Structure, spectroscopy, and thermal decomposition of 5-chloro-1,2,3,4-thiatriazole: A He i photoelectron, infrared, and quantum chemical study

Article abstract of DOI:10.1007/s11224-015-0655-z

5-Chloro-1,2,3,4-thiatriazole has been investigated in the gas phase for the first time by mid-infrared and He I photoelectron spectroscopy. The ground-state geometry has been obtained from quantum chemical calculations at the CCSD(T) and B3LYP levels using aug-cc-pVTZ basis set. Ionization potentials have been determined and the electronic structure has been discussed within the frame of molecular orbital theory. IR and photoelectron spectroscopies, supported by quantum chemical calculations at the B3LYP and SAC-CI levels, provide a detailed investigation into the vibrational and electronic character of the molecule. Thermal stability of 5-chloro-1,2,3,4-thiatriazole has been studied both experimentally and theoretically. Flash vacuum thermolysis of the molecule produces fast quantitatively N₂, ClCN, and sulfur. Theoretical calculations at the CCSD(T)//B3LYP level predict competitive decomposition routes, starting either with a retro-cycloaddition reaction leading to N₂S and ClCN or with a ring opening to chlorothiocarbonyl azide intermediate, to produce finally N₂, S, and ClCN. Calculations also predict that N₂S is reactive and decomposes in bimolecular reactions to N₂ and S₂.

Full text of DOI:10.1007/s11224-015-0655-z

Struct Chem
DOI 10.1007/s11224-015-0655-z
ORIGINAL RESEARCH
Structure, spectroscopy, and thermal decomposition
of 5-chloro-1,2,3,4-thiatriazole: a He I photoelectron,
infrared, and quantum chemical study
1
1
1
2,3
Tibor Pasinszki
•
D a´ niel Dzsotj a´ n
•
G a´ bor Vass
•
Jean-Claude Guillemin
Received: 27 July 2015 / Accepted: 30 July 2015
Ó Springer Science+Business Media New York 2015
Abstract 5-Chloro-1,2,3,4-thiatriazole has been investi-
gated in the gas phase for the first time by mid-infrared and
He I photoelectron spectroscopy. The ground-state geometry
has been obtained from quantum chemical calculations at the
CCSD(T) and B3LYP levels using aug-cc-pVTZ basis set.
Ionization potentials have been determined and the elec-
tronic structure has been discussed within the frame of
molecular orbital theory. IR and photoelectron spectro-
scopies, supported by quantum chemical calculations at the
B3LYP and SAC-CI levels, provide a detailed investigation
into the vibrational and electronic character of the molecule.
Thermal stability of 5-chloro-1,2,3,4-thiatriazole has been
studied both experimentally and theoretically. Flash vacuum
thermolysis of the molecule produces fast quantitatively N2,
ClCN, and sulfur. Theoretical calculations at the CCSD(T)//
B3LYP level predict competitive decomposition routes,
starting either with a retro-cycloaddition reaction leading to
Keywords Structure Á Thermolysis Á IR Á UPS Á Ab
initio Á DFT
Introduction
1,2,3,4-Thiatriazoles are well-known heterocyclic com-
pounds, with a history going back to the nineteenth century
[1–3]. A wide range of organic derivatives have been pre-
pared following the 1950s [3–7]. Thiatriazoles are known to
be thermally unstable and decompose upon mild heating or
even at room temperature to sulfur, nitrogen, and nitriles.
Their thermal stability, however, strongly depends on the
substituent attached to the ring carbon atom. Aryl and sub-
stituted amino- and thio-derivatives are relatively stable and
have found applications due to their interesting biological
properties, including fungicidal, antitubercular, antiviral,
anticancer, and muscle stimulant activity [7]. Alkyl, aralkyl,
and alkoxy derivatives are unstable, isolated only at low
temperature, and often present explosion danger at ordinary
conditions [3–7]. The only known halogen derivative, the
N S and ClCN or with a ring opening to chlorothiocarbonyl
2
azide intermediate, to produce finally N , S, and ClCN.
2
Calculations also predict that N S is reactive and decom-
2
poses in bimolecular reactions to N and S .
2
2
5-chloro-1,2,3,4-thiatriazole is highly explosive [8]. It has
been used in nucleophilic reactions for the synthesis of
substituted amino- and alkoxy-thiatriazoles [6–8]. Very little
is known experimentally about the structure of unstable
thiatriazoles, and 5-chloro-1,2,3,4-thiatriazole has never
been studied by any experimental or theoretical methods to
date. Structure and properties of a few relatively stable aryl-,
amino-, and thio-derivatives have been studied by X-ray
crystallography, NMR, IR, UV, and Raman spectroscopy
(see reviews [6, 7] and references cited therein).
The paper is dedicated to Magdolna Hargittai on the occasion of her
0th birthday.
7
&
Tibor Pasinszki
pasinszki@chem.elte.hu
1
2
Institute of Chemistry, E o¨ tv o¨ s Lor a´ nd University,
Budapest 112, P.O. Box 32, 1518, Hungary
´
Ecole Nationale Sup e´ rieure de Chimie de Rennes, CNRS,
In this work, we present the gas-phase characterization of
5-chloro-1,2,3,4-thiatriazole molecule, the study of its
decomposition in the gas phase, and an investigation of its
electronic and geometric structure by quantum chemical
UMR 6226, 11 All e´ e de Beaulieu, CS 50837,
3
5708 Rennes Cedex 7, France
3
Universit e´ europ e´ enne de Bretagne, Rennes, France
123

Struct Chem
methods and gas-phase spectroscopy. The latter includes He
I ultraviolet photoelectron spectroscopy (UPS) and mid-in-
frared (IR) spectroscopy.
functions were checked, and both wave functions were found
to be stable. The CCSD T1 diagnostic, using the
CCSD(T) geometry, was 0.019, and single point
CASSCF(10,10) calculations indicated that apart from the
main HF configuration (weight 89 %), there was no other
important configuration (weight for any other configuration
was smaller than 3 %). Harmonic and anharmonic vibra-
tional wave numbers were calculated at the B3LYP level,
and infrared intensities were calculated using the harmonic
force field. Vertical ionization energies (IEs) were calculated
using the symmetry adapted cluster/configuration interac-
tion (SAC-CI) and the Outer Valence Green’s Function
Experimental procedure
5-Chloro-1,2,3,4-thiatriazole was synthesized and evaporated
for gas-phase investigations by adapting a known literature
procedure [8], as follows. Temperatures of all equipments
(flasks and separating funnels) and solvents were kept at 0 °C
during the synthesis, and an inert nitrogen atmosphere was
used. 2 g (30.8 mmol) of sodium azide was dissolved in 50 ml
of water, and the solution was cooled down to 0 °C. 3.54 g
(
OVGF) methods using the geometry obtained at the
CCSD(T) level. Lowest energy paths for decomposition of
-chloro-1,2,3,4-thiatriazole were calculated at the
(
30.8 mmol) of thiophosgene was added to the solution drop-
wise in 30 min, and the suspension was stirred for additional
h. The reaction mixture was extracted two times with 25 ml
5
CCSD(T)//B3LYP level. The minima and the connecting
lowest energy paths between minima were calculated at the
B3LYP level using an intrinsic reaction coordinate (IRC)
approach which was also manually checked by proceeding
along the given reaction coordinate and simultaneously
relaxing all other bond lengths and angles. Stability check
was performed for all calculated structures. In order to obtain
the total energies, single point energy calculations were done
on top of B3LYP geometries at the CCSD(T) level. Gibbs
free energies (G) were obtained by correcting the
CCSD(T) total energy with zero-point vibrational energy
2
of cold diethyl ether, and the combined organic phase was dried
over anhydrous magnesium sulfate for at least an hour. Drying
agent was filtered off on a pre-cooled funnel. The solution is
transferred into a flask, and the ether solvent is removed in
vacuum at 0 °C. The flask is then connected via a vacuum
stopcock to a vacuum system (practically to spectrometers) and
pumped for about 3 h to remove all traces of side products,
unreacted thiophosgene, and water, while keeping the tem-
perature of the flask at 0 °C.
The thermolysis of gaseous 5-chloro-1,2,3,4-thiatriazole
(ZPE) and thermal corrections calculated at the B3LYP
was carried out in a quartz tube (6 mm i.d.) heated along
level. DG80K values, for example, represent energy differ-
ence between ZPE-corrected total energies.
3
0 cm using an electrical furnace. The effluent from the tube
leddirectlyintotheIRcellorphotoelectronspectrometer. The
distance between furnace and detection point was 40 cm.
All calculations were done using the aug-cc-pVTZ basis
set. Only valence electrons were correlated in
CCSD(T) and SAC-CI calculations. All calculations were
performed with the GAUSSIAN-09 quantum chemistry
package [10]. References to original theoretical methods
are listed in the program package manual [11]. For char-
acterization of the normal vibrational modes of 5-chloro-
-
1
The IR spectrum (resolution 1.0 cm ) of gaseous
-chloro-1,2,3,4-thiatriazole was recorded on a Bruker IFS
8 FTIR spectrometer equipped with a 22-cm single-pass
5
2
glass cell. The cell, with KBr windows, gave a spectral range
-
1
from 400 to 4000 cm . The effluent from the sample con-
tainer was pumped continuously through the cell using a
rotary vacuum pump while maintaining the temperature of
the container at 0 °C and the pressure in the cell at 0.3 mbar.
The He I ultraviolet photoelectron spectrum (UPS) of the
gaseous thiatriazolederivative and itspyrolysisproductswere
recorded using an Atomki ESA-32 photoelectron spectrom-
eter described in detail elsewhere [9]. Photoelectron spectra
wererecordedusingtheconstanttransmissionenergymodeof
the electron energy analyzer and were calibrated with the
1,2,3,4-thiatriazole, the total energy distribution (TED),
which provides a measure of the internal coordinate con-
tributions, was determined [12, 13].
Results and discussion
Calculated equilibrium structure and stability
?
2
Ar ( P
) spin–orbit doublet. The resolution of the ana-
/2,1/2
3
Calculated structural data of 5-chloro-1,2,3,4-thiatriazole
are presented in Table 1 and the structure and numbering
of atoms are shown in Fig. 1. CCSD(T) and B3LYP results
are in good agreement with each other, the largest differ-
2
lyzer was 30 meV (fwhm for the Ar P3 line).
/2
Computational details
˚
ence in bond length and bond angles is 0.014 A and 0.7°,
The geometry of the ground-state neutral 5-chloro-1,2,3,4-
thiatriazole molecule was calculated using the CCSD(T) and
B3LYP methods. The stability of HF and B3LYP wave
respectively. According to calculations, the molecule is
planar, with CS symmetry, and has singlet electronic
ground state. The singlet ground state is more stable than
1
23

Struct Chem
Table 1 Calculated equilibrium structure of 5-chloro-1,2,3,4-
thiatriazole
˚
Bond lengths/A
Bond angles/°
S
1
–N
2
1.710 (1.713)
1.281 (1.267)
1.367 (1.353)
1.309 (1.301)
1.715 (1.715)
1.706 (1.707)
S
1
C
5
N
4
113.3 (112.7)
110.2 (110.9)
116.8 (117.0)
110.9 (110.7)
88.9 (88.6)
N
N
N
2
3
4
–N
–N
3
4
5 4 3
C N N
N
N
N
2
3
2
N
3 4
N
–C
5
N
2 1
S
S
1
–C
5
S
1
C
5
Fig. 1 Structure of 5-chloro-1,2,3,4-thiatriazole and numbering of
atoms
C
5
–Cl
6
S
C
1 5
Cl
6
123.8 (124.2)
Calculated at the CCSD(T) and B3LYP (in parenthesis) levels using
the aug-cc-pVTZ basis set. See Fig. 1 for numbering of atoms
?
7?8 ? 9?10), respectively. These latter molecules
either decompose in a consecutive step via ClCNS or
1
directly to N2, S, and ClCN or isomerize to ClSCN.
-
1
the lowest energy triplet excited state by 282 kJ mol
o
(
DG ) at the B3LYP level. Bond orders of 5-chloro-
0
K
ClSCN, the thermodynamically most stable pseudohalide
isomer, is possibly the best candidate for identification
among the decomposition products of 5-chloro-1,2,3,4-thi-
atriazole. The overall barrier at the rate-determining step for
the retro-cycloaddition and azide routes is very similar, 110,
1
,2,3,4-thiatriazole ring have been calculated by comparing
the calculated bond lengths of the thiatriazole with those of
molecules having typical single/double CN, NN, CS, and
˚
NS bonds [H C–NH (1.464 A)/H C=NH (1.264 A), H N–
˚
NH2 (1.433 A)/trans-HN=NH (1.235 A), H C–SH
˚
3
2
2
2
˚
3
-1
o
˚ ˚ ˚
1.830 A)/H C=S (1.611 A) and H N–SH (1.733 A)/HN=S
2 2
˚
1.570 A), calculated at the B3LYP level], using the Gor-
112, and 104 kJ mol
(DG298K) at TSs 2, 13, and 9,
(
(
respectively, thus these decomposition routes are competi-
tive. Barriers are relatively low and explain thermal insta-
bility for 5-chloro-1,2,3,4-thiatriazole at room and elevated
temperatures.
dy’s rule [14]. N –N and N –C bonds, nominally double
5
2
3
4
bonds, are between a single and a double bonds (bond order
.81 and 1.78, respectively), and S –N , N –N , and C –S
1
1
1
2
3
4
5
N S is expected to be one of the key intermediates in the
2
bonds, nominally single bonds, are shorter than a S–N, N–
N, or C–S single bond (bond order 1.11, 1.35, and 1.48,
respectively). This tendency to bond order equalization is
in agreement with the expected aromaticity of thiatriazoles
decomposition of thiatriazoles, and it is a crucial question
whether we can identify this molecule among pyrolysis
[
6, 7]. This corroborates with the calculated nucleus inde-
Table 2 Energetics of the decomposition of 5-chloro-1,2,3,4-
thiatriazole
pendent chemical shift values of NICS(0) = -11.1 and
NICS(1) = -11.2 (B3LYP/aug-cc-pVTZ). Negative NICS
values indicate aromaticity. The S –N bond of the mole-
o
o
Number
Description
ClCN
DG298K (DG0K)
1
2
1
3
S
0 (0)
110 (113)
-55 (-14)
97 (167)
74 (75)
cule is close to a single bond, the weakest of ring bonds
concerning bond orders, this being the point of cleavage
upon thermal ring opening to the corresponding chloroth-
iocarbonyl azide.
2
TS (1 $ 3)
3
NNS ? ClCN
NN ? S ? ClCN
TS (1 $ 6)
4
5
Thermal stability is a key issue for thiatriazoles, and the
unimolecular stability is determined by the lowest energy
path leading to bond dissociation or isomerization. Ener-
getics of the lowest energy decomposition paths for
6
ClC(S)N
TS (6 $ 8)
ClC(S)N
3
33 (36)
7
77 (78)
8
3
43 (47)
5
-chloro-1,2,3,4-thiatriazole are summarized in Table 2 and
9
TS (8 $ 10)
ClC(NS) ? NN
TS (10 $ 12)
ClCNS ? NN
TS (6 $ 14)
ClNCS ? NN
TS (14 $ 10)
ClSCN ? NN
TS (14 $ 16)
104 (110)
-99 (-55)
40 (85)
shown in Fig. 2. Considering the initiating step, decompo-
sition starts either with a retro-cycloaddition reaction lead-
10
11
12
13
14
15
16
17
ing to N S and ClCN via transition state 2 or by a ring
2
-98 (-56)
112 (118)
-146 (-101)
18 (62)
opening to chlorothiocarbonyl azide via TS 5. N S
2
1
decomposes in a second step to singlet sulfur atom, S( D),
and nitrogen molecule (1 ? 2?3 ? 4, see Fig. 2). The
formation of ground-state triplet sulfur, S( P), in this process
3
-221 (-175)
19 (64)
is spin forbidden. Chlorothiocarbonyl azide, 6, can isomer-
ize with rotation around the C–N bond to azide 8, and both
azide decompose with N2 loss to chloroisocyanate
Calculated at the CCSD(T)//B3LYP/aug-cc-pVTZ level. Energies (in
-1
kJ mol ) are relative to that of the 5-chloro-1,2,3,4-thiatriazole. See
Fig. 2
(1 ? 5?6 ? 13 ? 14) and chlorothiazirine (1 ? 5?6
123

Struct Chem
Fig. 2 Decomposition of
-chloro-1,2,3,4-thiatriazole to
5
1
2
ClCN, N , and S. Gibbs free
energies are relative to that of
the 5-chloro-1,2,3,4-
thiatriazole. Calculated at the
CCSD(T)//B3LYP/aug-cc-
pVTZ level
products of 5-chloro-1,2,3,4-thiatriazole in this work (see
below). N S has been identified among gas-phase ther-
2
molysis products of 5-phenyl-1,2,3,4-thiatriazole previ-
ously [15, 16]. However, its identification in the gas phase
is based on a special experimental setup, namely on a very
short distance between furnace and detection point,
allowing limited time for bimolecular reactions. N S, an N-
2
sulfide, is very reactive and has been shown to decompose
in bimolecular reactions in the gas phase to N and S [15].
2
2
To obtain information about the bimolecular reaction of
N S, calculations have been performed at the CCSD(T)//
2
B3LYP level (see Fig. 3 and Table 3). In principle,
bimolecular reactions between all possible species should
be taken into account; however, N-sulfides and thiazirine
are expected to be the most reactive and the only short-
lived species in our experiment [16, 17]. Therefore, we
focus on these reactions. The lowest energy bimolecular
decomposition path for two N S molecules is found to
2
proceed via S–S bond formation, where two molecules of
N S produce two molecules of nitrogen and singlet S
2
2
directly in a ‘‘tail to tail’’ (NNS…SNN) reaction after
Fig. 3 Bimolecular decomposition of N S, ClCNS, and ClC(NS),
2
o
and the structure of transition states. Gibbs free energies are relative
to that of reacting species. Calculated at the CCSD(T)//B3LYP/aug-
cc-pVTZ level
passing over a kinetic energy barrier of DG
= 76 -
2
98K
-
1
o
-1
kJ mol (DG = 46 kJ mol ). There is no intermediate
0
K
in this reaction, and the two N–S bonds simultaneously
break, releasing nitrogen and S . The kinetic energy barrier
2
been calculated (Fig. 3). For comparison, the ‘‘tail to tail’’
reaction between two ClCNS molecules has also been
computed. We note that this latter reaction was investi-
gated by us earlier at the B3LYP/6-31G** level [18] and
results are in agreement with the present work. Calcula-
tions predict similar decomposition mechanism for all of
these investigated bimolecular reactions, with the
of this reaction is relatively small, which clearly explains
the reactivity and instability of N S at room temperature if
2
molecules interact with each other. Chloronitrile sulfide
and chlorothiazirine, ClCNS and ClC(NS), are also
expected to be reactive and to take part in sulfur atom
transfer reactions similarly to N S [16, 17], thus bimolec-
2
ular reactions between these molecules and N S have also
2
1
23

Struct Chem
Table 3 Energetics of the
bimolecular decomposition of
o
o
o
o
Reactants
TS: DG298K (DG0K
)
Products
P: DG298K (DG0K)
2
N S, ClCNS, and ClC(NS)
1
? S
1
? ClCN ? S
1
? ClCN ? S
N
N
N
2
2
2
S ? N
2
S
76 (46)
53 (21)
55 (19)
68 (36)
2 N
2
2
-296 (-261)
-253 (-218)
-252 (-220)
-211 (-176)
S ? ClCNS
N
2
2
2
2
a
S ? ClC(NS)
N
1
2 ClCN ? S
ClCNS ? ClCNS
2
-
1
Calculated at the CCSD(T)//B3LYP/aug-cc-pVTZ level. Energies (in kJ mol ) are relative to that of the
reacting species
TS transition state, P products. See Fig. 3
a
298K
An intermediate is located on the singlet potential energy surface with a small barrier of DG-
o
-1
o
-1
1
=
24 kJ mol (DG0K = 26 kJ mol ) to decomposition to ClCN and S
2
formation of S , nitrogen, and/or ClCN. The kinetic energy
2
barrier for all of these processes is small, between 53 and
o
-
1
6
8 kJ mol
(DG 98K). Calculations thus predict that
2
experimental observation of these species at room or higher
temperatures requires the prevention of bimolecular reac-
tions; e.g., working pressures and contact time should be
kept low.
Gas-phase IR spectrum
The IR spectrum of gaseous 5-chloro-1,2,3,4-thiatriazole is
shown in Fig. 4, with the experimental and calculated
vibrational wave numbers (including calculated IR inten-
sities and the TED) listed in Table 4. The molecule has
nonlinear planar structure; thus, it has twelve normal
modes of vibration, nine of which are in the molecular
Fig. 4 Gas-phase IR spectrum of 5-chloro-1,2,3,4-thiatriazole
0
00
plane (a ) and three are out-of-plane (a ). All vibrational
modes are infrared active. The calculated wave numbers
and IR intensities are in good agreement with experiment
and support the band assignments. The calculated values
indicate that ten of the fundamentals should give rise to IR
out-of-plane ring deformations, possessing C-type bands,
1
-
have very small IR intensity (1 km mol , see Table 4),
they could be identified by their prominent Q branch at 642
-
1
and 543 cm . Detailed assignment of the IR spectrum is
given in Table 4, and the simplified assignment below is
based on the major internal coordinate contribution. Total
energy distribution (TED) of the normal vibrational modes
indicates that vibrations are strongly mixed.
-
1
bands above the 400 cm cutoff of the instrument used in
this experiment; however, one of them (m ) is not observed
due to low IR intensity.
8
The asymmetry parameter j of 5-chloro-1,2,3,4-thiatri-
azole, calculated using the computed B3LYP rotational
constants, is -0.78, thus the molecule is a prolate asym-
metric rotor (q* = 2.28 and b = 2.03). The experimental
5-Chloro-1,2,3,4-thiatriazole exhibits no fundamental IR
-
bands above 1400 cm , and the most characteristic fin-
1
gerprint of the molecule comprises the two medium and the
-1
strong intensity bands at 1375, 1226, and 1107 cm ,
0
fundamentals of a symmetry are of A-type, B-type, or
0
0
A/B-hybrid type, while those of a symmetry modes are
C-type bands with pronounced Q-branches. Based on
molecular constants above and equations published previ-
ously [19], the calculated PR separations for pure A-, B-,
corresponding to C=N and N=N ring stretches and one of
the ring in-plane deformations, respectively. The second
ring in-plane deformation (m
and may be assigned to the very weak band at 1043 cm
According to calculations, weak bands at 897, 707, and
) has very small IR intensity,
4
-
1
.
-
1
and C-type bands are 13, 10, and 19 cm , respectively.
The PQR structure is clearly observed on almost all IR
bands in the gas-phase IR spectrum of 5-chloro-1,2,3,4-
thiatriazole. The experimental PR separations for all A-,
-
1
611 cm can be assigned to the N–N, C–S, and S–N ring
stretches, respectively. There are two weak IR bands in the
-
1
spectrum at 1146 and 1008 cm whose assignment is
ambiguous. Based on calculations, they are assigned to
combination bands (see Table 4).
-
1
B-, and A/B-type bands are in the range of 10–12 cm , in
good agreement with the predicted separations. Although
123

Struct Chem
-1
Table 4 Experimental and calculated vibrational wave numbers (cm ) of 5-chloro-1,2,3,4-thiatriazole
a
b,c
Calc.
b,d
e
Exp.
Int.
Assignment and description
TED
0
0
0
0
0
0
1
1
1
1
1
1
8
7
6
6
5
375 Q m
226 Q m
146 Q w
107 Q s
1367 (1400) a
1251 (1304) a
1166 (1184) a
1076 (1099) a
1003 (1037) a
982 (1022) a
46
82
m
m
m
m
m
m
m
m
m
m
m
m
m
m
1
2
5
3
4
7
5
6
C = N ring st
N = N ring st
C
5
N
4
st(79), N
2 3
N st(12)
2 3
N N st(86)
9
? m ?
142
1
ip ring def
ip ring def
r.b.(40), C st(17), S
Cl
5 6
1 5
C st(15)
043 (?) vw
008 Q, w
97 w
3 4
r.b.(60), N N st(33)
8
? m ?
0
900 (916) a
11
4
N–N ring st
C–S ring st
N
3
N
4
st(58), r.b.(14)
0
07 w
681 (697) a
1 5
S C st(55), r.b.(38)
0
0
0
0
0
0
0
0
42 Q (?) vw
11 Q w
641 (662) a
563 (586) a
551 (557) a
427 (436) a
266 (268) a
237 (240) a
1
10 oop ring def
S–N ring st
11 oop ring def
oop r.t.(93)
st(85), C
oop r.t.(93), C
19
1
7
S
1
N
2
5
Cl
6
st(14)
wag(35)
43 Q (?) vw
5
Cl
6
n.o.
n.o.
n.o.
0.01
2
8
9
Cl–C st
C
5
Cl
6
5
6
st(48), r.b.(20), S
bend(84), S st(11)
wag(58), oop r.t.(35)
1 2
N st(10)
ip ClC bend
Cl
6
C
1 5
C
0
1
12 oop ClC wag
C
5
Cl
s strong, m medium, w weak, v very. st stretching, r.b. ring bend, r.t. ring torsion, wag wagging, oop out-of-plane, ip in-plane, def deformation
a
b
12
Gas phase. Position of the most intense Q-band or the band center is given. Calculated at the B3LYP/aug-cc-pVTZ level. Isotopes: C, N,
14
35
32
c
Cl, S. Asymmetric top parameters: j = - 0.7823, r = 17.3707. Anharmonic vibrational wave numbers. Harmonic wave numbers are in
parenthesis. In km mol . Calculated using the harmonic force field. Total vibrational energy distribution from force field analysis based on
harmonic force constants. Contributions larger than 10 % are provided
d
-1
e
He I photoelectron spectrum
The He I photoelectron spectrum of 5-chloro-1,2,3,4-thia-
triazole, together with calculated molecular orbital plots, is
shown in Fig. 5. Experimental and calculated ionization
energies are listed in Table 5. We note that 5-phenyl-
1
,2,3,4-thiatriazole is the only thiatriazole derivative whose
photoelectron spectrum was published to date [15], but
assignment of the spectrum was not provided. The
assignment of the photoelectron spectrum of 5-chloro-
1
,2,3,4-thiatriazole below is based on SAC-CI and OVGF
calculations, and on the comparison with known spectra of
relevant five-membered thiadiazoles [20, 21].
The ground-state electronic structure of 5-chloro-
1
,2,3,4-thiatriazole is A . The sequence of molecular
1
1
0
2
00
2
0
2
0
2
orbitals (MOs) deduced are …(7a ) (1a ) (8a ) (9a ) (2-
0
0
2
0
2
0
2
0
2
00
2
00
2
a ) (10a ) (11a ) (12a ) (3a ) (4a ) . A possible starting
point to describe the electronic structure is to consider the
MOs of a five-membered aromatic ring, modified with an
exocyclic chlorine atom. Therefore, three p orbitals, three
nitrogen ‘‘lone pair’’ orbitals (n ), and one sulfur ‘‘lone
N
pair’’ orbital (n ) can be deduced from the thiatriazole
S
moiety as low IE MOs, as well as five high IE r orbitals
corresponding to five r bonds of the ring. These MOs are
augmented, and mix to some extent, with orbitals of the
chlorine atom attached to the thiatriazole frame. Chlorine
Fig. 5 He I photoelectron spectrum of 5-chloro-1,2,3,4-thiatriazole
and schematics of the corresponding MOs
1
23

Struct Chem
Table 5 Experimental and
calculated vertical ionization
energies (eV) of 5-chloro-
b
Experimental
SAC-CI
OVGF
Orbital character
a
0
0
1
1
1
1
1
1
1
0.52
10.44 [0.95 (12a ), 0.11 (10a )]
0
10.99
11.20
11.80
11.56
12.91
14.05
13.90
16.01
16.32
16.91
r (n
N
)
)
1
,2,3,4-thiatriazole
0
00
1.4
10.90 [- 0.93 (4a ), -0.27 (3a )]
0
p
3
0
1.8
11.08 [0.89 (11a ), -0.34 (10a )]
00
r (n
N
0
0
2.88
3.8
11.35 [0.93 (3a ), -0.28 (4a )]
0
p
2
0
0
12.58 [0.83 (10a ), 0.36 (11a ), -0.28 (9a )]
0
r (nCl
r (n
r (nCl
r (n
)
0
0
5.6
13.48 [0.88 (9a ), 0.32 (10a ), 0.17 (7a )]
0
N
)
0
6.8
13.86 [0.96 (2a )]
)
0
0
0
1
1
1
5.61 [- 0.91 (8a ), 0.21 (7a ), -0.15 (9a )]
0
S
)
0
6.09 [- 0.93 (1a )]
0
p
1
0
0
6.74 [0.89 (7a ), 0.27 (8a ), -0.19 (9a )]
r (C–Cl)
Calculated at the SAC-CI and OVGF//CCSD(T)/aug-cc-pVTZ level
a
-1 b
.
Adiabatic ionization energy. Cationic vibrational wave number: 810 ± 50 cm
for single excitations and SAC-CI coefficients (|c
Openshell occupancy
i
| [ 0.1) are provided in square brackets
‘
‘lone pair’’ orbitals (n ) are expected to have low IEs.
Cl
delocalization over the entire r framework; calculations
indicate that the corresponding MO has some C–Cl char-
acter (see Fig. 5).
Photoelectron bands corresponding to high IE r orbitals
are not expected in the investigated IE region. MOs in
general are delocalized over the entire molecular frame
(
see Fig. 5), but in order to keep discussion simple and to
Gas-phase thermolysis
provide the main character of the MO, we use notations
above.
The thermal decomposition of 5-chloro-1,2,3,4-thiatriazole
is interesting not only from the viewpoint of high energy
materials, but the generation of small reactive species, such
Five bands are observed in the photoelectron spectrum
of 5-chloro-1,2,3,4-thiatriazole (Fig. 5), and according to
calculations these bands originate from ionization of ten
MOs (Table 5). The first photoelectron band has a complex
structure, and calculations predict that it can be assigned to
ionization from two ring p and two nitrogen ‘‘lone pair’’
as N S and chloro-pseudohalides. Thermolysis of the thi-
2
atriazole was carried out in the gas phase in an empty
quartz tube. The effluent from the tube was continuously
monitored by UPS and IR. Decomposition of thiatriazole
commenced at 150 °C, and destruction was complete at
300 °C (see photoelectron spectra in Fig. 6). Spectro-
orbitals (n ). The low IE side of the band shows vibrational
N
fine structure with a weak adiabatic transition. The band
shape thus indicates a geometrical change due to ionization
and that a bonding electron is removed during ionization.
The vibrational fine structure is not entirely resolved, but
our best estimates indicate ionic vibrational wave numbers
scopies indicated the formation of ClCN and N as major
2
products, and the precipitation of elemental sulfur on the
glassware was visually observed. Only two weak photo-
electron bands at 10.53 and 11.42 eV indicate the forma-
tion of trace amounts of other side products. Side products
have not been detected in IR. The photoelectron band at
10.53 eV is tentatively assigned to ClSCN on the basis of
previously published photoelectron spectrum of ClSCN
-
1
of 810 ± 50 cm . This value, comparing to the wave
numbers of the neutral molecule (see above), may be
assigned to the N–N ring stretch of the ground-state radical
cation. The assignment of the second band at 12.88 eV to
one of the chlorine lone pair MOs is unambiguous con-
sidering the relatively narrow band shape and comparing
the spectrum to those of mono- and dichloro-1,2,5-thiadi-
azoles [20, 21]. The next band at 13.8 eV is assigned to two
[22]. It is interesting to note that N S was not detected by
2
UPS or IR spectroscopy in our experiments, neither was S2,
the primary product of bimolecular reactions of N2S
molecules. In contrast, N S and S were observed by UPS
2
2
MOs, one is the third nitrogen lone pair, n , and the second
N
as products of the gas-phase thermolysis of 5-phenyl-
1,2,3,4-thiatriazole [15]. It was commented in this latter
is the second chlorine lone pair MO. The next band at
1
5.6 eV is assigned again to two orbitals, to the lowest
work that N S was very reactive and a bimolecular
2
energy p orbital and n . This latter, according to calcula-
decomposition of N S was also occurring during thermol-
2
S
tions, is strongly mixed with the r framework. The cor-
responding n band is observed in the 13–15 eV region of
ysis and effluent flow between the end of pyrolysis tube
and intersecting ionizing beam (even at a short distance of
S
the photoelectron spectra of 1,2,5-thiadiazoles [20, 21].
The last band in the spectrum at 16.8 eV is assigned to one
MO. An unambiguous assignment is not possible due to the
about 1 cm) [15]. Although it is likely that N S forms
2
during the thermolysis of 5-chloro-1,2,3,4-thiatriazole, we
could not identify it. N S is a reactive intermediate and it
2
123

Struct Chem
Acknowledgments We thank the Hungarian Scientific Research
Fund for financial support to the project (grant no. OTKA K101164).
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The electronic, geometric, and vibrational properties, as
well as thermal decomposition of 5-chloro-1,2,3,4-thiatri-
azole, have been investigated in the gas phase by infrared
spectroscopy, photoelectron spectroscopy, and theoretical
calculations. According to calculations, the molecule has
planar structure and CS symmetry. It is predicted to
decompose, via the formation of N S or chlorothiocarbonyl
2
azide intermediates, to N , ClCN, and sulfur. The infrared
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and photoelectron spectroscopy has provided information
on the fundamental vibrations and on the valence occupied
levels of the neutral molecule, and on the sequence of the
low-lying cationic states. 5-chloro-1,2,3,4-thiatriazole is
thermally unstable, and gas-phase pyrolysis leads to N2,
ClCN, and sulfur.
(
103:4423–4427
1
23