Syntheses, spectral, X-ray and DFT studies of 5-benzyl-N-phenyl-1,3,4- thiadiazol-2-amine, 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine and 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole obtained by Mn(II) catalyzed reactions

Article abstract of DOI:10.1016/j.molstruc.2013.09.051

New compounds 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (Bptha, 1), 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine (Pthp, 2) and 2-(5-methyl-1,3,4- thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (Mtmth, 3) have been synthesized and characterized with the aid of elemental analyses, IR, NMR and single crystal X-ray data. The structure of compounds 1, 2 and 3 are stabilized via intramolecular as well as intermolecular hydrogen bonding and crystallize in monoclinic system with space group P 1, P21/n and P 1, respectively. During the course of reaction, the substituted thiosemicarbazide/thiohydrazide get cyclized into the corresponding thiadiazole in the presence of manganese(II) nitrate via loss of H₂O to yield compounds 1 and 2. However condensation occurred in the case of 5-methyl-1,3,4-thiadiazole-2-thiol which yielded 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (3) by loss of one mole of H₂S from two moles of 5-methyl-1,3,4- thiadiazole-2-thiol in the presence of manganese(II) acetate. The geometry optimization has been performed using DFT method and geometrical parameters thus obtained for the compounds have been compared with their single crystal X-ray data. The negative values of HOMO and LUMO energies for the molecules indicate that they are stable. The electronic transition from the ground state to the excited state due to a transfer of electrons from the HOMO to LUMO levels is mainly associated with the πa??π transition.

Full text of DOI:10.1016/j.molstruc.2013.09.051

Journal of Molecular Structure 1054–1055 (2013) 251–261
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Syntheses, spectral, X-ray and DFT studies of 5-benzyl-N-phenyl-1,3,4
-thiadiazol-2-amine, 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine and 2
-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole
obtained by Mn(II) catalyzed reactions
a
a
b
b
a
R.K. Dani a, M.K. Bharty a, , S.K. Kushawaha , S. Paswan , Om Prakash , Ranjan K. Singh , N.K. Singh
⇑
a Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
b Department of Physics, Banaras Hindu University, Varanasi 221005, India
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Substituted thiosemicarbazide/thiohydrazide get cyclized into thiadiazole in the presence of manganese(II) salt.
The compounds are stabilized through intramolecular and weak intermolecular hydrogen bonding.
X-ray crystallographic results are well produced by DFT calculations.
The HOMO and LUMO energies of the molecules are negative indicating that the all compounds are stable.
Lower energy gap for the compound 2 indicates better NLO properties as compared to compounds 1 and 3.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2013
Received in revised form 26 September 2013
Accepted 26 September 2013
Available online 4 October 2013
New compounds 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (Bptha, 1), 2-(5-phenyl-1,3,4-thiadiazol-
2-yl) pyridine (Pthp, 2) and 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (Mtmth,
3) have been synthesized and characterized with the aid of elemental analyses, IR, NMR and single crystal
X-ray data. The structure of compounds 1, 2 and 3 are stabilized via intramolecular as well as intermo-
lecular hydrogen bonding and crystallize in monoclinic system with space group P 1, P21/n and P 1,
respectively. During the course of reaction, the substituted thiosemicarbazide/thiohydrazide get cyclized
into the corresponding thiadiazole in the presence of manganese(II) nitrate via loss of H2O to yield com-
pounds 1 and 2. However condensation occurred in the case of 5-methyl-1,3,4-thiadiazole-2-thiol which
yielded 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (3) by loss of one mole of H2S
from two moles of 5-methyl-1,3,4-thiadiazole-2-thiol in the presence of manganese(II) acetate. The
geometry optimization has been performed using DFT method and geometrical parameters thus obtained
for the compounds have been compared with their single crystal X-ray data. The negative values of
HOMO and LUMO energies for the molecules indicate that they are stable. The electronic transition from
the ground state to the excited state due to a transfer of electrons from the HOMO to LUMO levels is
Keywords:
1,3,4-Thiadiazole derivatives
Mn(II) catalyzed conversion
NLO
DFT calculation
mainly associated with the
pꢁ ꢁ ꢁp transition.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
cyclization of acyclic precursor such as N0-acyl hydrazine-carbo-
dithioic acid alkyl ester [10,11]. 1,3,4-Thiadiazoles are used as
starting material for the synthesis of numerous types of chemical
compounds including sulfa drugs, biocides, fungicides, dyes and
chemical reaction accelerators [12]. Thiadiazoles containing
mercapto, hydroxyl and amino substituents can exist in tautomeric
forms and have found many industrial applications and posses
surprising chemical properties including their capacity for forming
complexes with metal ions [13]. These properties prompted us to
synthesize a new type of heterocyclic compound with a presump-
tion that incorporation of phenyl and pyridine rings in the thiadi-
azole moiety would have much better solubility in non-aqueous
1,3,4-Thiadiazole and its derivatives have found applications as
bioactive agents such as antimicrobials, antituberculostatic,
anti-inflammatory, analgesic, antifungal, antipyretic and anticon-
vulsant [1–9]. The synthesis of this type of heterocyclic compounds
has attracted considerable attention and several methods have
been developed for their syntheses, for example the oxidative
⇑
Corresponding author. Tel.: +91 542 6702447.
E-mail address: mkbharty@bhu.ac.in (M.K. Bharty).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.09.051

252
R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
medium. Here we report the metal assisted cyclization of
4-phenyl-1-(2-phenylacetyl) thiosemicarbazide and N0-ben-
zothioylpicolinohydrazide to 5-benzyl-N-phenyl-1,3,4-thiadiazol-
2-amine and 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine, respec-
tively, during an attempt to prepare their complexes with
manganese (II) nitrate. The conversion of 5-methyl-1,3,4-thiadia-
zole-2-thiol to 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-
1,3,4-thiadiazole has been achieved under mild condition using
manganese(II) acetate. The spectroscopic, single crystal X-ray
structure, DFT studies, FMOs analysis and NLO properties of the
resulting compounds are presented in this paper.
5-methyl-1,3,4-thiadiazole-2-thiol (Sigma Aldrich) were used as
received. The 2-phenyl acetohydrazide was prepared by refluxing
equimolar amount of methyl-2-phenyl acetate with hydrazine hy-
drate. All the solvents were purchased from Merck Chemicals, In-
dia, and used after purification. Carbon, hydrogen, nitrogen and
sulfur contents were estimated on a CHN Model CE-440 Analyser
and on an Elementar Vario EL III Carlo Erbo 1108. IR spectra were
recorded in the 4000–400 cmꢂ1 region as KBr pellets on a Varian
Excalibur 3100 FT-IR spectrophotometer. 1H and 13C NMR spectra
were recorded in DMSO-d6 on a JEOL AL300 FT NMR spectrometer
using TMS as an internal reference.
2. Experimental section
2.2. Synthesis
2.1. Materials and physical measurements
2.2.1. Synthesis of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine
(Bptha, 1)
Commercial reagents were used without further purification
and all experiments were carried out in open atmosphere.
Methyl-2-phenyl acetate, picolinic acid hydrazide, hydrazine
hydrate (SD Fine Chemicals, India), phenyl isothiocyanate and
A mixture of 2-phenyl acetohydrazide (1.220 g, 10 mmol) and
phenyl isothiocyanate (1.2 mL, 10 mmol) in absolute ethanol
(20 mL) was refluxed for 6 h at 70 °C. The solid 4-phenyl-1-(2-
phenylacetyl) thiosemicarbazide obtained upon cooling was
O
H
N
H
O
N
O
NH2-NH2.H2O
PhNCS
EtOH
N
O
H
HN
EtOH
S
CH3
NH2
.
Mn(NO3)2 4H2O
N
N
N
H
C
H2
S
Scheme 1. Synthesis of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (Bptha).
O
O
NH2
H
O
N
N
+
1N NaOH
AcOH
N
H
S
H
N
OH
N
S
S
.
EtOH-H2O
Mn(NO3)2 4H2O
N
S
N
N
Scheme 2. Synthesis of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (Pthp).
CH3
N
N
N
N
CH3CN
MeOH
N
N
S
+
N
HS
CH3
H3C
SH
H3C
S
Mn(OAc)2.4H2O
S
S
S
N
Scheme 3. Synthesis of 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (Mtmth).

R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
253
Table 1
Crystallographic data for the compound 1, 2 and 3.
Parameters
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
T (K)
C
15H13N3S
C13H9N3S
239.30
Monoclinic
P21/n
C6H6N4S3
230.36
Monoclinic
P1
267.35
Monoclinic
P1
293(2)
293(2)
293(2)
k, Mo K
a (Å)
b (Å)
c (Å)
b (°)
a
(Å)
0.71073
0.71073
0.71073
5.6970(6)
6.7973(10)
12.0017(13)
102.569(10)
453.62(10)
2
11.3408(19)
7.0518(10)
8.5427(10)
103.887(14)
663.22(17)
2
14.1797(15)
16.5084(17)
15.970(2)
110.866(13)
3493.2(7)
12
V (Å3)
Z
qcalcd (g/cm3)
1.339
0.233
1.365
0.256
1.686
0.770
l
(mmꢂ1
)
F(000)
280
1488
236
Crystal size (mm3)
0.23 ꢃ 0.21 ꢃ 0.18
3.41–29.23
ꢂ14 6 h 6 15
ꢂ9 6 k 6 9
ꢂ10 6 l 6 11
7232
0.27 ꢃ 0.23 ꢃ 0.21
3.31–29.05
ꢂ25 6 h 6 24
ꢂ23 6 k 6 22
ꢂ19 6 l 6 18
6574
0.28 ꢃ 0.24 ꢃ 0.20
3.00–29.16
ꢂ7 6 h 6 7
ꢂ8 6 k 6 9
ꢂ15 6 l 6 16
1880
h range for data collections (°)
Index ranges
No. of reflections collected
No. of independent reflections (Rint
)
3805
3953
1595
No. of data/restrains/parameters
7926/0/343
1.226
0.0681, 0.1099
0.1320, 0.1052
0.280, ꢂ0.283
8026/0/460
1.295
0.0661, 0.1567
0.1383, 0.1765
0.345, ꢂ0.283
3762/0/235
1.712
0.0411, 0.1194
0.0691, 0.1412
0.408, ꢂ0.376
Goodness-of-fit on F2
b
R1a, wR2 [(I > 2
r
(I)]
b
R1a, wR2 (all data)
Largest difference in peak/hole (e Åꢂ3
)
a
R1
=
R
||Fo| ꢂ |Fc||
R|Fo|.
1=2
R2 ¼ ½
R
wðjF2oj ꢂ jF2c jÞ2=
R
wjF2oj ꢄ
.
2
b
Table 2
Table 4
Bond length (Å) and angles (°) for 1.
Bond length (Å) and angles (°) for 2.
Bond length (Å)
Bond angles (°)
Bond length (Å)
Bond angles (°)
(Exp.)
(Cal.)
(Exp.)
(Cal.)
(Exp.)
(Cal.)
(Exp.)
(Cal.)
S(1)AC(8)
S(1)AC(9)
N(1)AN(2)
N(1)AC(8)
N(2)AC(9)
N(3)AC(9)
C(10)AN(3)
1.719(7)
1.736(7)
1.393(9)
1.293(8)
1.292(9)
1.294(9)
1.404(9)
1.771
1.772
1.368
1.291
1.302
1.366
1.409
C(8)AS(1)AC(9)
S(1)AC(8)AN(1)
S(1)AC(9)AN(2)
C(9)AN(2)AN(1)
C(9)AN(3)AC(10)
C(11)AC(10)AN(3)
C(7)AC(8)AS(1)
86.8(3)
114.9(6)
113.6(6)
112.9(6)
126.7(6)
121.4(3)
119.5(5)
85.7
114.2
113.7
112.7
129.5
123.6
123.9
S1AC8
S1AC7
N1AC8
N1AN2
N2AC7
C9AN3
N3AC13
1.700(5)
1.699(6)
1.188(7)
1.372(6)
1.317(6)
1.308(7)
1.333(8)
1.748
1.756
1.303
1.354
1.309
1.341
1.332
C8AN1AN2
C7AN2AN1
N1AC8AC9
N1AC8AS1
C9AC8AS1
N2AC7AC6
C3AC4AC5
111.5(5)
110.8(5)
122.9(4)
119.5(5)
117.4(4)
123.2(5)
123.9(7)
113.41
113.52
124.37
113.75
121.86
122.94
120.15
filtered off and washed with water and ether ((50:50 v/v). The mix-
ture of 4-phenyl-1-(2-phenylacetyl) thiosemicarbazide (0.285 g,
1 mmol) and Mn(NO3)2ꢁ4H2O (0.251 g, 1 mmol) was heated for
4 h at 45 °C. After cooling, the reaction mixture was filtered off
and kept for crystallization whereupon yellow crystals of 1 suitable
for X-ray analyses were obtained after 15 days. Yield: 85%. m.p.
220 °C. Anal. Found. C, 67.34%; H, 4.88%; N, 15.69%; S, 11.96%. Calc.
for C15H13N3S (267.35): C, 67.33%; H, 4.86%; N, 15.71%; S, 11.97%.
IR (KBr, cmꢂ1):
m(NH) 3270; m(C@N) 1590; m(NAN) 1097; m(CAS)
794. 1H NMR (DMSO-d6; d ppm): 11.48 (s,1H, NH), 3.34 (2H,
ACH2). 7.90–6.86 (m, aromatic protons). 13C NMR (DMSO-d6; d
ppm): 179.65 (CAS), 168.35 (C@O), 121.68–144.32 (aromatic car-
bons), 36.95 (CH2).
Table 3
Hydrogen bond parameters [Å and °] for 1.
DAH. . .
A
d(DAH)
d(H. . .A)
d(D. . .A)
<(DHA)
Intermolecular hydrogen bonding
C22AH22A. . .N1
C22AH22B. . .N1
C22AH22A. . .N6
N6AH6. . .N2
0.970
0.970
0.970
0.860
0.860
0.971
0.969
0.969
2.398
2.694
2.684
2.117
2.115
2.416
2.691
2.711
2.994
2.994
3.649
2.959
2.958
3.002
3.002
3.676
119.18
93.38
2.2.2. Synthesis of 2-(5-phenyl-1,3,4-thiadiazol-2-yl)pyridine (Pthp, 2)
The N0-benzothioylpicolinohydrazide was prepared by drop-
173.33
165.91
166.73
118.48
99.21
wise addition of benzodithioyl acetic acid into
a methanol
suspension of picolinic acid hydrazide (2.742 g, 20 mmol) in the
presence of 1N NaOH with continuous stirring for 1 h. The white
solid obtained was filtered, washed with water, dried and
recrystallized from MeOH–CHCl3 mixture (50:50 v/v). N0-ben-
zothioylpicolinohydrazide (0.257 g, 1 mmol) was dissolved in
water ethanol mixture (50:50, v/v) and an aqueous solution of
Mn(NO3)2ꢁ4H2O (0.251 g, 1 mmol) was added to the above solution
and stirred for 1 h. Yellow clear solution was filtered off and kept
for crystallization. Light yellow crystals of 2 suitable for X-ray
N3AH3A. . .N5
C7AH7A. . .N4
C7AH7B. . .N4
C7AH7B. . .N3
173.00
Intramolecular hydrogen bonding
C5AH5. . .N1
0.930
0.930
0.930
0.930
2.575
2.667
2.589
2.652
3.070
3.112
3.091
3.112
113.67
110.94
114.36
111.23
C11AH11. . .N2
C20AH20. . .N4
C26AH26. . .N5

254
R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
Fig. 1. ORTEP diagram of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (1) showing atomic numbering scheme with ellipsoid of 30% probability.
Table 5
Table 7
Hydrogen bond parameters (Å and °) for 2.
Hydrogen bond parameters (Å and °) for 3.
DAH. . .
A
d(DAH)
d(H. . .A)
d(D. . .A)
<(DHA)
DAH. . .
A
d(DAH)
d(H. . .A)
d(D. . .A)
<(DHA)
Intermolecular hydrogen bonding
Intermolecular hydrogen bonding
C23AH23. . .N1
C27AH27. . .N8
0.930
0.927
2.718
2.605
3.548
3.387
149.12
142.13
C7AH7A. . .N5
C7AH7A. . .N6
C6AH6A. . .N3
C6AH6B. . .N4
0.958
0.958
0.961
0.961
2.554
2.456
2.346
2.546
3.501
3.395
3.284
3.476
169.67
158.51
165.07
162.76
Intramolecular hydrogen bonding
C10AH10. . .N1
C1AH1. . .N2
C5AH5. . .S1
0.930
0.931
0.930
2.663
2.642
2.692
2.917
2.920
3.094
96.35
97.97
106.92
(230.36): C, 31.25%; H, 2.61%; N, 24.31%; S, 41.68%. Found: IR
(KBr, cmꢂ1):
m(C@N) 1602; m(NAN) 1065; m
(CAS) 727. 1H NMR
(DMSO-d6; d ppm): 2.50 (3H, ACH3. 13C NMR (DMSO-d6; d ppm):
190.31 (CAS), 168.80 (C@N), 38.66 (CH3).
Table 6
Bond length (Å) and angles (°) for 3.
Bond length (Å)
Bond angles (°)
3. Crystal structure determination
(Exp.)
(Cal.)
(Exp.)
(Cal.)
S2AC3
N2AN1
S2AC4
N1AC2
N3AN4
N3AC4
S1AC2
N2AC3
S1AC3
S3AC4
S3AC5
C5AN4
1.778(11)
1.383(2)
1.749(14)
1.291(2)
1.369(17)
1.272(2)
1.751(17)
1.287(17)
1.686(13)
1.744(15)
1.713(14)
1.301(2)
1.758
1.369
1.775
1.293
1.367
1.297
1.767
1.296
1.756
1.753
1.758
1.299
C3AN2AN1
S1AC3AS2
N2AC3AS2
N2AC3AS1
C5AS3AC4
N4AC5AS3
S3AC4AS2
N3AC4AS2
N3AC4AS3
C1AC2AS1
C4AN3AN4
N4AC5AC6
110.2(12)
119.0(6)
124.4(10)
116.4(9)
84.8(7)
116.1(9)
129.1(10)
116.7(11)
114.2(10)
128.0(12)
112.2(12)
125.1(11)
113.2
119.4
126.6
113.9
85.4
114.1
130.0
115.1
113.7
125.7
112.9
123.2
Crystals suitable for X-ray analyses of the compounds 1, 2 and 3
were grown at room temperature. The crystal data were collected
on an Oxford Gemini diffractometer equipped with a CrysAlis CCD
software using a graphite mono-chromated Mo Ka (k = 0.71073 Å)
radiation source at 293 K for 1, 2 and 3. Multi scan absorption cor-
rection was applied to the X-ray data collection for all the com-
pounds. The structures were solved by direct methods (SHELXS-
08) and refined against all data by full matrix least-square on F2
using anisotropic displacement parameters for all non-hydrogen
atoms. All hydrogen atoms were included in the refinement at geo-
metrically ideal position and refined with a riding model [14]. The
MERCURY package and ORTEP-3 for Windows program were used
for generating structures [15,16].
analyses were obtained by slow evaporation of the above solution
over a period of 15 days. Yield: 78%; m.p. 180 °C. Anal. Found: C,
65.21%; H, 3.71%; N, 17.56%; S, 13.40%. Calc. for
C
13H9N3S
4. Results and discussion
(239.30) C, 65.19%; H, 3.76%; N, 17.55%; S, 13.39%. IR (KBr,
cmꢂ1):
m(C@N) 1576; m(NAN) 1073; m
(CAS) 764. 1H NMR
The compounds 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine
(1) and 2-(5-phenyl-1,3,4-thiadiazol-2-yl) pyridine (2) were ob-
tained by the reactions of 4-phenyl-1-(2-phenylacetyl) thiosemi-
carbazide and N0-benzothioylpicolinohydrazide, respectively,
with Mn(NO3)2ꢁ4H2O whereas compound 3 was obtained by the
reaction of 5-methyl-1,3,4-thiadiazole-2-thiol with Mn(OAc)2ꢁ4H2-
O via condensation of two moles of thiadiazole-2-thiol. The
Schemes 1–3 depict the formation of the compounds 1, 2 and 3.
All the compounds are air stable, non-hygroscopic crystalline sol-
ids, which are soluble in common organic solvents and can be kept
at room temperature over a prolonged period without any sign of
decomposition. The compounds 1, 2 and 3 melt at 220, 180 and
192 °C, respectively. Thus, the new route employed for the synthe-
ses of above compounds do not only impart good yields but is also
adequate for growing their crystals.
(DMSO-d6; d ppm): 7.20–8.68 (m, aromatic protons). 13C NMR
(DMSO-d6; d ppm): 197.55 (CAS), 165.40 (C@N), 125.26–143.18
(aromatic carbons).
2.2.3. Synthesis of 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-
1,3,4-thiadiazole (Mtmth, 3)
Mn(OAc)2ꢁ4H2O (0.125 g, 0.5 mmol) was added to a methanol–
acetonitrile suspension of 5-methyl-1,3,4-thiadiazole-2-thiol
(0.101 g, 1 mmol) and the reaction mixture refluxed for 2 h. The
clear solution obtained upon cooling was filtered off and kept for
crystallization. Colorless crystals of 3 suitable for X-ray analyses
were obtained by slow evaporation of the above solution over a
period of 10 days. Yield: 70%. m.p. 192 °C. Anal. Found: C,
31.24%; H, 2.60%; N, 24.33%; S, 41.66%. Calc. for C6H6N4S3

R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
255
Fig. 2. Intermolecular CAHꢁ ꢁ ꢁN and NAHꢁ ꢁ ꢁN interactions forming a supramolecular architecture.
Fig. 3. Optimized geometry and charge distribution on 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (1).
Fig. 4. MEP plot of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (1).
4.1. IR spectra
The IR spectra of the compounds 1 and 2 in KBr show absorp-
tions (cmꢂ1) due to the stretching modes of NH (3270, only for
1), C@N (1590, 1576), CAS (794, 764) and NAN (1097, 1073). The
bands at 764 and 677 cmꢂ1 in compound 2 are due to the pyridine
and phenyl rings, respectively. The IR spectrum of the compound 3
Fig. 5. Contour map of 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (1).
4.2. 1H and 13C NMR spectra
The 1H NMR spectrum of Bptha (1) exhibits two signals at d
11.48 and 3.34 ppm for the amine and CH2 protons, respectively
and the phenyl ring protons appear as a multiplet between d
7.90 and 6.86 ppm. The 13C spectrum of Bptha (1) shows signals
is expected to give rise to characteristic bands due to
(NAN), and
(CAS) which occur at 1602, 1065 and 812 cmꢂ1
respectively [17].
m(C@N),
m
m
,

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R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
Fig. 6. ORTEP diagram of 2-(5-phenyl-1,3,4-thiadiazol-2-yl)pyridine (2) showing atomic numbering scheme with ellipsoid of 30% probability level.
Fig. 7. Intermolecular CAHꢁ ꢁ ꢁN interactions and intramolecular CAHꢁ ꢁ ꢁN and CAHꢁ ꢁ ꢁS interactions in 2.
Fig. 8. Optimized and charge distribution on 2-(5-phenyl-1,3,4-thiadiazol-2-yl)pyridine.
at d 179.65, 168.35 and 121.68–144.32 ppm due to CAS, C@N and
phenyl ring carbons. The NMR data thus indicates that the com-
pound 5-benzyl-N-phenyl-1,3,4-thiadiazol-2-amine (1) is present
in the cyclic form. The 1H NMR spectrum of Pthp (2) exhibits sig-
nals for aromatic protons between d 7.20 and 8.68 ppm. The 13C
spectrum of 2 shows signals at d 197.55 and 165.40 ppm due to
CAS and C@N carbons, respectively. The phenyl and pyridine ring
carbons appear in the region of 125.26–143.18 ppm. The 1H NMR
spectrum of Mtmth (3) in DMSO-d6 shows signal at 2.50 ppm
due to methyl protons. The signals appearing at 190.31, 168.80
and 38.66 ppm in the 13C NMR spectrum of 3 are due to the ˜CAS,
˜C@N and CH3 carbons, respectively.
4.3. Crystal structure descriptions of 1, 2 and 3
The crystallographic data and structural refinement details for
compounds 1, 2 and 3 are given in Table 1. Selected bond distances
and bond angles are compiled in Tables 2, 4 and 6. Fig 1, 6 and 11

R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
257
the bond angle from 120 ° in the 1,3,4-thiadiazole ring is a com-
mon feature in five-membered rings. The values found for bond an-
gles and lengths measured for 1, 2 and 3 are similar to those
reported in related compounds [18,19]. The dihedral angle be-
tween the thiadiazole and the phenyl rings is found to be 43.85°
for compound 1 indicating that both rings are tilted with respect
to each other whereas for compound 2, the dihedral angle between
the thiadiazole and the phenyl rings is 5.05° and thiadiazole and
pyridine rings is 10.74° suggesting that the phenyl and pyridine
rings are almost coplanar to the thiadiazole ring. For compound
3, the dihedral angle between both thiadiazole rings is 1.48° indi-
cating that both rings are almost coplanar. Bond lengths and angles
in the phenyl ring and thiadiazole ring for compounds 1 and 2 are
generally normal. The geometrical parameters relevant for these
interactions are given in Tables 3, 5 and 7. In the solid state, the
compound 1 is stabilized via intermolecular CAHꢁꢁꢁN and NAHꢁꢁꢁN
interaction between thiadiazole ring nitrogen and CH2 and NH
hydrogen atoms substituted at 2 and 5 positions of the thiadiazole
ring, which leads to the formation of linear chain structure (Fig. 2).
In addition, the structure of compound 2 is stabilized by CAHꢁꢁꢁN
interaction occurring between thiadiazole ring nitrogen with
hydrogens of phenyl ring. The compound 2 is stabilized via inter-
molecular CAHꢁꢁꢁN interaction between thiadiazole ring and
hydrogen atoms of phenyl ring of a nearby molecule (Fig. 7. The
compound 3 is stabilized via intermolecular CAHꢁꢁꢁN interaction
between thiadiazole ring nitrogen and hydrogen atoms of methyl
group of a nearby molecule (Fig. 12). The two neighboring mole-
cules are also connected via SꢁꢁꢁN interaction occurring between
thiadiazole ring sulfur and nitrogen in the range of 3.110–
3.348 Å. The compound 2 also contains SꢁꢁꢁN as intramolecular
interaction. The above observed intermolecular and intramolecular
interactions have stabilized the molecular crystal packing of com-
pounds 1, 2 and 3.
Fig. 9. MEP plot of 2-(5-phenyl-1,3,4-thiadiazol-2-yl)pyridine.
Fig. 10. Contour map of 2-(5-phenyl-1,3,4-thiadiazol-2-yl)pyridine.
4.4. Geometry optimization of 1, 2 and 3
All calculations and the geometry optimization of 5-benzyl-N-
phenyl-1,3,4-thiadiazol-2-amine (1), 2-(5-phenyl-1,3,4-thiadiazol
-2-yl)pyridine (2) and 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-
methyl-1,3,4-thiadiazole (3) have been performed with Gaussian
03 and Gauss View 4.1 [20] program packages using DFT method
with functional B3LYP and basis set DFT/B3LYP/6-31+G(d,p)
[21,22]. The input geometries of compounds for the DFT calcula-
tions were generated from single crystal X-ray data. The optimized
geometrical parameters are listed in Tables 2, 4 and 6. The slight
disagreement in the bond lengths and angles are due to the fact
show the ORTEP diagrams of the compounds 1, 2 and 3 with their
atomic numbering scheme. The CAS, NAN and CAN bond distances
of compounds 1, 2 and 3 are shorter than the corresponding single
bond distance. This fact suggests multiple bond order and confers
aromaticity to the ring and CAS bond in thiadiazole ring involves
carbon atom with sp2 hybridization. This result is similar to those
found for 2,5-bis(2-pyridyl)-1,3,4-thiadiazole [18], 5-(4-hydroxy-
phenyl)-1,3,4-thiadiazole-2-thiol [19] and 4(5-ethyl sulfanyl-
1,3,4-thiadiazole-2-yl)-pyridinium perchlorate [11]. Deviation of
Fig. 11. ORTEP diagram of 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (3) showing atomic numbering scheme with ellipsoid of 30% probability.

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R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
Fig. 12. Intermolecular CAHꢁ ꢁ ꢁN and Sꢁ ꢁ ꢁN interactions and intramolecular Sꢁ ꢁ ꢁN interactions.
Fig. 13. Optimized and charge distribution on 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thiadiazole (3).
that the experimental results are for the solid phase and the theo-
retical calculations are for the gas phase. In the solid state, the exis-
tence of a crystal field along with the intermolecular interactions
connect the molecules together, which results in the differences
in bond parameters between the calculated and experimental val-
ues. The optimized geometries and charge distribution on atoms of
molecule are shown in Fig 3, 8 and 13.
4.5. Molecular electrostatic potential and contour maps
The molecular electrostatic potential (MEP) is a useful feature to
study reactivity given that an approaching electrophile will be at-
tracted to negative regions (the electron distribution where effect
is dominant). In the majority of the MEP, while the maximum neg-
ative region with preferred site for electrophilic attack indications
as red color, the maximum positive region which preferred site for
nucleophilic attack symptoms as blue color. The importance of
MEP lies in the fact that it simultaneously displays molecular size,
shape as well as positive, negative and neutral electrostatic poten-
tial regions in terms of color grading and is very useful in research
of molecular structure with its physicochemical property relation-
ship [23]. The resulting surface simultaneously displays molecular
size and shape and electrostatic potential value. MEP plot of com-
pounds 1, 2 and 3 are shown in Fig. 4, 9 and 14 respectively. The
MEP is a plot of electrostatic potential mapped onto the constant
electron density surface. The different values of the electrostatic
potential at the surface are represented by different colors. Poten-
tial increases in the order red < orange < yellow < green < blue. The
color code of these maps is in the range between ꢂ0.05431 a.u.
(deepest red) and 0.05431 a.u. (deepest blue) in compound, where
blue indicates the strongest attraction and red indicates the stron-
gest repulsion. Regions of negative V(r) are usually associated with
the lone pair of electronegative atoms. As can be seen from the
MEP map of the molecule, while regions having the negative
Fig. 14. MEP plot of 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-thia-
diazole (3).

R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
259
However, the H atoms at the CH3 group have smaller values than
on the H atoms in the ring. From this result, we can say that the
H atoms indicate the strongest attraction and N atoms indicate
the strongest repulsion.
A contour map is a two-dimensional XY plot of a three-dimen-
sional XYZ surface showing lines where the surface intersects
planes of constant elevation (Z). The contour maps are also used
to show lines of constant density or brightness, such as electro-
static potentials. The contour maps are also calculated by DFT/
B3LYP/6-31+G(d,p) method at the 0.02 isovalues and 0.004 density
values at the same level of calculations of the MEP mapped surface
of the molecules. The contour maps of the molecules are shown in
Fig 5, 10 and 15. The contour maps of the compounds show that
there is more electron density around the thiadiazole ring nitrogen.
4.6. Frontier molecular orbital analysis
Frontier molecular orbitals (FMOs) refer to the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) which are very popular quantum chemical param-
eters that determine the molecular reactivity and offer qualitative
prediction of the excitation properties and the ability of electron
transport [24,25]. The outermost higher energy orbital (HOMO)
acts as an electron donor while the lowest energy orbital (LUMO)
acts as an electron acceptor. The energies of these orbitals of the
molecule calculated by using DFT/B3LYP method are found to be
negative (Table 8), which indicate that the molecules are stable
[26]. The isodensity surface plots of HOMO and LUMO for the com-
pounds are shown in Fig. 16. The positive phases are red and the
negative ones are green. It can be seen from Fig. 16 that the elec-
tron density of HOMO in case of compound 1 is localized on the
thiadiazole and N-phenyl rings, while the LUMO is localized on
the entire three rings (Fig. 16). The electron density of HOMO in
case of compound 3 is localized on the thiadiazole ring and methyl
group, while the LUMO is not localized on the methyl group. On the
other hand, the HOMO–LUMO electron density for the compound 2
is located over the aryl group and thiadiazole ring. Compound 2
Fig. 15. Contour map of 2-(5-methyl-1,3,4-thiadiazole-2-ylthio)-5-methyl-1,3,4-
thiadiazole (3).
Table 8
Calculated Frontier Molecular Orbitals energies (eV) for the compounds 1, 2 and 3.
Compound Energies (Hartree)
HOMO
LUMO
D
E
Degree of
hardness
g
(Hartree) (eV)
1
2
3
ꢂ0.03693 ꢂ0.21556 0.17863
ꢂ0.08250 ꢂ0.23770 0.13520
ꢂ0.06061 ꢂ0.24451 0.18390
4.86077 2.43039
3.67898 1.83949
5.00417 2.50209
potential are over the electronegative atom (nitrogen atom), the
regions having the positive potential are over the hydrogen atoms.
A maximum positive region localized on the H atoms in the ring.
HOMO
LUMO
1
2
3
Fig. 16. Ground state isodensity surface plots for molecular orbitals of the compounds 1, 2 and 3.

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R.K. Dani et al. / Journal of Molecular Structure 1054–1055 (2013) 251–261
makes larger contribution to HOMO and LUMO than those of com-
pounds 1 and 3, due to the presence of thiadiazole ring acting as a
zole-2-thiol. The compounds are stabilized through intramolecular
and weak intermolecular hydrogen bonding. X-ray crystallographic re-
sults are well produced by DFT calculations. MEP mapped surfaces and
the contour maps of molecule are computed by same method. The MEP
and the contour map show the negative potential (red region) sites on
nitrogen atoms and positive potential (blue region) sites around the
hydrogen atoms. The HOMO and LUMO energies of the molecules are
negative indicating that the all compounds are stable. The lower energy
gap for the compound 2indicates better NLO properties as compared to
compounds 1 and 3.
p-conjugation bridge. The above results indicate that compound 2
has better chemical activity and may effectively carry out intramo-
lecular charge transfer upon excitation. The electronic transition
from the ground state to the excited state takes place due to a
transfer of electrons from the HOMO to LUMO levels and is mainly
associated with the
pꢁ ꢁ ꢁp transition. The chemical hardness and
softness of a molecule is a good indication of the chemical stability
of a molecule. From the HOMO_LUMO energy gap, one can find
whether the molecule is hard or soft. The molecules having a large
energy gap are known as hard and molecules having a small energy
gap are known as soft molecules. The soft molecules are more
polarizable than the hard ones, because they need small energy
for excitation. The hardness value of a molecule can be determined
by the formula [27]
Acknowledgements
Dr. S.K. Kushawaha is thankful to the Department of Science
and Technology, New Delhi, India for the award of a Young Scien-
tist Project (No. SR/FT/CS-110/2011). R.K. Dani thanks UGC, New
Delhi for the award of RGNF (SRF).
g
¼ fꢂeHOMO
þ eLUMOg=2
where eHOMO and eLUMO are the energies of the HOMO and LUMO
Appendix A. Supplementary material
molecular orbitals. The value of for the title compounds 1, 2
g
and 3 are 2.43039, 1.83949 and 2.50209 eV, respectively. From
the above values, it can be concluded that the compounds 1, 2
and 3 are hard materials and out of these, the compound 2 is a less
hard material than compounds 1 and 3.
CCDC 943021, 948324 and 943022 contain the supplementary
crystallographic data for compounds 1, 2 and 3. These data can
be obtained free of charge from the Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.molstruc.2013.09.051.
4.7. Non-linear optical (NLO) properties
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