(1)–N(1)–S(1)
C(2)–C(3)–S(1)
C(2)–C(3)–H2A
C(2)–H2A–H2B
C(4)–C(5)–H(4)
C(5)–C(4)–H(5)
N(1)–C(1)–H1A
N(2)–C(1)–H2C
N(3)–O(2)–O(3)
N(3)–O(2)–O(1)
N(3)–O(3)–O(1)
D–HÁ Á ÁA
d(DÁ Á ÁH)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
<(DHA)
120.76
122.60
116.63
107.84
110.14
108.45
119.64
120.03
120.00
120.00
128.89
116.44
114.65
119.94
122.91
116.85
115.25
111.32
110.45
121.60
121.05
120.56
119.25
122.14
116.70
116.25
i
i
N1–H1AÁ Á ÁO2
N1–H1AÁ Á ÁO1
0.86
0.86
0.86
0.86
0.86
2.211
2.558
2.125
1.980
2.237
3.046
3.236
2.915
2.804
3.015
163.84
136.36
152.52
160.03
150.49
ii
i
N1–H1BÁ Á ÁO2
N2–H2CÁ Á ÁO1
iii
N2–H2DÁ Á ÁO3
Symmetry transformations used to generate equivalent atoms.
i
x À 1, y À 1, z.
ii
Àx + 1, Ày, Àz + 1.
iii
Àx, Ày + 1, Àz + 1.
Ref. [20].
isothiouronium ion forms well defined crystalline charge transfer
CT) salt with anion nitrate through well-defined N–HÁ Á ÁO and C–
(
HÁ Á ÁO hydrogen bonds. The present study aims at identifying the
direction of specific N–HÁ Á ÁO hydrogen bond between the-NH
2
À
group and O in the anion and also sacking in the solid state
responsible for NLO activity in these crystals. The molecular struc-
ture of SBTN was solved by direct method as implemented in SHEL-
XS-97 (Shledrick 1997) and refined by full matrix-least-squares
refinement using SHELXL-97 (Shledrick 1997) program. The inten-
sity data were collected for À6 6 h 6 6; À9 6 k 6 9; À28 6 l 6 28.
The final refinement converged to an R-value of 0.0309. The final
difference Fourier map had the maximum and minimum values
Fig. 3. Optimized structure of SBTN at B3LYP/6-311+G(d,p).
Table 3
The optimized bond length, bond angle and electron density (q) (at bond critical
point) in (a.u) of the SBTN Complex calculated using B3LYP methods with 6-
À3
of the residual electron density 1.08 and À0.79 e Å , respectively.
3
11+G(2d,2p) basis set.
Nature
Bond length
Bond angle
Electron density
Hydrogen bonding
N
N
11–H21Á Á ÁO25
10–H22Á Á ÁO25
1.816
1.641
142.074
150.310
0.035154
0.056793
The bond angles for O1–N3–O3 or O25–N12–O24 is 128.7 (4);
O1–N3–O2 or O25–N12–O26 is 116.7 (4); O3–N3–O2 or O24–
N12–O25 is 114.6 (3), which indicates slight deviation in the bond
angle of O1–N3–O3 or O25–N12–O24. The molecule is stabilized
N–HÁ Á ÁN, and N–HÁ Á ÁO intramolecular interactions between the
S-benzylisothiouronium moiety and the nitrate moiety. The bond
length of both C8–N1 and C8–N2 is 1.3073 Å and 1.3038 Å respec-
tively. The values are intermediate between carbon–nitrogen dou-
ble and single bond lengths. This partial double nature of C–N bond
in S-benzyl isothiouronium ion has been attributed to the reso-
nance effect. The distance N–O bonds in the anion vary from
1.2088 Å to 1.2702 Å. The C8–S1 or bond length is 1.7331 Å
whereas C7–S1 bond length is slightly longer than the previous
one, which has been determined to 1.8186 Å. The crystal packing
is stabilized by N–HÁ Á ÁO and C–HÁ Á ÁO intermolecular interactions
[20]. In SBTN, the orientation of the S-benzylisothiouronium and
nitrate ions facilitates the formation of expected weak hydrogen
bonds between the hydrogen at N1 of S-benzylisothiouronium
molecule (AIM) analysis. All the calculations have been performed
using the Gaussian 03 W program [19].
Results and discussion
Single crystal X-ray diffraction analysis
Single crystal X-ray analysis shows that SBTN crystal belongs to
monoclinic system with space group P2
cell dimensions of the crystal is a = 5.8569 Å, b = 7.5931 Å,
c = 23.9488 Å; = 90°; b = 93.3°, are shown in Table 1. The anal-
ysis was carried out using Nonius CAD-4/MACII 3 diffractometer
with Mo K (0.71073 Å) radiation. An ORTEP (Oak Ridge Thermal
Ellipsoid Plot) of SBTN crystal structure is shown in Fig. 2. S-benzyl
1
/c symmetry and the unit
a = c
a

4
P. Hemalatha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
Table 5
Observed and calculated FTIR wavenumbers for SBTN crystal.
À1
À1
Observed wavenumber (cm ) FTIR
Wavenumber (cm
425
)
Ir (Rel. Intensity)
Assignments
4
25(m)
74(m)
0.64
25.08
86.46
8.17
1.37
12.15
0.02
12.33
26.92
37.69
86.07
9.46
129.81
134.06
17.69
0.45
79.95
127.14
6.60
0.16
0.12
1.43
0.05
81.69
0.22
7.43
Lattice vibrations
4
49
473
92
s
s
s
m
(Ring deformation)
(Ring deformation)
(Ring deformation)
(CS) + ring deformation
4
4
5
5
15(s)
68(s)
521
543
(CNH) deformation
6
54
712
48
768
17
826
48
870
75
909
c
c
c
c
c
c
(NCS)
1
+
c
(CCS)
2 1
+ d(NOO)
7
7
8
8
9
08(s)
(CCS)
(NCS)
2
+ d(CNH)
2
7
1
+
+
c
c
(CCS)21
(CCS) + d(NOH)
2
67(s)
(CCS)
(NCS)
(NCS)
2
8
2
2
2
25(s)
8
b(NCS)
1
+ b(CCS)
2
+ d(CNH)
1
70(ms)
09(w)
d(CNH)
1
8
c
c
c
c
c
c
(NCS)
1
+
c
(CCS)
2
+ d(CNH)
2
(CCH)
(CCH)
(CCH)
(CCH)
(CCH)
2 1
+ ring(CCH)
+ ring(CCC)
+ ring(CCC)
9
9
9
39
64
82
2
2
1
2
2
1
2
3
2 2
+ d(NO )
1042(w)
1040
058
1
b(SCH)
b(SCH)
1
1
1
1
065(w)
100(m)
140(m)
160(w)
1059
1102
1135
1157
b(CNH)
b(CNH)
b(CNH)
b(CNH)
1
1
2
2
+
+
c
c
(CCH)
2
(NOH)
1
1
192
+
c
(NOH)
2
1
1
230(m)
248(m)
1241
1247
1.22
61.79
0.10
0.49
0.14
0.49
3.40
45.42
2.11
c
c
c
c
c
c
(CCH)
(CCH)
(CCH)
(CCH)
(CCH)
(CCH)
2
2
2
2
1
1
1
262
265
1
300(m)
1292
1356
1389
1402
1433
b(CCH)
b(CCH)
b(CCH)
2
1
2
+ c(CCH)
1
1
1
1
480(w)
548(s)
639(vs)
1487
1577
1620
69.48
13.48
18.62
17.79
7.72
1.40
3.46
2.78
0.68
1.14
2.82
0.60
0.46
0.53
3.28
3.71
8.68
t
(CN)+b(CCH)
2
b(CCH)
1
m
m
m
m
m
s
s
s
s
s
(CNH
(CNH
2
)
)
2
1
1
1
1
1
625
630
726
745
783
2
1
+ b(CCC)
2
(CC)
(CC)
(CC)
1
1
1
+ b(CCH)
2
d(NH
d(NH
2
)
)
2
1
3
820(ms)
072(ms)
1838
3027
2
1
m
m
m
m
m
m
m
m
m
m
m
s
s
s
(CH)
(CH)
(CH)
1
3
3
3
3
3
3
3
030
140
144
251
259
260
266
1
5
2 1
as(NH )
s
s
s
s
s
(NH
(NH
2
)
)
1
2
2
(CH)
(CH)
(CH)
5
3
1
3
280(w)
3277
4.49
32.45
14.57
3
3
343
355
as(NH
as(NH
2
)
)
1
2
2
s, strong; m, medium; vs, very strong; w, weak; vw, very weak;
m, stretching;
m
s
, symmetric stretching; as, asymmetric
m
stretching; b, in-plane-bending; , out-of-plane bending; d, scissoring;
c
x, wagging;
q
, rocking; t, twisting; s, torsion.
moiety and O1 of nitrate ion whereas the one formed between the
hydrogen at N2 of the same moiety and O1 of the nitrate ion is
comparatively stronger as has been evidenced by the data given
in Table 2. The molecular association through intermolecular
hydrogen bonding is presumably the root cause for the SHG prop-
erty in this crystal.
tor and S-benzylisothiouronium as proton donor. The bond angles
for N11–H21Á Á ÁO25 and N10–H22Á Á ÁO25 bonds in SBTN complex
are 142.074° and 150.310° respectively. An understanding of the
nature of a hydrogen bond can be analyzed through the study of
electron density at the bond critical point (BCP) of the various
hydrogen bonds involved. The values of electron density (q) (in
atomic units) at BCP of N–HÁ Á ÁO bond calculated using AIM ap-
proach are presented in Table 4. The electron densities at the val-
ues indicate the presence of hydrogen bond in the complex in
accordance with Popelier’s criteria [21]. It is expected that the
strong bonds are usually associated with higher electron density
values, indicating higher structural stability, as is observed for
the N10–H12Á Á ÁO25 bond in the SBTN complex, which has higher
DFT calculations
The optimized structure of SBTN crystal at DFT level is shown in
Fig. 3. Table 3 lists the bond lengths and bond angles of the struc-
ture of SBTN crystal calculated using B3LYP method at 6-
3
11+(2d,2p) basis set. In this case oxygen acts as the proton accep-

P. Hemalatha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
5
H22Á Á ÁO25 hydrogen bond when compared with N11–H21Á Á ÁO25
hydrogen bond, due to which the former hydrogen bond length
is shorter than the latter. Further the charge transfer energy E(
is found to be 28.34 kcal/mol in the case of N10–H22Á Á ÁO25, which
is large when compared with N11–H21Á Á ÁO25 hydrogen bond with
1
1
2
0
2)
8
.35 kcal/mol. Both the large charge transfer and charge transfer
8
6
4
2
0
energy in N10–H22Á Á ÁO25 could be attributed to the bond angle
of the H-bond which is 150.31° while for N11–H21Á Á ÁO25 H-bond
it is142.07°. A remarkable deviation of calculated hydrogen bond-
ing distance and angle with experimental value is noticed. It is due
to the fact that anharmonicity with the real system, which includes
neighborhood interactions also.
FTIR spectral analysis
The FTIR spectrum of SBTN was recorded using SHIMADZU FTIR
À1
spectrometer in the region 4000–400 cm and is shown in Fig. 4
(
Provided in Supplementary part) The –NH
2
symmetric and asym-
À1
2
00
400
600
800
1000
metric stretches in the range 3545–3433 cm are in agreement
with experimental value of 3280 cm is assigned to NH
À1
2
stretch-
Wavelength (nm)
ing vibrations. The aromatic CC stretch known as semicircle
stretching predicted at 1630 at DFT levels of calculation is in agree-
Fig. 5. UV–Vis spectrum of SBTN crystal.
À1
ment with observed value at 1639 cm . This very sharp band with
intensity of 7.2 is assigned to CC stretching vibrations [23]. The CN
stretching wavenumbers in the side chains is a difficult task, be-
cause there are problems in identifying these wavenumbers from
electron density (0.056793 a.u) and strong hydrogen bond
1.641 Å). The correlation between the hydrogen bond length and
(
À1
other vibrations. The band observed at 1480 cm is assigned to
electron density is inverse, that is, an increase in bond length cor-
responds to a decrease in the electron density, which is expected,
since increase in distance results in reduced orbital overlap and
hence low electron density along the bond. DFT calculations pre-
dict N10–H22Á Á ÁO25 to be the strong hydrogen bond present in
SBTN, which is in accordance with experimental results given in
Table 4. The AIM analysis augments the former result indicating
a large electron density value for the hydrogen bond.
À1
CN stretch. The peaks lying in between 3300 and 3000 cm are
due to hydrogen bonded NH and aromatic CH stretching vibra-
tions. The aromatic ring skeletal vibration is observed at
À1
À1
1
454 cm . The peak at 703 cm is due to aromatic C–H out of
À1
plane bending. The peak at 1639 cm represents the NH
2
bending.
À1
The NO stretch is seen as a well resolved peak at 1548 cm . The CS
stretching is different from other vibrations in both aliphatic and
aromatic sulfides [23]. Normally, thiocarbonyl group exhibit weak
to medium peaks. In the present study the band observed at
À1
À1
Natural bond orbital analysis
568 cm and a sharp band at 708 cm are assigned to thiocar-
bonyl stretch and bend vibrations. The absorption peaks at
À1
À1
It is known that, the hydrogen bonds (H-bonds) are formed be-
tween proton donor and proton acceptor, and the amount of
charge transfer is significant for the elongation and contraction
of hydrogen bonds. In order to study the charge transfer between
the donor and acceptor natural bonding analysis (NBO) [22] was
carried out. The value of NBO occupation numbers for the proton
donor antibonds (H–Y) and proton acceptor lone pairs n(X) for
N11–H21Á Á ÁO25 and N10–H22Á Á ÁO25 bonds in SBTN shows the
bonding nature. The occupation numbers for N11–H21and N10–
H22 proton donor are found to be 0.04574 e and 0.09429 e respec-
tively. The corresponding value for N–H in monomer is found to be
1384 cm
and 825 cm
are due to NO symmetric stretching
and NO bending vibrations, respectively. The fundamental modes
of vibrations of assignment for SBTN crystal is shown in Table 5.
UV–Vis spectral analysis
Optical transparency of 1 mm thick plate SBTN crystal was re-
corded using a Shimadzu 3100 PC UV–Vis spectrophotometer.
The recorded transmission spectrum is shown in Fig. 5. The trans-
mission range extends from 200 nm in the UV to 1000 nm in the IR
region. The crystal has a good transmission in the entire visible
range. The extended transmission window will enable higher har-
monic generation and wavelength extension by cascaded fre-
quency conversion process. From the spectrum, it is evident that
0
.00614 e. This increase in occupation number from monomer to
complex is due to charge transfer from the proton donor O25 of ni-
trate ion. The amount of charge transfer is large (0.09429e) in N10–
Table 6
Electronic transistion state of SBTN at TD-DFT/6-31G(d,p) level.
À1
No
State of excitation
Energy (cm
)
Wavelength (nm)
Oscillator strength (f)
State dipole (Debye)
Wavelength (nm) observed
1
2
3
4
5
6
7
8
9
Singlet
Singlet
Singlet
Singlet
Singlet
Singlet
Singlet
Singlet
Singlet
Singlet
21427.5
26518.0
32905.3
36874.6
41502.8
42603.9
43363.9
46658.2
46838.1
48044.6
466.7
377.1
303.9
271.2
240.9
234.7
230.6
214.3
213.5
208.1
0.002522
0.002375
0.000151
0.004776
0.042371
0.067188
0.116801
0.029070
0.083272
0.239041
4.999392
5.241358
8.484894
3.582994
6.188184
14.529789
16.313392
23.210926
7.453551
1.957221
–
–
–
–
–
–
–
–
–
208
1
0

6
P. Hemalatha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
Table 7
to the protons of the amino groups. The four protons of two amino
NMR assignments for SBTN crystal.
groups appear at the same frequency due to resonance. The multi-
plet at 7.72787 ppm is due to aromatic protons. The identification
of functional groups and their assignments confirms the structure
of SBTN crystal.
Chemical shift (
r
) value in ppm
Assignments
4
.2864
Methylene protons of S-benzyl
isothiouronium nitrate moiety
Amino protons of the same moiety
Aromatic protons
4
7
.6704
.2737
Electron charge density
The gross atomic charges of SBTN with chemical shifts are given
in Table 8. This data would be used to explain the preferred posi-
tion of nucleophilic attack of this molecule. In the present study,
we employed DFT levels to calculate the atomic charges. The gross
SBTN crystal has UV cutoff around 208 nm, which is sufficiently
low for SHG laser radiation at 1064 nm, and has transmission over
2
5% and above 382 nm the material observed to be highly trans-
2
atomic charges at the carbon C9 attached to NH and oxygen atoms
parent with transmission over 64%.
in NO are electron deficient compared to other carbon atoms. In
3
Further, the optical and electronic properties of SBTN were
investigated by means of TD-DFT calculations at the molecular le-
vel. The main orbital compositions of the computed lower lying 10
singlet excited states and transition features of the molecule ob-
tained at the B3LYP/6-311G+(2d,2p) level. It is observed from the
general, electron deficient atoms have the higher d value than an
electron rich atom. The carbon attached to N and O has lesser elec-
tron densities than the other carbons and hence these are more un-
shielded as shown in Table 8. So the C9 position is the preferred
nucleophilic center. The net electron densities on all hydrogen
atoms attached to the carbon are electropositive. In general, elec-
tron deficient atoms are more unshielded and hence they absorb
at downfield. The chemical shift at d 128.8 was assigned to the
C–NH2.
Table 6, that the lowest energy absorption found at 208 nm be-
Ã
longs to the dipole allowed
p
–p
transitions from HOMO to LUMO,
corresponding to 208.1 nm with the largest oscillator strength cal-
culated at the B3LYP/6-311+G(d,p) levels. The charge transfer is
Ã
not from HOMO to LUMO but from allowed
p
–p
transitions
[
24]. The calculated outcomes are in good agreement with experi-
Thermo gravimetric analysis
mental data and provide deep insight into their electronic and
charge transfer characteristics of SBTN crystals.
Thermal analysis is used to provide quantitative information on
weight losses due to decomposition and/or evaporation of low
molecular material as a function of time and temperature, thus
greatly facilitating the interpretation of thermal degradation pro-
cesses. Computational quantum chemistry provides additional
data, which can be utilized successfully for the interpretation of
experimental results which may be used in the description and
prediction of primary fragmentation processes. The thermal stabil-
ity of SBTN was identified by thermo gravimetric (TG) and differen-
tial thermal analysis (DTA). The thermal analyses were carried out
simultaneously employing Perkin Elmer thermal analyzer between
28 and 800 °C at a heating rate of 10.0 °C/min in nitrogen atmo-
NMR spectral analysis
Nuclear magnetic resonance is a very versatile technique in the
identification of organic and semi-organic compounds. It is also an
important tool for examination of electronic structure. The H NMR
spectrum of SBTN crystal is shown in Fig. 6 (provided in Supple-
mentary part). The assignments of various absorptions of the com-
pound are shown in Table 7. It is observed from the figure that a
1
2
singlet at 4.2864 ppm is due to –CH protons present in the com-
pound. The intense singlet peak at 4.6704 ppm has been assigned
Table 8
Gross atomic charge density of SBTN.
No.
Atom
Gross atomic charge
Calculated chemical shift
Observed chemical
1
13
shift (d) 1H NMR
(
d) H and C NMR
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
C
S
C
N
N
C
C
C
À0.22093
0.215301
0.168963
À0.3948
À0.3935
À0.07799
À0.07861
À0.07998
À0.08793
À0.0133
À0.08176
0.091891
0.116474
0.261495
0.223167
0.250575
0.228101
0.082671
0.08305
31.2
70.2
128.7
127.2
128.7
128.8
137
128.8
3.70
4.80
2.0
C
C
C
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
H
H
H
H
H
H
H
H
H
H
H
O
N
O
O
1.572
2.0
7.14
7.07
7.14
7.07
7.06
2.0
7.425
7.372
7.267
0.084544
0.094308
0.078738
À0.23737
0.20102
2.0
À0.21885
À0.29529

P. Hemalatha et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 1–7
7
balance between competing molecular and supramolecular CT ef-
fects, thus to create SHG active material which is 45% efficiency
of standard NLO material.
Acknowledgments
The authors thank Prof. E.M. Subramanian, Department of
Chemistry, Pachaiyappas College for Men, Kanchipuram, Tamil
Nadu, India, for valuable discussions. The authors are thankful to
Sophisticated Analytical Instrumentation Facility (SAIF), IIT Ma-
dras, Chennai, and NIT, Trichy for recording spectral
measurements.
Appendix A. Supplementary material
Fig. 7. Thermogram (TGA/DTA) of SBTN crystal.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.01.080.
sphere. The SBTN sample weighing 6.698 mg was taken for the
analysis and the thermogram is illustrated in Fig. 7. The material
is moisture free and stable up to 190 °C with the elimination of
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Conclusion
1
The single crystal of SBTN is monoclinic with space group P2 /c
and it exhibits second-order NLO susceptibility due to intermolec-
ular hydrogen bonding. The geometry of the crystal was analyzed
by X-ray diffraction studies and compared with DFT methods.
2001.
The better linearity in the NH
2
with NO
3
bond may have aided
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for large charge transfer and charge transfer energy. The presence
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been identified by FTIR and NMR spectral studies. The optical
transmittance was studied using UV–Vis spectrum and lower cut
off at 200 nm makes the crystal for NLO applications. The thermal
analysis shows the grown crystal is thermally stable up to 208 C.
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