Electronic properties of 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thiones: An experimental and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2009.05.049

The syntheses of 1-(4-methoxyphenyl)-4-methyl-1H-tetrazol-5(4H)-thione 1a, 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thione 1b, 1-(4-chlorophenyl)-4-methyl-1H-tetrazol-5(4H)-thione 1c, 1-methyl-4-(4-nitrophenyl)-1H-tetrazol-5(4H)-thione 1d were carried out and their electronic absorption spectra were obtained in cyclohexane, THF, and acetonitrile. The UV spectra of 1a-d showed a modest dependence on the polarity of the solvent. The change of substituent on the tetrazolethione ring from a strongly electron donating group (p-C₆H₄OMe, 1a), to a moderately electron donating (C₆H₅, 1b) to a weakly electron withdrawing group (p-C₆H₄Cl, 1c) also produced minimal effect on the electronic properties of 1a-c. However, the presence of a strongly electron withdrawing group (p-C₆H₄NO₂, 1d) on the heterocyclic ring produced a marked change in the UV spectrum. Time-dependent density functional calculations revealed that all the bands result from π → π* excitations with some degree of intramolecular charge transfer (ICT) within the molecules. Our studies further showed that as the acceptor strength is increased in the order: 1a (p-C₆H₄OMe) < 1b (C₆H₅) < 1c (p-C₆H₄Cl) < 1d (p-C₆H₄NO₂), the ICT also increases. In accordance with the experimental observations, the calculated transitions also showed modest dependence on the polarity of the solvent.

Full text of DOI:10.1016/j.molstruc.2009.05.049

Journal of Molecular Structure 933 (2009) 38–45
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Electronic properties of 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thiones:
An experimental and theoretical study
*
Sundeep Rayat , Radhika Chhabra, Olajide Alawode, Aditya S. Gundugola
Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses of 1-(4-methoxyphenyl)-4-methyl-1H-tetrazol-5(4H)-thione 1a, 1-methyl-4-phenyl-1H-
tetrazole-5(4H)-thione 1b, 1-(4-chlorophenyl)-4-methyl-1H-tetrazol-5(4H)-thione 1c, 1-methyl-4-(4-
nitrophenyl)-1H-tetrazol-5(4H)-thione 1d were carried out and their electronic absorption spectra were
obtained in cyclohexane, THF, and acetonitrile. The UV spectra of 1a–d showed a modest dependence on
the polarity of the solvent. The change of substituent on the tetrazolethione ring from a strongly electron
donating group (p-C6H4OMe, 1a), to a moderately electron donating (C6H5, 1b) to a weakly electron with-
drawing group (p-C6H4Cl, 1c) also produced minimal effect on the electronic properties of 1a–c. However,
the presence of a strongly electron withdrawing group (p-C6H4NO2, 1d) on the heterocyclic ring produced
a marked change in the UV spectrum. Time-dependent density functional calculations revealed that all
Received 14 March 2009
Received in revised form 29 May 2009
Accepted 29 May 2009
Available online 6 June 2009
Keywords:
UV/Vis spectroscopy
Density functional calculations
Charge transfer
Frontier molecular orbitals
Tetrazolethiones
the bands result from
p ? p* excitations with some degree of intramolecular charge transfer (ICT) within
the molecules. Our studies further showed that as the acceptor strength is increased in the order: 1a (p-
C6H4OMe) < 1b (C6H5) < 1c (p-C6H4Cl) < 1d (p-C6H4NO2), the ICT also increases. In accordance with the
experimental observations, the calculated transitions also showed modest dependence on the polarity
of the solvent.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
in situ photogeneration of carbodiimides that subsequently react
with carboxylic acids to form deep UV resists [10]. Therefore, the
Widespread applications of tetrazolic compounds and their
derivatives in industry [1–13], agriculture [14] and medicine [15]
have stimulated interest in the study of their structure and reactiv-
ity. Several reports have appeared in literature that discuss the pho-
tochemical properties of these interesting ring systems [16–20]. For
instance, the photodecomposition of 5-thio substituted tetrazoles,
i.e. tetrazolethiones is known to proceed via three pathways involv-
ing (1) the expulsion of dinitrogen to form azridinethiones, (2) the
cleavage of the ring to form isothiocyanates and azides, and (3) the
concurrent expulsion of dinitrogen and sulfur to form carbodii-
mides (Scheme 1) [21]. It has been reported that photoproducts
from the parent tetrazoles can be influenced by the nature of the
substituents present on the ring [22–28], however, the effect of
substituents on the photodecomposition of their 5-thio derivatives
is not known. We were interested in exploring whether differences
in the electronic properties of various tetrazolethiones could allow
one pathway to be favored over the other. Insights into the elec-
tronic properties of these ring systems could subsequently be used
to design new compounds that would decompose only by the de-
sired pathway resulting in the formation of needed photoproducts.
For instance, tetrazolethione scaffolds have been proposed for the
ability to drive the photodecompositon of tetrazolethiones in the
direction of carbodiimide formation would be of interest in the
use of these ring systems for photolithographic applications.
In this paper, we present the experimental and theoretical
investigations of the electronic properties of a series of 1-methyl-
4-phenyl-1H-tetrazole-5(4H)-thiones. We synthesized 1-(4-meth-
oxyphenyl)-4-methyl-1H-tetrazol-5(4H)-thione 1a, 1-methyl-4-
phenyl-1H-tetrazole-5(4H)-thione 1b, 1-(4-chlorophenyl)-4-methyl-
1H-tetrazol-5(4H)-thione 1c, and 1-methyl-4-(4-nitrophenyl)-1H-
tetrazol-5(4H)-thione 1d (Scheme 2). Since, the substituent and
solvent effects on the spectral properties of organic compounds
are well known [29–35], the substituents on the tetrazolethione
ring were chosen to study the effect of a strongly electron donating
(p-C6H4OMe, 1a), moderately electron donating (C6H5, 1b), weakly
electron withdrawing (p-C6H4Cl, 1c) and a strongly electron with-
drawing group (p-C6H4NO2) on the electronic properties of tetra-
zolethiones, and the absorption spectra 1a–d were obtained in
solvents of varying polarity e.g. cyclohexane, tetrahydrofuran
(THF) and acetonitrile. Density functional methods were employed
to obtain the ground state geometries of 1a–d and the frontier
molecular orbitals were analyzed to gain insights into their elec-
tronic structure. Since, the time-dependent density functional the-
ory (TDDFT) has been widely employed to determine the excitation
energies of organic compounds, and the polarized continuum
* Corresponding author. Tel.: +1 785 532 6660.
E-mail address: sundeep@ksu.edu (S. Rayat).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.05.049

S. Rayat et al. / Journal of Molecular Structure 933 (2009) 38–45
39
glycol (PEG) was added to the matrix and sample to provide brac-
keting calibration ions. High resolution electrospray ionization
spectra were acquired on a time of flight mass spectrometer. The
instrument was operated at 10,000 resolution (W mode) with dy-
namic range enhancement that attenuates large intensity signals.
The cone voltage was 60 eV. Spectra were acquired at 16666 Hz
pusher frequency covering the mass range from 100 to 1200 amu
and accumulating data for 2 s per cycle. Mass correction for the ex-
act mass determination was carried out automatically with the
lock mass feature in the MassLynx data system.
Scheme 1. Products formed during the irradiation of matrix-isolated 1-methyl-1H-
tetrazole-5(4H)-thione [21].
2.1.1. 1-(4-Methoxyphenyl)-1H-tetrazol-5(4H)-one (3a) [42,43]
Trimethylsilyl azide (TMSA) (0.773 g, 6.7 mmol) was added to
4-methoxyphenylisocyanate (2a) (0.5 g, 3.35 mmol). The mixture
was refluxed at 100 °C for 24 h. The unreacted TMSA was removed
under reduced pressure and the white residue was dried under
vacuum. Compound 3a (0.581 g, 90% yield) was obtained as a
white solid: Rf 0.45 (40:60 hexane:ethylacetate); 1H NMR
(400 MHz, DMSO): d 3.81 (s, 3H, OCH3), 7.10 (d, J = 8.98 Hz, 2H),
7.71 (d, J = 9.17 Hz, 2H); 13C NMR (400 MHz, DMSO): d 55.5,
114.5, 122.0, 127.1, 150.5, 158.5. LRMS (ESI): m/z 210 (M+NH4+),
193(M+H+), 165, 150, 137, 134, 122, 110.
Similarly, compound 3c was prepared [42].
2.1.2. 1-(4-Chlorophenyl)-1H-tetrazol-5(4H)-one (3c)
Compound 3c (1.63 g, 64% yield) was obtained from 4-chloro-
phenylisocyanate (2c) (2.0 g, 13.02 mmol) as a white solid which
was recrystallized from ethanol: Rf 0.55 (40:60 hexane:ethylace-
tate); 1H NMR (400 MHz, DMSO): d 6.80 (d, J = 9.16 Hz, 2H), 7.07
(d, J = 9.15 Hz, 2H); 13C NMR (400 MHz, DMSO): d 121.0, 129.5,
131.7, 133.1, 150.1. LRMS (ESI): m/z 214 (M+NH4+), 197 (M+H+),
173, 169, 158, 141, 134.
Scheme 2. Tetrazolethiones 1a–d.
model (PCM) that considers the effect of solvent on the vertical
excitation energies has become quite popular [36–41], we em-
ployed TDDFT with the integral equation formalism of the PCM
model to determine the nature of electronic transitions that give
rise to bands in the UV spectra of 1a–c. For 1d, a combination of
TDDFT and CIS (Configuration Interaction Singles) approach was
used to gain insights into the absorption spectra. This is the first
study that attempts to assign the UV spectral bands of tetrazolethi-
ones by employing computational methods.
2.1.3. 5-Chloro-1-phenyl-1H-tetrazole (5) [44]
A mixture of sodium azide (3.0 g, 46.15 mmol) and tetrabutyl-
ammonium bromide (0.650 g, 2.02 mmol) in water (8.7 mL) was
added to a solution of phenyl-1,1-dichloroisocyanide (4) (5.0 g,
28.73 mmol) in toluene (42.4 mL). The mixture was stirred at room
temperature and monitored by TLC until the starting material dis-
appeared. After 2.5 h the aqueous layer was saturated with NaCl
and the organic layer was extracted with toluene thrice and dried
over Na2SO4. The organic layer was concentrated under reduced
pressure. The flask was then kept at ꢀ5 °C until the crystals of 5
started to appear (approximately 2–5 min) which were recovered
by filtration. The supernatant was further concentrated, cooled to
ꢀ5 °C to recover more crystals. The latter process was repeated
twice. Compound 5 (3.8 g, 73% yield) was obtained as white crys-
talline material: Rf 0.3 (80:20 hexane:ethylacetate); 1H NMR
(400 MHz, CDCl3): d 7.58–7.65 (m, 5H, Ph); 13C NMR (400 MHz,
CDCl3): d 124.7, 130.1, 131.1, 133.0, 145.7. LRMS (ESI): m/z 198
(M+NH4+), 181 (M+H+), 153, 138, 118, 92, 77.
2. Experimental and computational details
2.1. General procedures
Thin layer chromatography was carried out on 250
plates and UV-light was used as a visualizing agent. Standard col-
umn chromatography was performed using 63–200 m silica gel.
lm silica gel
l
1H and 13C NMR spectra were recorded on 400 MHz and
200 MHz spectrometers as indicated. The carrier frequencies were
199.98 MHz (1H) and 50.29 MHz (13C) for the 200 MHz and
399.78 MHz (1H) and 100.53 MHz (13C) for 400 MHz spectrome-
ters, respectively. Number of scans used was 32 for 1H and for
13C NMR spectra, it ranged from 3–5 K depending on the sample
concentration. Both the 1H and 13C spectra were recorded with
longer relaxation time (10 s). Chemical shifts and the coupling con-
stants are reported in parts per million and Hertz, respectively. The
infrared frequencies are reported in cmꢀ1. Low resolution mass
spectra were obtained on a mass spectrometer equipped with an
electrospray ion source operated in the positive ion mode and con-
nected to a triple quadrupole mass analyzer. Collision-induced dis-
sociation of mass selected ions was performed using nitrogen as a
target gas at a collision energy of 25 eV. Fast atom bombardment
mass spectrometry experiments were performed using an argon
gun operated at 6 KeV and 0.8 mA emission. Nitrobenzyl alcohol
was used as the matrix. Exact mass FAB experiments were carried
out at 10 k resolution using linear voltage scans under data system
control in multi-channel analyzer mode (MCA). Polyethylene
2.1.4. 1-Phenyl-1H-tetrazol-5(4H)-one (3b) [45]
5 (3.7 g, 20.49 mmol) was added to a mixture of 2 M aqueous
NaOH (55.7 mL) and ethanol (18.6 mL). The mixture was refluxed
at 70 °C until the starting material disappeared (approximately
2 h). The reaction mixture was cooled in ice that resulted in the
separation of a side product as a white solid that was removed
by filtration. Acidification of the filtrate with concentrated HCl
(pH 3–5) afforded a thick white precipitate that was filtered,
washed with water and air dried. Compound 3b (3.15 g, 95% yield)
was obtained as a white solid: Rf 0.22 (40:60 hexane:ethylacetate);
1H NMR (400 MHz, DMSO): d 7.42 (t, J = 7.51 Hz, 1H), 7.56 (t,
J = 7.60 Hz, 2H), 7.85 (d, J = 7.68 Hz, 2H); 13C NMR (400 MHz,
DMSO): d 119.5, 127.515, 129.4, 134.1, 150.2. LRMS (ESI): m/z
180 (M+NH4+), 163 (M+H+), 135, 120, 107, 92, 80.

40
S. Rayat et al. / Journal of Molecular Structure 933 (2009) 38–45
2.1.5. 1-(4-Nitrophenyl)-1H-tetrazol-5(4H)-one (3d) [45]
20 h at 110 °C until the starting material disappeared. The reaction
mixture after filteration was dried over Na2SO4 and concentrated
under reduced pressure. Purification by column chromatography
(SiO2, hexane:EtOAc, 9:1) gave 1a (0.243 g, 75% yield) as a white
crystalline solid: Rf 0.56 (60:40 hexane:ethylacetate); mp 134–
135 °C; IR (KBr) 543, 561, 742, 825, 1028, 1041, 1080, 1114,
1191, 1256, 1302, 1326, 1369, 1446, 1514, 1591, 1612, 2840,
Fuming HNO3 (3.5 mL) was added to 3b (1.0 g, 6.2 mmol). The
mixture was refluxed at 100 °C for 30 min during which formation
of a yellow solid was observed. The reaction mixture was poured
over ice and cooled for 2 h to afford a thick yellow solid which
was filtered, washed with plenty of water and air dried. Compound
3d (0.749 g, 59% yield) was obtained as a yellow solid: Rf 0.18
(20:80 hexane:ethylacetate); IR (KBr) 587, 688, 704, 720, 751,
861, 1032, 1055, 1112, 1337, 1352, 1375, 1502, 1512, 1598,
2926, 2965, 2998, 3076 cmꢀ1 1H NMR (200 MHz, CDCl3): d 7.82
.
(d, J = 9.16 Hz, 2H), 7.05 (d, J = 9.16 Hz, 2H), 3.98 (s, 3H), 3.88 (s,
3H); 13C NMR (200 MHz, CDCl3): d 35.2, 55.8, 114.6, 125.7, 127.8,
160.5, 163.9. HRMS (ESI): exact mass calculated for (M+H+)
C9H11N4OS+ = 223.0654, found 223.0645.
1611, 1742, 3090, 3121, 3270 cmꢀ1 1H NMR (200 MHz, DMSO): d
.
8.19 (d, J = 9.17 Hz, 2H), 8.43 (d, J = 9.15 Hz, 2H); 13C NMR
(400 MHz, DMSOC): d 119.0, 125.1, 139.2, 145.4, 150.0. LRMS
(ESI): m/z 225 (M+NH4+), 208 (M+H+), 165, 121, 105, 91.
Similarly, Compounds 1b–d were synthesized [47].
2.1.6. 1-(4-methoxyphenyl)-4-methyl-1H-tetrazol-5(4H)-one (6a)
[43,46]
2.1.11. 1-Methyl-4-phenyl-1H-tetrazole-5(4H)-thione (1b)
Synthesis of 1b was carried out from 6b (4.0 g, 22.7 mmol). The
refluxing time in P2S5 was 48 h. Purification by column chromatog-
raphy (SiO2, hexane:EtOAc, 88:12) gave 1b (2.3 g, 53% yield) as a
yellow solid: Rf 0.77 (40:60 hexane:ethylacetate); 1H NMR
(400 MHz, DMSO): d 3.91 (s, 3H, CH3) 7.68-7.55 (m, 3H), 7.87 (d,
J = 7.14 Hz, 2H); 13C NMR (400 MHz, DMSO): d 34.7, 124.3, 129.3,
129.8, 134.4, 163.2. HRMS (ESI): exact mass calculated for
(M+H+) C8H9N4S+ = 193.0548, found 193.0545.
A solution of dimethyl sulfate (0.40 g, 2.08 mmol) and methy-
lene chloride (5 mL) was added to
a mixture of 3a (0.4 g,
2.08 mmol), tetrabutylammonium bromide (0.13 g, 0.40 mmol),
20% NaOH (15 mL) and methylene chloride (10 mL). The reaction
mixture was stirred at room temperature for 3 h. The organic layer
was separated and washed with water thrice to remove tetrabutyl-
ammonium bromide and dried over Na2SO4, concentrated under
reduced pressure and dried under vacuum. Compound 6a (0.30 g,
91% yield) was obtained as a white crystalline solid: Rf 0.51
(40:60 hexane:ethylacetate); mp 98–99 °C; IR (KBr) 511, 574,
729, 744, 824, 1028, 1048, 1155, 1252, 1357, 1424, 1519, 1618,
2.1.12. 1-(4-chlorophenyl)-4-methyl-1H-tetrazole-5(4H)-thione (1c)
Synthesis of 1c was carried out from 6c (0.4 g, 2.0 mmol). The
refluxing time in P2S5 was 20–22 h. Purification by column chro-
matography (SiO2, hexane:EtOAc, 93:7) gave 1c (0.36 g, 84% yield)
as white crystalline solid: Rf 0.62 (60:40 hexane:ethylacetate); mp
137–139 °C; IR (KBr) 544, 571, 718, 828, 1016, 1080, 1095, 1201,
1368, 1389, 1494, 1590, 1721, 2855, 2924, 2953, 3077,
1721, 2847, 2945, 2975, 3011, 3088 cmꢀ1 1H NMR (400 MHz,
.
CDCl3): d 3.70 (s, 3H, CH3), 3.86 (s, 3H, OCH3), 7.01 (d, J = 9.17 Hz,
2H), 7.80 (d, J = 9.17 Hz, 2H); 13C NMR (200 MHz, CDCl3): d 31.6,
55.7, 114.7, 121.6, 128.0, 149.6, 159.2. HRMS (ESI), exact mass cal-
culated for (M+Na+) C9H10N4O2Na+ = 229.0702, found 229.0712.
Similarly, compounds 6b–d were prepared [46].
3097 cmꢀ1 1H NMR (200 MHz, CDCl3) d: 3.98 (s, 3H, CH3), 7.53
.
(d, J = 9.15 Hz, 2H), 7.99 (d, J = 8.80 Hz, 2H); 13C NMR (200 MHz,
CDCl3) d: 35.0, 124.7, 129.6, 133.4, 135.5, 163.6. HRMS (FAB), exact
mass calculated for (M+H+) C8H8ClN4S+ = 227.0158, found
227.0161.
2.1.7. 1-Methyl-4-phenyl-1H-tetrazol-5(4H)-one (6b)
Compound 6b (0.30 g, 91% yield) was obtained from 3b (0.27 g,
2.15 mmol) as a crystalline colorless solid: Rf 0.81 (40:60 hex-
ane:ethylacetate); 1H NMR (400 MHz, DMSO): d 3.62 (s, 3H, CH3),
7.44 (t, J = 7.40 Hz, 1H), 7.58 (t, J = 7.03 Hz, 2H), 7.85 (d,
J = 8.27 Hz, 2H); 13C NMR (400 MHz, DMSO): d 31.2, 119.4, 127.7,
129.5, 134.2, 148.8. HRMS (ESI), exact mass calculated for
(M+Na+) C8H8N4ONa+ = 199.0596, found 199.0602.
2.1.13. 1-Methyl-4-(4-nitrophenyl)-1H-tetrazole-5(4H)-thione (1d)
Synthesis of 1d was carried out from 6d (0.561 g, 2.54 mmol).
The refluxing time in P2S5 was 24 h. Purification by column chro-
matography (SiO2, hexane:EtOAc, 9:1) gave 1d (0.303 g, 50% yield)
as a yellow crystalline solid: Rf 0.73 (40:60 hexane:ethylacetate);
mp 141–143 °C; IR (KBr) 549, 568, 684, 702, 749, 854, 1038,
1068, 1109, 1328, 1340, 1365, 1494, 1529, 1593, 1614, 2853,
2.1.8. 1-(4-chlorophenyl)-4-methyl-1H-tetrazol-5(4H)-one (6c)
Compound 6c (0.47 g, 88% yield) was obtained from 6c (0.5 g,
2.54 mmol) as a white solid: Rf 0.62 (40:60 hexane:ethylacetate);
mp 91–93 °C; IR (KBr) 512, 575, 708, 729, 832, 839, 1014, 1092,
1138, 1387, 1423, 1496, 1597, 1732, 2854, 2928, 2958, 3054,
2963, 3093, 3120 cmꢀ1 1H NMR (200 MHz, DMSO): d 3.92 (s, 3H,
.
CH3), 8.33 (d, J = 9.42 Hz, 2H), 8.49 (d, J = 8.42 Hz, 2H); 13C NMR
(200 MHz, DMSO): d 34.8, 124.6, 124.8, 139.2, 147.2, 163.2. HRMS
(FAB): exact mass calculated for (M+H+) C8H8N5O2S+ = 238.0399,
found 238.0387.
3106 cmꢀ1 1H NMR (200 MHz, CDCl3) d: 3.72 (s, 3H, CH3), 7.47 (d,
.
J = 8.80 Hz, 2H), 7.93 (d, J = 8.80 Hz2H); 13C NMR (400 MHz, CDCl3):
d 31.7, 120.5, 129.8, 133.4, 133.5, 149.3. HRMS (ESI), exact mass cal-
culated for (M+Na+) C8H7ClN4ONa+ = 233.0206, found 233.0215.
2.2. Computational details
All calculations were carried out with Gaussian 03 package of
programs [48]. B3LYP functional was used which combines Becke’s
three parameter exchange functional with the correlation func-
tional of Lee, Yang, and Parr [49,50]. All calculations employed 6-
311+G* and basis sets. All the geometry optimizations were fol-
lowed by vibrational analyses to ensure the positive sign of all
eigenvalues of the Hessian matrix and to confirm that the station-
ary point found was a true minimum on the potential energy sur-
face. Vertical excitation energies were computed using time-
dependent density functional theory (TDDFT) [51] at optimized
geometries. In the case, where TDDFT failed (1d), Configuration
Interaction Singles (CIS) [52,53] was employed to determine the
nature of electronic transitions at RHF geometries. The solvent ef-
fects on the ground state geometries and excitation energies were
2.1.9. 1-Methyl-4-(4-nitrophenyl)-1H-tetrazol-5(4H)-one (6d)
Compound 6d (0.719 g, 96% yield) was obtained from 6d
(0.71 g, 3.38 mmol) as a yellow-brown solid: Rf 0.6 (20:80 hex-
ane:ethylacetate); 1H NMR (200 MHz, DMSO): d 3.64 (s, 3H, CH3),
8.20 (d, J = 9.16 Hz, 2H), 8.45 (d, J = 9.15 Hz, 2H); 13C NMR
(400 MHz, DMSO): d 31.3, 119.0, 125.3, 139.2, 145.6, 148.6. HRMS
(ESI): exact mass calculated for (M+H+) C8H8N5O3+ = 222.0627,
found 222.0634.
2.1.10. 1-(4-methoxyphenyl)-4-methyl-1H-tetrazole-5(4H)-thione
(1a) [47]
P2S5 (0.76 g, 3.42 mmol) was added to a solution of 6a (0.3 g,
1.46 mmol) in dry toluene (10 mL). The mixture was refluxed for

S. Rayat et al. / Journal of Molecular Structure 933 (2009) 38–45
41
considered using the integral equation formalism of the polarized
continuum model (IEFPCM). Electronic structures were analyzed
using Natural Population Analysis with the inclusion of solvent ef-
fects. The molecular orbitals were visualized using GaussView.
3. Results and discussion
3.1. Synthesis of 1,4-disubstituted tetrazolethiones 1a–d
The 1-(4-methoxyphenyl)-1H-tetrazol-5(4H)-one (3a) and 1-
(4-chlorophenyl)-1H-tetrazol-5(4H)-one (3c) were synthesized by
the treatment of corresponding isocyanates 2a and 2c with trime-
thysilylazide [42]. 1,3-dipolar cycloaddition of phenyl-1,1-dichlo-
roisocyanide (4) with sodium azide resulted in the formation of
5-chloro-1-phenyl-1H-tetrazole (5) [44]. 5 was converted to tet-
razolone 3b on treatment with NaOH [45]. 3b was nitrated to give
3d by a method previously carried out on a similar compound [45].
3a–d were methylated to yield 1,4-disubstituted tetrazolones 6a–d
[46] which on treatment with P2S5 yielded the 1,4-disubstituted
tetrazolethiones 1a–d (Scheme 3) [47].
3.2. Absorption spectra
The absorption spectra of 1a–d were recorded in cyclohexane,
THF, and acetonitrile. The compounds 1a–c exhibited four bands,
k1, k2, k3 and k4 in cyclohexane and MeCN, however, k4 could not
be observed in THF due to solvent interference (Fig. 1). 1d exhib-
ited only three bands k1, k2, and k4 (Fig. 2). The latter could not
be observed in THF. The extinction coefficient for k4 of 1c and 1d
could not be determined in cyclohexane as the absorption spec-
trum changed in this region at low concentration.
Fig. 1. Experimental absorption spectra of 1a in cyclohexane (bottom), tetrahy-
drofuran (middle) and acetonitrile (top).
In case of 1a–c, k1 and k2 underwent small blue-shifts as the
polarity of the solvent was increased from cyclohex-
ane ? THF ? MeCN. k3 showed a slight red shift in 1a while a blue
shift was observed for 1b and 1c as the solvent polarity increased.
k4 of 1a and 1c showed a blue shift and that of 1b underwent a red
shift from cyclohexane ? THF ? MeCN. In 1d, k1 was slightly blue
shifted with increasing solvent polarity while k2 was unaffected. k4
of 1d underwent a blue shift from cyclohexane to MeCN. The UV
spectra of 1d in cyclohexane also produced shoulders to k1 and
k2 at 341 and 244 nm, respectively. The absorption spectra of 1a–
d showed no significant dependence on the polarity of the solvent
(Table 1, Fig. 1).
The effect of changing the substituent on the tetrazolethione
ring from strongly electron donating, p-C6H4OMe group (1a) to
moderately electron donating, C6H5 group (1b) produced a red
shift in the position of k1 in cyclohexane and THF (5 nm and
8 nm, respectively), and a slight blue shift in MeCN (1–2 nm). k2 re-
mained unchanged in cyclohexane and THF, however, it was red
shifted in MeCN (a shift of 3 nm and 5 nm, respectively). Both k3
and k4 showed a blue shift from 1a to 1b in cyclohexane, THF
and acetonitrile. On replacing C6H5 (1b) with a weakly electron
withdrawing, p-C6H4Cl group (1c), we observed red shift in the po-
sition of k1 (3–12 nm), k2 (1–2 nm), k3 (5–8 nm) and k4 (3–8 nm) in
all solvents. Notably, the presence of a strongly electron withdraw-
ing, p-C6H4NO2 group on the tetrazolethione (1d) had a significant
effect on the position of k1 and k2. The former was substantially red
shifted (ꢁ34–40 nm) and the latter showed a blue shift (9–17 nm)
relative to 1c in the solvents considered. However, k4 produced a
small blue shift from 1c to 1d in cyclohexane and THF. We con-
clude that there was a modest effect on the absorption spectra of
tetrazolethione upon changing the substituent from p-C6H4OMe
to C6H5 to p-C6H4Cl, however, the presence of p-C6H4NO2 produced
Scheme 3. Synthesis of tetrazolethiones 1a–d.

42
S. Rayat et al. / Journal of Molecular Structure 933 (2009) 38–45
[18,19,43]. The dihedral angle between the tetrazolethione ring
and the phenyl ring was between 33.5°–47.0°. The ground state di-
pole moment in the gas phase is calculated to be 2.5, 1.1, 2.0 and
5.6 D for 1a–d, respectively. The relatively high dipole moment
in 1d clearly indicates unsymmetrical charge distribution in the
ground state.
The NBO analysis reveal that 1a–d are highly polarized in the
ground state. The sum of the charges on N1, N2, N3, N4, C5 and
S13 of the tetrazolethione ring, and the C14, H15, H16, and H17 of
the methyl group in the presence of various solvents is shown in
Table 2 (Refer to Scheme 2 for atom numbering). This fragment be-
comes more positive on going from 1a to 1b to 1c to 1d which indi-
cates that the increase in the acceptor strength from p-C6H4OMe
(1a) < C6H5 (1b) < p-C6H4Cl (1c) < p-C6H4NO2 (1d) may facilitate
an intramolecular charge transfer of the type shown in Scheme 4.
The latter is also manifested in the dihedral angle, between the tet-
razolethione and the aromatic ring, that decreases from 1a
(47.0°) > 1b (43.3°) > 1c (41.2°) > 1d (33.5°) indicating an approach
to planarity (Fig. 3).
3.3.2. Frontier molecular orbitals
The electronic structure of 1a–d was analyzed in the presence of
cyclohexane, THF and acetonitrile. The isodensity plots of the fron-
tier molecular orbitals obtained at B3LYP/6-311+G* in acetonitrile
are depicted in Fig. 1s (Appendix A) and they all exhibit
p-type
symmetry. The HOMO-3 for 1a–c and HOMO-2 of 1b and 1d are
exclusively localized over the aromatic ring while the HOMO-2 of
1a and 1c and HOMO-3 of 1d are delocalized over the entire mol-
ecule. The HOMO-1 is mainly localized on the tetrazolethione ring
and has major contribution from the C5–S13
p bond (Refer to
Fig. 2. Experimental absorption spectra of 1a–d in acetonitrile.
Scheme 2 for the numbering of atoms). The HOMOs of 1a–c are
delocalized over the entire molecule showing some bonding con-
tributions from the aromatic ring while the HOMO of 1d is almost
identical to HOMO-1 and is localized on the heterocycle. All
a marked change in the positions of k1 and k2, while k4 was unaf-
fected (Table 1, Fig. 2).
HOMOs have significant contribution from C5–S13
p bond.
3.3. Theoretical calculations
The LUMOs exhibit a bonding character at the N1–C7, C8–C9 and
C11–C12 bonds that supports the resonance structures 1a–d0
(Scheme 4). The LUMO of 1d has major contribution from the aro-
matic ring. The LUMO+1 of 1a–d is mainly localized over the tetra-
3.3.1. Ground state geometries
To gain insights into the electronic properties of 1a–d, we per-
formed DFT calculations employing 6-311+G*. The optimized
ground state geometries of 1a–d at B3LYP/6-311+G* and their cor-
responding bond lengths are shown in Fig. 3. The molecules display
C1 symmetry which is supported by their crystal structures
zolethione ring, while the LUMO+2 is a
p* orbital exclusively
localized over the aromatic ring. LUMO+3 of 1a–d is delocalized
over the entire molecule.
Table 1
Energies (E (k)/eV (nm)) and extinction coefficients (log e
/Mꢀ1 cmꢀ1) for the peaks observed in the absorption spectra of 1a–d in cyclohexane, tetrahydrofuran and acetonitrile.
Cyclohexane
THF
MeCN
T(k)
log
e
T(k)
log
e
T(k)
log e
1a
k1
k2
k3
k4
4.34 (286.0)
4.63 (267.5)
5.46 (227.0)
6.23 (199.0)
3.81
3.96
4.03
4.31
4.39 (282.3)
4.69 (264.1)
5.44 (228.0)
3.77
3.89
4.03
4.47 (277.4)
4.99 (254.2)
5.40 (230.0)
6.29 (197.0)
3.86
4.03
4.08
4.46
1b
k1
k2
k3
k4
4.26 (291.0)
4.65 (267.0)
5.63 (221.0)
6.42 (193.0)
3.82
3.97
4.02
4.06
4.27 (290.0)
4.69 (264.1)
5.58 (222.0)
3.79
3.94
4.03
4.49 (276.1)
4.78 (259.4)
5.84 (217.0)
6.36 (195.0)
3.79
3.95
4.12
4.42
1c
k1
k2
k3
k4
4.18 (297.0)
4.63 (268.0)
5.41 (229.0)
6.61(201.0)
3.85
3.91
4.47
4.22 (294.0)
4.68 (265.0)
5.46 (227.0)
3.76
3.82
4.23
4.34 (286.0)
4.75 (261.0)
5.51 (225.0)
6.26 (198.0)
3.97
4.05
4.19
4.47
1d
k1
k2
k4
3.75 (331.0) 3.63 (341.3)
4.94 (251.0) 5.08 (244.0)
6.26 (198.0)
3.83
4.14
3.72 (333.2)
4.93 (252.0)
3.88
4.24
3.80 (326.4)
4.95 (250.4)
6.39 (194.0)
3.66
4.09
4.34

S. Rayat et al. / Journal of Molecular Structure 933 (2009) 38–45
43
Fig. 3. B3LYP/6-311+G** optimized ground state structures 1a–d showing bond lengths and the dihedral angle (in red) between the tetrazolethione ring and the phenyl ring
(Cartesian coordinates are given in Appendix A).
density functional calculations were carried out and thirty low ly-
ing singlet excited states of 1a–d were calculated. The vertical exci-
tation energies, corresponding wavelengths, oscillator strengths,
MO character and the transition type for the most intense transi-
tions calculated at TDDFT/6-311+G* in cyclohexane, THF and MeCN
are reported in Tables 2s–5s (Appendix A).
The analysis of TDDFT wavefunction of 1a indicates that k1
which is experimentally observed at 4.34, 4.39 and 4.47 eV in
cyclohexane, THF and MeCN (Table 1) corresponds to a transition
from HOMO-1 or HOMO to the LUMO and is theoretically calcu-
lated to be at 4.10, 4.16 and 4.19 eV, respectively (Table 1s). k2
which is experimentally found at 4.63, 4.69 and 4.99 eV in cyclo-
hexane, THF and MeCN is comprised of two or three excited states
consisting of transitions from HOMO-2 to LUMO, HOMO-1/HOMO
to LUMO+1 and HOMO to LUMO+2. k2 is calculated at 4.75, 4.71
and 4.70 eV by TDDFT in cyclohexane, THF and MeCN, respectively.
k3 is predicted at 5.52, 5.72 and 5.73 eV and experimentally ob-
served at 5.46, 5.44 and 5.40 eV in cyclohexane, THF and acetoni-
trile. This band is formed of many excited states consisting of
transitions from several MOs of similar energy including HOMO-
Table 2
Sum of the NBO charges on the individual atoms of the tetrazolethione ring and the
methyl group of 1a–d at B3LYP/6-311+G*.
C6H12
THF
MeCN
1a
1b
1c
1d
ꢀ0.288
ꢀ0.276
ꢀ0.267
ꢀ0.233
ꢀ0.293
ꢀ0.283
ꢀ0.272
ꢀ0.236
ꢀ0.294
ꢀ0.286
ꢀ0.272
ꢀ0.237
Scheme 4. Resonance structures of 1a–d.
The order of the molecular orbitals shown was unaffected in
3 ? LUMO/LUMO+1,
HOMO-2 ? LUMO+2,
and
HOMO-1/
cyclohexane or THF, except that the HOMO-2 and HOMO-3 of 1d
in acetonitrile changed to HOMO-3 and HOMO-2 in cyclohexane,
respectively. The HOMO-1 ? LUMO and HOMO ? LUMO gaps are
not notably different for 1a–c in the three solvents (Table 3). Over-
all, a stabilization of the frontier molecular orbitals is observed as
the acceptor strength increases in the order: 1a (p-C6H4OMe) < 1b
(C6H5) < 1c (p-C6H4Cl) < 1d (p-C6H4NO2). The most dramatic effect
is on the LUMO of 1d which is significantly stabilized in all solvents
that results into a substantial narrowing of the HOMO-1 ? LUMO
and HOMO ? LUMO gaps of 1d as compared to 1a–c.
HOMO ? LUMO+3. k4 appears experimentally at 6.23 and 6.29 eV
in cyclohexane and acetonitrile and is calculated by TDDFT to be
at 6.57 and 6.59 eV, respectively, consisting of transition from
HOMO-3 ? LUMO+2.
Similarly, the experimental and predicted values of k1, k2, k3 and
k4 for 1b and 1c in the three solvents are given in Table 1, Tables
2s–3s. Note that with the exception of k1, all bands are composed
of several excited states consisting of transitions between several
molecular orbitals of comparable energy.
Furthermore, the analyses of the MOs indicate that all bands ob-
served in the UV spectra of 1a–c are
p ? p* in nature. k1 is accom-
3.3.3. Vertical excitation energies
In order to determine the nature of the electronic transitions
that give rise to bands in the UV spectra of 1a–d, time-dependent
pained by intramolecular charge transfer (CT) from the heterocycle
to the phenyl ring of respective compounds 1a–c; note that both
HOMO-1 and HOMO are localized on the tetrazolethione while
LUMO is delocalized over the entire molecule. Similarly, k2 and k3
correspond to a
some degree of charge transfer within the molecule. k4 is a
* transition localized on the phenyl ring in 1a–c.
TDDFT calculated k2 and k4 of 1d at 4.97 and 6.45 eV in cyclo-
hexane (Table 4s), which were experimentally observed at 4.94
and 6.26 eV (Table 1). These correspond to a * transition
p ? p* transition which are also associated with
Table 3
HOMO/HOMO-1 ? LUMO gaps in 1a–d calculated at B3LYP/6-311+G* in eV.
p
? p
HOMO ? LUMO
HOMO-1 ? LUMO
C6H12
THF
MeCN
C6H12
THF
MeCN
p
? p
1a
1b
1c
1d
4.73
4.71
4.63
3.50
4.77
4.80
4.71
3.47
4.79
4.84
4.74
3.47
4.87
4.79
4.71
3.58
4.99
4.91
4.82
3.55
5.05
4.96
4.88
3.55
delocalized over the entire molecule. k2 results from the excita-
tions from HOMO-1/HOMO to LUMO+3 while k4 involved promo-
tion of an electron from HOMO-3 to LUMO+2/LUMO+3. However,
the TDDFT wavefunction failed to predict k1. Note that 1d contains

44
S. Rayat et al. / Journal of Molecular Structure 933 (2009) 38–45
a strongly electron withdrawing (p-C6H4NO2) group and we expect
k1 to be a charge transfer band (the phenyl ring of 1d is a typical
example of a donor–acceptor substituted aromatic ring which is
susceptible to efficient intramolecular charge transfer). Since the
charge transfer is inadequately described by TDDFT employing
the current exchange–correlation functionals [54], we believe that
this level of theory is unable to correctly predict k1 of 1d. In order
to confirm the charge transfer nature of k1, we calculated vertical
excitation energies at CIS/6-311+G*. The MOs calculated at CIS
were similar to that of TDDFT. Although, CIS overestimated the en-
ergy of k1 (5.63 eV), it correctly predicted the charge transfer nat-
ure of k1 that results from excitation from HOMO-3/HOMO-1/
HOMO to LUMO. Note that HOMO-3 is delocalized over the entire
molecule and, HOMO-1 and HOMO are exclusively localized on the
tetrazolethione ring, while the LUMO is localized on the aromatic
ring and the nitro group supporting the resonance structure 1d’
shown in Scheme 4.
dence on the solvent, we suggest that the solvent polarity will min-
imally affect the photophysical properties of tetrazolethiones. Our
calculations also reveal that transitions to the second or higher ex-
cited states (S2, S4, S6, etc.) are characterized by oscillator strengths
that are orders of magnitude higher than the oscillator strength of
the first excited state (S1). This supports that the higher excited
states (S2, S4, S6, etc.) may play a crucial role in the photochemistry
of 1a–d.
Our results further suggest that the effect of substituents on the
electronic properties of tetrazolethiones is modest. Future studies
will be focused on changing the position of the substituent on
the aromatic ring of 1a–d, and evaluating the effect on the elec-
tronic properties of tetrazolethiones.
Acknowledgements
The authors acknowledge Professor Christine Aikens for valu-
able discussions during the preparation of this paper, Kansas State
University and Terry C. Johnson Cancer Center for providing finan-
cial support for this project, the National Center for Supercomput-
ing Applications at University of Illinois at Urbana-Champaign for
the computational resources and the Kansas Lipidomics Research
Center for providing the mass spectra on representative
compounds.
Due to the similar reasons, TDDFT is unable to predict k1 of 1d in
THF and acetonitrile while it correctly calculates k2 and k4. Again,
we employed CIS to confirm the charge transfer nature of k1 in
these solvents. The experimental and predicted values are depicted
in Table 1 and Table 4s.
3.3.4. Intramolecular charge transfer
In accordance with the experimental observations, the calcu-
lated transitions show modest dependence on the polarity of the
solvent. From the examination of the molecular orbitals (Fig. 1s,
Appendix A), it is further evident that as the electron accepting
ability of the group attached to the tetrazolethione is increased
in the order: 1a (p-C6H4OMe) < 1b (C6H5) < 1c (p-C6H4Cl) < 1d (p-
C6H4NO2), the CT of the type shown in Scheme 4 is facilitated.
Appendix A. Supplementary data
Molecular orbitals for B3LYP optimized geometries of 1a–d in
acetonitrile (Fig. 1s); Experimental absorption spectra and TDDFT
calculated transitions for 1a–d in cyclohexane, THF and acetoni-
trile (Figs. 2s–5s); TDDFT vertical excitation energies, oscillator
strengths, MO character and transition type of 1a–d in the three
solvents (Tables 1s–4s); 1H NMR and 13C NMR spectra of 1a, 1c–
d, and Cartesian coordinates of optimized 1a–d at B3LYP/6-
311+G* and of 1d at RHF/6-311+G*. Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/
j.molstruc.2009.05.049.
The latter involves promotion of
a electron from HOMO-1/
HOMO ? LUMO and is most pronounced in 1d due to the delocal-
ization of the negative charge on to the nitro group (note that
HOMO-1 and HOMO are exclusively localized on the heterocycle,
while the LUMO is mostly localized on p-C6H4NO2 group and has
bonding characteristics at the N1–C7, C8–C9 and C11–C12 bonds).
The significant red shift of the CT band (k1) in the calculated and
observed UV spectra of 1d as opposed to 1a–c, is due to the sub-
stantial stabilization of the LUMO of 1d (Table 3). In 1b and 1c,
charge transfer from HOMO-1/HOMO ? LUMO is clear as the
HOMO-1 and HOMO are exclusively localized on the heterocycle,
while the LUMO is delocalized over the entire molecule, however,
it is certainly not of the same magnitude as in 1d. In 1a, the charge
transfer is minimal (HOMO-1/HOMO ? LUMO) and can only be
noticed by the bonding nature of the N1–C7 bond.
Note that the absorption spectra of related heterocycles, 1-
substituted-tetrazolones and 1-substituted-tetrazolethiones have
previously been reported and is known to produce two bands
[55–57]. The presence of an intramolecular CT in the UV spectra
of these compounds was proposed, however, not further investi-
gated [56]. Therefore, this study provides insights into the elec-
tronic structure of 1-substituted-tetrazolones and 1-substituted-
tetrazolethiones involving an intramolecular CT.
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