UV-induced transformations of matrix-isolated 1,3,4-thiadiazole-2-thiones

Article abstract of DOI:10.1002/poc.1585

Monomers of 5-mercapto-1,3,4-thiadiazole-2-thione (bismuthiol) were studied using an experimental matrix-isolation technique as well as by carrying out theoretical quantum chemical calculations. The calculations, performed using the quadratic configuration interaction method with single and double excitations (QCISD)/6-31++G(d,p)//DFT(B3LYP)/ 6-311 ++G(2d,p), predict that the thione-thiol tautomer of bismuthiol should be significantly (by more than 19 kJ mol-1) more stable than other tautomeric forms. Accordingly, only the signatures of the thione-thiol tautomer were observed in the FT-IR spectrum of bismuthiol, recorded directly after deposition of an Ar matrix. UV (l> 320 nm) irradiation induced the conversion of the thione-thiol tautomer into the dithiol form. Analogous investigations were carried out for two related compounds: 5-methyl-1,3,4-thiadiazole-2-thione and 5-methylthio-1,3,4-thiadiazole-2-thione. For these two species, only the thione tautomeric forms were observed after deposition of Ar matrices. These tautomers were predicted (by QCISD calculations) to be more stable (by at least 19kJmol-1) than other tautomeric forms. Upon UV irradiation, the most stable thione forms of these compounds were transformed into the corresponding thiol tautomers. Direct observation of the thione! thiol phototautomeric processes provides a clear proof that intramolecular proton transfer reaction can occur in molecules, such as bismuthiol, in spite of the increased NH...S distance, in comparison to other phototautomerizing species studied so far. All the isomers of the studied compounds (substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311 ++G(2d,p) level of theory for possible isomeric structures. Copyright

Full text of DOI:10.1002/poc.1585

Research Article
Received: 2 March 2009,
Revised: 27 May 2009,
Accepted: 27 May 2009,
Published online in Wiley InterScience: 24 July 2009
(
www.interscience.wiley.com) DOI 10.1002/poc.1585
UV-induced transformations of
matrix-isolated 1,3,4-thiadiazole-2-thiones
a
a
a
Hanna Rostkowska , Leszek Lapinski and Maciej J. Nowak *
Monomers of 5-mercapto-1,3,4-thiadiazole-2-thione (bismuthiol) were studied using an experimental matrix-isolation
technique as well as by carrying out theoretical quantum chemical calculations. The calculations, performed using the
quadratic configuration interaction method with single and double excitations (QCISD)/6-31RRG(d,p)//DFT(B3LYP)/
6
-311RRG(2d,p), predict that the thione–thiol tautomer of bismuthiol should be significantly (by more than
S1
19 kJ mol ) more stable than other tautomeric forms. Accordingly, only the signatures of the thione–thiol tautomer
were observed in the FT-IR spectrum of bismuthiol, recorded directly after deposition of an Ar matrix. UV (l > 320 nm)
irradiation induced the conversion of the thione–thiol tautomer into the dithiol form. Analogous investigations
were carried out for two related compounds: 5-methyl-1,3,4-thiadiazole-2-thione and 5-methylthio-1,3,4-thiadiazole-
2
-thione. For these two species, only the thione tautomeric forms were observed after deposition of Ar matrices.
S1
These tautomers were predicted (by QCISD calculations) to be more stable (by at least 19 kJ mol ) than other
tautomeric forms. Upon UV irradiation, the most stable thione forms of these compounds were transformed into the
corresponding thiol tautomers. Direct observation of the thione ! thiol phototautomeric processes provides a
clear proof that intramolecular proton transfer reaction can occur in molecules, such as bismuthiol, in spite of the
increased NH. . .S distance, in comparison to other phototautomerizing species studied so far. All the isomers of
the studied compounds (substrates and products of the photoreactions) were identified by comparison of their IR
spectra with the spectra calculated at the DFT(B3LYP)/6-311RRG(2d,p) level of theory for possible isomeric structures.
Copyright ß 2009 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: IR spectra; matrix isolation; phototautomerism; proton transfer; tautomerism
NHꢀꢀꢀS distance calculated for 5-mercapto-1,3,4-thiadiazole-2-
INTRODUCTION
thione (a molecule with five-membered heterocyclic ring) is
˚
Photoinduced, intramolecular proton-transfer processes were
previously observed for a number of heterocyclic compounds
2.93 A. Hence, it was interesting to experimentally verify if the
thione ! thiol phototautomeric reaction would occur, in spite of
the increased NHꢀꢀꢀS distance, also for 5-mercapto-1,3,
4-thiadiazole-2-thione and molecules of similar structure.
In the current work, tautomerism of monomeric 5-mercapto-1,
3,4-thiadiazole-2-thione, 5-methyl-1,3,4-thiadiazole-2-thione, and
5–methylthio-1,3,4-thiadiazole-2-thione (see Scheme 2) was
studied using a matrix-isolation technique. In order to investigate
the unimolecular phototransformations of these compounds, the
matrix-isolated molecules were exposed to UV light. Photo-
induced transformations were monitored by means of IR
spectroscopy. The identification of the photogenerated species
was based on comparison of the IR spectra recorded after UV
irradiation with the spectra simulated theoretically for possible
structures of photoproducts.
[1]
such as 2(1H)-pyridinethione, 4(3H)-pyrimidinethione and 3(2H)-
[
2]
[3]
[4]
[5]
pyridazinethione, 2-thiouracils, 4-thiouracils, 6–thiopurine,
[6]
and 2–quinolinethione, isolated in low-temperature inert gas
matrices. In the structures of these molecules, the thiocarbonyl
—
C—S group, attached directly to the heterocyclic ring, was placed
in the a position with respect to the NH group. Upon UV irradiation
of the monomers of the compounds listed above, the thione
tautomeric form was converted into the thiol tautomer (see
Scheme 1a). Analogous thione ! thiol photoreactions were
[
7]
found for simple thioamides such as thiourea and thioaceta-
[
8]
mide (see Scheme 1b).
For the heterocyclic molecules with a six-membered ring, the
distance between an exocyclic sulfur atom and the hydrogen
˚
atom of the vicinal NH group is 2.672–2.801 A (according to our
DFT(B3LYP)/6-311þþG(2d,p) calculations). Very similar (2.724–
˚
2
.804 A) are the NHꢀꢀꢀS distances in non-cyclic molecules of
thiourea, thioacetamide, and thiosemicarbazide. It can be expected
that analogous distances should be larger for compounds with
*
Correspondence to: M. J. Nowak, Institute of Physics, Polish Academy of
Sciences, Al. Lotnikow 32/46, 02–668 Warsaw, Poland.
E-mail: mjnow@ifpan.edu.pl
—
C—S and NH groups attached to a five-membered ring (Scheme 1).
The increase of the NHꢀꢀꢀS distance should be caused by internal
strain in a five-membered ring. Calculations carried out at the
DFT(B3LYP)/6-311þþG(2d,p) level confirm these expectations. The
a
H. Rostkowska, L. Lapinski, M. J. Nowak
Institute of Physics, Polish Academy of Sciences, Warsaw, Poland
J. Phys. Org. Chem. 2010, 23 56–66
Copyright ß 2009 John Wiley & Sons, Ltd.

PHOTOTRANSFORMATIONS OF 1,3,4-THIADIAZOLE-2-THIONES
COMPUTATIONAL
The geometries of the isomeric forms of compounds, considered
in the current work, were optimized using the hybrid Hartree–
Fock and density functional theory method DFT(B3LYP) with
[
9]
Becke’s three-parameter exchange functional and the gra-
[
10,11]
dient-corrected functional of Lee, Yang, and Parr.
6
The
-311þþG(2d,p) basis set was applied in these calculations. All
the geometries optimized at DFT(B3LYP)/6-311þþG(2d,p) level
corresponds to minima on the potential energy surfaces. At the
optimized geometries, single-point calculations of electronic
energies (Eel) were carried out using the quadratic configuration
interaction method with single and double excitations
[
12]
(
QCISD),
In the previous work
applying the 6-31þþG(d,p) basis set.
[13]
on tautomeric equilibria in 2-
Scheme 1. Examples of photoinduced intramolecular proton-transfer
reactions observed for six-membered heterocyclic thione compounds
pyridinone, 4-pyrimidinone, 2-pyrazinone, 2-quinolinone, and
2-quinoxalinone, we have demonstrated that, for these hetero-
cyclic compounds, the single point QCISD/cc-pVDZ calculations
˚
and for simple thioamides. NHꢀꢀꢀS distances (A) in the optimized structures
of: (a) 3(2H)-pyridazinethione, (b) thioacetamide, and (c) 5-mercapto-1,3,
(
performed at geometry optimized at the DFT(B3LYP)/cc-pVTZ
4
6
-thiadiazole-2-thione. The structures were optimized at the DFT(B3LYP)/
level) reproduce the experimental relative energies of tautomers
-311þþG(2d,p) level.
ꢂ1
with an accuracy not worse than ꢁ3 kJ mol . For the archetype
molecule of 2-pyridinone, the relative energy of tautomers
computed at the QCISD/cc-pVDZ//DFT(B3LYP)/cc-pVTZ level
ꢂ1
(
2.5 kJ mol ) was found to differ only very slightly from the
ꢂ1
corresponding value (3.1 kJ mol ) predicted at the QCISD/
TZV(2df,2dp)//QCISD/TZV(2df,2dp) level; both values being in
EXPERIMENTAL
[
14]
The compounds used in this study, 5-mercapto-1,3,4-thiadiazole-
a
nice agreement with the experimental estimate
ꢂ1
2
1
-thione, 5-methyl-1,3,4–thiadiazole-2-thione, and 5-methylthio-
,3,4-thiadiazole-2-thione, were supplied by Aldrich in the form
2.5 ꢁ 0.6 kJ mol
.
The harmonic vibrational frequencies and IR intensities were
calculated at the DFT(B3LYP)/6-311þþG(2d,p) level. To correct for
the systematic shortcomings of the applied methodology (mainly
for anharmonicity), the predicted vibrational wavenumbers were
scaled down by a uniform factor of 0.98. The theoretical normal
modes, calculated for the tautomers of the studied compounds,
were analyzed by carrying out the potential energy distribution
(PED) calculations, performed according to the procedure
described in Ref. [15]. The internal coordinates applied in this
analysis were defined following the recommendations of Pulay et
of off-white to yellow powder (1,3,4-thiadiazole-2,5-dithiol –
CAS number 1072-71-5; 5-methyl-1,3,4-thiadiazole-2-thiol – CAS
number 29490-19-5; and 5-methylthio-1,3,4-thiadiazole-2-thiol –
CAS number 6264-40-0). In order to prepare Ar matrices
containing isolated monomers of studied 1,3,4-thiadiazole-2-
thiones, solid sample of a compound was heated in a miniature
glass furnace placed in the vacuum chamber of a continuous-flow
helium cryostat. The approximate temperatures of the furnace
during the deposition of 5-methyl-1,3,4-thiadiazole-2-thione,
[
16]
5-methylthio-1,3,4-thiadiazole-2-thione, and 5-mercapto-1,3,4-
al.
These coordinates are listed in Tables S1–S6 in the
thiadiazole-2-thione were 330, 340, and 380 K, respectively.
Samples of a compound isolated in solid argon were formed by
codeposition of the compound vapors and argon onto a CsI
window cooled to 10 K. The spectral purity (6.0) matrix gas argon
used in the present experiments was supplied by Linde AG. The
infrared spectra were recorded with the Thermo Nicolet Nexus
Supporting Information. The obtained elements of PED matrices
greater than 10% are given in Tables S7–S12 in the Supporting
Information.
The barrier for rotation of the SH group in the thione–thiol
tautomer of 5-mercapto-1,3,4-thiadiazole-2-thione in the electro-
nic ground state was calculated using the minimum energy path
approach. At each of the points, intermediate between the
minima corresponding to the rotameric structures, all geometry
parameters (except for the dihedral angle H-S-C-S, which served
as a driving coordinate) were optimized using the MP2
670 FT-IR spectrometer equipped with a KBr beam splitter and a
DTGS detector.
Matrices were irradiated with UV light from a high pressure
mercury lamp HBO 200 fitted with a water filter and cut–off
filters WG 295 and WG 320 transmitting light with l > 295 or
l > 320 nm. The typical time of irradiation was 2–3 h.
[
17]
method
and the 6-311þþG(2d,p) basis set. The calculations
in the current work were carried out using the GAUSSIAN 03
program.
[
18]
RESULTS AND DISCUSSION
Tautomerism of 5-mercapto-1,3,4-thiadiazole-2-thione
The 5-mercapto-1,3,4-thiadiazole-2-thione molecule may theor-
etically exist in three different tautomeric forms: dithione I,
thione–thiol II, and dithiol III (see Scheme 3). Repulsive
interactions between hydrogen atoms attached to the vicinal
Scheme 2. Structures and atom numbering of the most stable tautomeric
forms of: (a) 5-mercapto-1,3,4-thiadiazole-2-thione, (b) 5-methyl-1,3,4-
thiadiazole-2-thione, and (c) 5-methylthio-1,3,4-thiadiazole-2-thione.
J. Phys. Org. Chem. 2010, 23 56–66
Copyright ß 2009 John Wiley & Sons, Ltd.
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H. ROSTKOWSKA, L. LAPINSKI AND M. J. NOWAK
stretching vibrations of the SH group, the spectrum of
5
-mercapto-1,3,4-thiadiazole-2-thione displays a weak absorp-
ꢂ1
tion at 2591 cm . Occurrence of these bands suggests that the
thione–thiol tautomer II is the form adopted by the molecules of
5
-mercapto-1,3,4-thiadiazole-2-thione in low-temperature Ar
matrices.
The experimental spectrum of matrix-isolated molecules is
Scheme 3. Tautomeric forms of 5-mercapto-1,3,4-thiadiazole-2-thione.
juxtaposed (in Fig. 1) with the spectra simulated at the
DFT(B3LYP)/6-311þþG(2d,p) level for the most stable forms,
IIa and IIb of 5-mercapto-1,3,4-thiadiazole-2-thione. As could be
seen, the experimental spectrum is quite well reproduced by the
theoretical spectra of both IIa and IIb. However, in some spectral
regions, the agreement between experiment and theory is better
for the spectrum calculated for isomer IIa than for the spectrum
predicted for IIb. Crucial support for identification of the form,
present in the matrix as IIa, comes from the analysis of the bands
due to the motions of the SH group. The bands corresponding to
the vibrations of the SH group should be most sensitive to the
conformation at this fragment of the molecule. In the spectral
range, characteristic of stretching vibrations of the SH group, only
nitrogen atoms in the ring of the dithione form I and the
destabilizing effect of repulsion between lone electron pairs of
the neighboring nitrogen atoms in the ring of the dithiol
tautomer III suggest that the thione–thiol form II, where such
destabilizing interactions are absent, should be the most stable.
This intuitive expectation was confirmed by quantum-chemical
calculations. Theoretical data on relative energies of all possible
isomers of the 5-mercapto-1,3,4-thiadiazole-2-thione (collected
in Table 1) show that most stable are the thione–thiol forms IIa
and IIb. According to the calculations using the QCISD/
6-31þþG(d,p)//DFT(B3LYP)/6-311þþG(2d,p) method, the IIa
ꢂ1
and IIb rotamers of the thione–thiol tautomer are very close
one experimental band was visible at 2591 cm . Alas, this
in energy. The energy difference predicted at this level of theory
observation does not exclude the possibility of the presence of
both thione–thiol rotamers in the matrix, because the absolute
intensity calculated for the band due to n SH vibration in rotamer
IIb is very low and this band might be too weak to be detected.
More conclusive information stems from the experimental
spectrum in the wavenumber region typical of bending
vibrations of SH groups. For conformer IIa, theoretical calcu-
ꢂ1
is as small as 1.6 kJ mol , in favor of conformer IIa. Similar energy
ꢂ1
difference (DEtotal ¼ 1.7 kJ mol ) was obtained when the MP2/
6
-311þþG(2d,p) approach was applied. The small energy
difference between the thione–thiol conformers IIa and IIb
suggests that both forms should be significantly populated in the
gas phase. With respect to tautomer II, the dithione tautomer of
ꢂ1
ꢂ1
C
2
symmetry I is higher in energy by more than 30 kJ mol , and
lations predict the b SH band at 881 cm and for conformer IIb
ꢂ1
the conformers of the dithiol tautomer III are higher in energy by
at 925 cm (wavenumbers scaled by 0.98). In the region around
ꢂ1
[19]
ꢂ1
more than 19 kJ mol . Previous calculations of Hipler et al.,
900 cm of the experimental spectrum (see the oval in Fig. 1),
ꢂ1
performed at a lower theory level, led to similar predictions.
only one band was observed at 892 cm , clearly nearer to the
The experimental IR spectrum of 5-mercapto-1,3,4-thiadiazole-
-thione isolated in an Ar matrix is presented in Fig. 1. This
predicted frequency of the b SH mode in the thione–thiol
conformer IIa. This observation suggests that in an Ar matrix, only
one thione–thiol form, IIa, is populated. Hence, the ratio of
population of IIa and IIb, frozen in a low-temperature matrix,
must be quite different from the relative populations of these
forms in the gas phase. On the basis of calculated energies of IIa
and IIb (Table 1), for 5-mercapto-1,3,4-thiadiazole-2-thione in the
gas phase, the population of IIb can be expected to be only
2
[
19]
spectrum is nearly the same as that reported by Hipler et al. In
ꢂ1
the high-frequency (3650–3350 cm ) range of the spectrum of
matrix-isolated 5–mercapto-1,3,4-thiadiazole-2-thione, the charac-
teristic single absorption band corresponding to the stretching
ꢂ1
vibration of the NH group was observed at 3445 cm . Moreover,
in the spectral region, characteristic of the bands due to the
ꢂ1
Table 1. Relative electronic (DE ), zero-point vibrational (DZPE), and total (DE
forms of 5-mercapto-1,3,4–thiadiazole-2-thione
¼ DE þ DZPE) energies (kJ mol ) of isomeric
el
total
el
I
IIa
IIb
IIIa
IIIb
IIIc
C
2
C
s
C
1
C
2v
C
1
C
2
DEel
*
24.99
8.35
33.34
0.0
0.0
0.0
1.44
0.16
1.60
30.51
35.77
ꢂ10.76
25.01
41.04
ꢂ10.60
30.44
DZPE**
ꢂ10.75
DEtotal ¼ DE þ DZPE
19.76
el
Torsional angles (deg)
H(8)—S(6)—C(5)—S(1)
H(9)—S(7)—C(2)—S(1)
180.00
29.24
180.00
180.00
22.33
ꢂ177.97
24.01
24.01
*
*
DEel calculated using the QCISD/6-31þþG(d,p) method at geometries optimized at the DFT(B3LYP)/6-311þþG(2d,p) level.
* DZPE calculated at the DFT(B3LYP)/6-311þþG(2d,p) level. Atom numbering as in Scheme 3.
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PHOTOTRANSFORMATIONS OF 1,3,4-THIADIAZOLE-2-THIONES
Figure 1. Comparison of: (A) the experimental spectrum of 5-mercapto-1,3,4-thiadiazole-2-thione isolated in an Ar matrix with the spectrum (B) of
isomer IIa and the spectrum (C) of isomer IIb theoretically simulated at the DFT(B3LYP)/6-311þþG(2d,p) level. The calculated wavenumbers were scaled
by the uniform factor of 0.98.
slightly smaller than the population of IIa. Therefore, by landing
of the molecules on the matrix surface and by their cooling to the
cryogenic temperatures, conversion of conformer IIb into IIa must
occur. Such ‘conformational cooling’ processes were previously
observed in matrix-isolation experiments on other com-
adopted by the molecule. The possible thione and thiol isomers
of 5-methyl-1,3,4–thiadiazole-2-thione are depicted in Table 2. In
the thiol tautomer V, the hydrogen atom of the SH group may be
directed towards the nitrogen atom (form Va) or toward the
endocyclic sulfur atom (form Vb). The results of the theoretical
QCISD calculations of the relative energies of isomers IV, Va, and
Vb are collected in Table 2. The calculations show that the thione
tautomer IV is the most stable form of the compound. This
structure is stabilized, analogously to the thione–thiol tautomer II
of 5-mercapto-1,3,4-thiadiazole-2-thione, by interactions within
[
20–23]
pounds.
Taking into account the experience gathered in
ꢂ1
those investigations, a barrier lower than 5 kJ mol should allow
conformational cooling processes to be effective while molecules
are frozen from the gas phase into a low-temperature matrix.
In the current work, the barrier for IIb ! IIa conversion was
calculated with the MP2 method. The potential energy curve
obtained using the minimum-energy-path approach is presented
in Fig. 2. According to the computed results, the minima of IIb
and IIa thione–thiol rotamers are separated by a barrier as low as
ꢂ1
3
6
.1 kJ mol . The barrier obtained in QCISD/6-31þþG(d,p)//DFT/
ꢂ1
-311þþG(2d,p) calculations is even lower, it is 2.0 kJ mol (see
Table S13 in the Supporting Information). Such a low barrier for
rotation of the SH group (involving mainly the movement of a
light particle, the proton), should allow effective conformational
cooling during trapping 5-mercapto-1,3,4-thiadiazole-2-thione
molecules from the gas phase into a low-temperature matrix.
Hence, only the most stable IIa conformer should be frozen into
an Ar matrix at 10 K. This expectation is in good agreement with
the analysis of the experimental spectrum of 5-mercapto-1,
3,4-thiadiazole-2-thione monomers isolated in solid Ar.
Full list of the bands observed in the IR spectrum of matrix-
isolated 5-mercapto-1,3,4–thiadiazole-2-thione and their assign-
ment to the theoretically predicted normal modes of thione–thiol
rotamer IIa is presented in Table S7 in the Supporting Information.
Figure 2. Potential energy curve for interconversion between rotamers
IIa and IIb of the thione–thiol tautomer of 5-mercapto-1,3,4-thiadiazole-
Tautomerism of 5-methyl-1,3,4-thiadiazole-2-thione
2
6
-thione. At each point, the electronic energy was calculated at the MP2/
fixed value of the dihedral angle
Substitution of the exocyclic mercapto group in 5-mercapto-1,3,
-311þþG(2d,p) level for
a
4-thiadiazole-2-thione by a methyl group led to the significant
H(8)-S(6)-C(5)-S(1), while all the remaining geometry parameters were
ꢂ1
reduction of the number of conformeric structures that could be
optimized. The height of the barrier is 3.11 kJ mol
.
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H. ROSTKOWSKA, L. LAPINSKI AND M. J. NOWAK
Table 2. Relative electronic (DEel), zero-point vibrational (DZPE), and total (DEtotal ¼ DEel þ DZPE) energies (kJ molꢂ1) of isomeric
forms of 5-methyl-1,3,4–thiadiazole-2-thione
IV
Va
Vb
DEel
*
0.0
0.0
0.0
30.19
ꢂ11.05
19.14
36.54
ꢂ11.25
25.29
DZPE**
DEtotal ¼ DE þ DZPE
el
*
*
DEel calculated using the QCISD/6-31þþG(d,p) method at geometries optimized at the DFT(B3LYP)/6-311þþG(2d,p) level.
* DZPE calculated at the DFT(B3LYP)/6-311þþG(2d,p) level.
ꢂ1
the experimental spectrum and lack of any band at ca. 2600 cm
the N—NH moiety (where the hydrogen atom of the NH group
interacts with the lone electron pair of the neighboring nitrogen
atom; Scheme 4a). The thiol rotamers, Va and Vb of
(corresponding to the n SH vibration), leaves no doubt about the
identification of form IV as the structure adopted by the
5-methyl-1,3,4-thiadiazole-2–thione molecules isolated in low-
temperature matrices. Assignment of the IR bands observed in
the experimental IR spectrum of 5-methyl-1,3,4-thiadiazole-
2-thione to the theoretical normal vibrations of thione tautomer
IV is proposed in Table S8 of the Supporting Information.
5
-methyl-1,3,4-thiadiazole-2-thione, are higher in energy by
ꢂ1
19.1 and 25.3 kJ mol , respectively. Because of such a big
energy gap between the thione and thiol forms of the molecule,
only the thione tautomer is expected to be trapped in an inert
low-temperature matrix. In the current work, monomers of
5-methyl-1,3,4-thiadiazole-2-thione were isolated in an Ar matrix.
The IR spectrum of this matrix is presented in Fig. 3. This spectrum
[
19]
is virtually the same as that previously reported by Hipler et al.
Tautomerism of 5-methylthio-1,3,4-thiadiazole-2-thione
The experimental spectrum shown in Fig. 3A is compared with
the theoretical spectrum of thione form IV predicted using the
DFT(B3LYP)/6-311þþG(2d,p) method (Fig. 3B). Good agreement
between the experimental and theoretical spectra, as well as
Similarly to the case of 5-methyl-1,3,4-thiadiazole-2-thione,
molecules of 5-methylthio-1,3,4-thiadiazole-2-thione may theor-
etically adopt two tautomeric forms: thione VI and thiol VII (see
Table 3). In the structure of the thiol tautomer VII, repulsive
ꢂ1
presence of the band at 3449 cm (due to the n NH vibration) in
Figure 3. Comparison of: (A) the experimental spectrum of 5-methyl-1,3,4-thiadiazole-2-thione isolated in an Ar matrix with the spectrum (B) of isomer
IV theoretically simulated at the DFT(B3LYP)/6-311þþG(2d,p) level. The calculated wavenumbers were scaled by the factor of 0.98.
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 56–66

PHOTOTRANSFORMATIONS OF 1,3,4-THIADIAZOLE-2-THIONES
Table 3. Relative electronic (DEel), zero-point vibrational (DZPE), and total (DEtotal ¼ DEel þ DZPE) energies (kJ molꢂ1) of isomeric
forms of 5-methylthio-1,3,4–thiadiazole-2-thione
VIa
VIb
VIIa
VIIb
VIIc
VIId
DEel
*
0
0
0
8.46
ꢂ0.0012
8.46
31.27
ꢂ10.54
20.73
36.34
ꢂ10.81
25.53
42.81
ꢂ10.77
32.04
47.21
ꢂ10.89
36.32
DZPE**
DEtotal ¼ DEel þ DZPE
*
*
DEel calculated using the QCISD/6-31þþG(d,p) method at geometries optimized at the DFT(B3LYP)/6-311þþG(2d,p) level.
* DZPE calculated at the DFT(B3LYP)/6-311þþG(2d,p) level.
interactions of lone electron pairs of the neighboring nitrogen
atoms in the ring are responsible for the lower stability of this
form with respect to the thione tautomer VI. This effect is seen
well in Table 3, where the results of QCISD calculations of relative
energies of isomeric forms of the 5-methylthio-1,3,4-thiadiazole-
only one nitrogen atom in the N–N fragment is protonated
(Scheme 4a). The tautomers with both nitrogen atoms proto-
nated (Scheme 4b) and the tautomers with none of the nitrogen
atoms protonated (Scheme 4c) were theoretically predicted as
significantly higher in energy. Analysis of the IR spectra of isolated
monomers of 5-mercapto-1,3,4-thiadiazole-2-thione and two
other 1,3,4-thiadiazole-2-thiones revealed that only the isomers
of these molecules with protonation pattern shown in Scheme 4a
are populated in low-temperature Ar matrices.
2-thione are collected. The thione isomer VIa was predicted as
the most stable form of the compound, whereas the energies
calculated for conformers of the thiol form VII were higher by
ꢂ1
more than 20 kJ mol . Therefore, in the IR spectrum of
5
-methylthio-1,3,4-thiadiazole-2-thione isolated in
a
low-
For similar reasons, also for other compounds, the tautomers
with the N–N fragment protonated as in Scheme 4a are favored
and the forms with protonation Schemes 4b and 4c are avoided.
temperature inert gas matrix, only the manifestations of thione
forms should be expected. However, the thione tautomer of
[
24]
5-methylthio-1,3,4-thiadiazole-2-thione may potentially exist in
Maleic hydrazide is, perhaps, the best known example of such
tautomerism. In this compound, the oxo-hydroxy form, following
the protonation Scheme 4a, is heavily favored. Similar tautomer-
ism was observed for an analog of maleic hydrazide with both
two conformeric forms, with CH group directed towards a
3
nitrogen atom (VIa) or towards an endocyclic sulfur atom (VIb).
The IR spectra simulated at the DFT(B3LYP)/6-311þþG(2d,p) level
for the thione isomers VIa and VIb are compared in Fig. 4 with the
experimental spectrum of 5-methylthio-1,3,4–thiadiazole-2-
thione monomers isolated in an argon matrix. Very good
agreement between experiment and theoretical predictions
corroborates the identification of the form trapped in the matrix
as the thione form VI. Comparison of the experimental spectrum
with the spectra simulated theoretically for the thione
conformers VIa and VIb shows that, though the two theoretical
spectra are not much different, the spectrum predicted for form
VIa bears more resemblance to that obtained experimentally.
Moreover, according to the theoretical predictions, form VIa is
the most stable conformer of the compound and is lower in
[
25]
oxygen atoms substituted with sulfur.
This compound,
3-mercapto-6(1H)-pyridazinethione, was found to adopt only
the thione–thiol tautomeric structure. Analogously, the oxo and
[
26,27]
the thione tautomers of 3(2H)-pyridazinone
and
[
2]
3(2H)-pyridazinethione (protonation Scheme 4a) are so much
more stable than the respective hydroxy and thiol forms
(protonation Scheme 4c) that no traces of the latter isomers
were observed in the gas phase and low-temperature matrices.
Other examples can be found in Ref. [28].
Photoprocesses induced by UV irradiation
ꢂ1
energy by 8.46 kJ mol than isomer VIb. Hence, the spectrum of
In the present study, molecules of 5-mercapto-1,3,4-thiadiazole-
2-thione isolated in a low-temperature Ar matrix were exposed to
UV (l > 320 nm) light. Upon UV irradiation, the infrared spectrum
of the matrix changed drastically (Fig. 5). All bands observed in
the spectrum recorded after deposition of the matrix decreased
evenly, while a set of new, relatively weak bands emerged. Nearly
5-methylthio-1,3,4–thiadiazole-2-thione, isolated in a low-
temperature matrix, has been ascribed to conformer VIa. An
assignment of IR bands observed in the spectrum of
5-methylthio-1,3,4-thiadiazole-2-thione isolated in an argon
matrix to the calculated normal modes of form VIa of
the compound is proposed in Table S9 of the Supporting
Information.
ꢂ1
total disappearance of the band at 3445 cm attributed to the
General remarks on tautomerism of
1
,3,4-thiadiazole-2-thiones
According to the theoretical calculations, the most energetically
stable structures of 5-mercapto-1,3,4-thiadiazole-2-thione, 5-methyl-1,
3,4-thiadiazole-2-thione, and 5-methylthio-1,3,4-thiadiazole-2-thione
Scheme 4. N–N moiety protonated with: (a) one hydrogen atom, (b) two
are the tautomeric forms II, IV, and VI, respectively. In all these forms,
hydrogen atoms, and (c) not protonated.
J. Phys. Org. Chem. 2010, 23 56–66
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H. ROSTKOWSKA, L. LAPINSKI AND M. J. NOWAK
Figure 4. Comparison of: (A) the experimental spectrum of 5-methylthio-1,3,4-thiadiazole-2-thione isolated in an Ar matrix with the spectrum (B) of
isomer VIa and the spectrum (C) of isomer VIb theoretically simulated at the DFT(B3LYP)/6-311þþG(2d,p) level. The calculated wavenumbers were
scaled by the factor of 0.98.
n NH motions, accompanied by the increase in the absorption at
wavenumbers characteristic of stretching vibrations of SH groups
suggests that the thione–thiol tautomer II of 5-mercapto-1,3,4-
thiadiazole-2-thione is subject to the phototautomeric reaction
leading to generation of the dithiol form III. This supposition is
further confirmed by the appearance of two new bands at
wavenumbers typical of the bending vibrations of SH groups, i.e.,
The extracted spectrum of the photoproduct generated upon
UV (l > 320 nm) irradiation of 5-mercapto-1,3,4-thiadiazole-2-
thione is presented in Fig. 6. This spectrum consists only of the
bands emerging upon UV irradiation. The experimental spectrum
is compared with the ‘stick’ spectrum calculated for the dithiol
form IIIa using DFT approach. The comparison between the
experimental spectrum and the spectra calculated for forms IIIa,
IIIb, and IIIc is provided in the Supporting Information (Figure S1).
Among the possible structures of the dithiol tautomer III, the
theoretical spectrum predicted for conformer IIIa reproduces
best the experimental spectrum of the photoproduct. Indeed,
form IIIa of C2v symmetry seems to be produced in the easiest
way, by simple proton shift from the NH group to the sulfur atom
ꢂ1
at 912 and 888 cm . During UV irradiation, both bands were
increasing at the same rate, which suggests that they originate
from the same photoproduct. The bands due to the b SH
ꢂ1
vibrations, observed in the infrared spectra at ca. 900 cm , are
the ‘fingerprints’ of the thiol tautomers. Such bands were
previously found in the spectra of the thiol tautomers photo-
ꢂ1 [7]
—
generated from: thiourea (at 909 and 888 cm ), 6-aza-2,4-
of the C—S moiety (see Scheme 5a). No rotation of SH groups
ꢂ1 [29]
dithiouracil (at 956, 902, and 900 cm ),
1
2,4-dithiopurine (at
needs to be involved in the phototransformation of IIa into IIIa.
An assignment of IR bands, observed in the spectrum of the
photoproduct generated from 5-mercapto-1,3,4–thiadiazole-2-
thione to the calculated normal modes of form IIIa, is proposed in
Table S10 in the Supporting Information.
ꢂ1 [30]
[1]
040 and 899 cm ), as well as in the spectrum of 2-pyridinethiol
isolated in a low-temperature Ar matrix, at 884 cm
ꢂ1
.
Even a perfunctory comparison of the spectra A and B pre-
sented in Fig. 6 shows that the experimental spectrum has
additional bands, which have no theoretical counterparts. The
ꢂ1
most pronounced is the band at 1528cm . Such a spectral posi-
tion is typical of the absorption due to antisymmetric stretching
vibrations of CS
matrices.
5
2
molecules, isolated in low temperature Ar
In the work on thermolysis reactions of
-mercapto-1,3,4-thiadiazole-2-thione, investigated using a matrix-
[31]
[19]
isolation technique, Hipler et al.
also observed this band and
ascribed it to the decomposition product CS . Apart from the band
Scheme 5. Phototautomeric reactions observed for: (a) 5-mercapto-1,
2
3
5
,4-thiadiazole-2-thione (b) 5-methyl-1,3,4-thiadiazole-2-thione and (c)
-methylthio-1,3,4-thiadiazole-2-thione.
due to CS
2
fragment, in the spectrum recorded in the current work
after UV irradiation of 5-mercapto-1,3,4-thiadiazole-2-thione, a
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 56–66

PHOTOTRANSFORMATIONS OF 1,3,4-THIADIAZOLE-2-THIONES
Figure 5. Infrared spectrum of 5-mercapto-1,3,4-thiadiazole-2-thione isolated in an Ar matrix recorded (A) after deposition of the matrix and (B) after 2 h
of UV (l > 320 nm) irradiation.
[
32]
spectral signature of HNCS photoproduct was also noticed at
agreement between the experimental and theoretical spectra
(Fig. 8) allowed a positive identification of the photoproduct as
the thiol form Va. Detailed assignment of the observed bands of
the photoproduct is proposed in Table S11 in the Supporting
Information. Apart from the bands ascribed to the spectrum of
ꢂ1
1982 cm
.
In the case of 5-methyl-1,3,4-thiadiazole-2-thione, UV irradia-
tion of an argon matrix containing monomers of the compound
resulted in a thione ! thiol phototautomeric reaction (see
Scheme 5b) analogous to that observed for 5-mercapto-1,3,
ꢂ1
conformer Va, a weak band at 1528 cm , indicating photo-
4
3
-thiadiazole-2-thione. Changes in the spectrum of 5-methyl-1,
,4-thiadiazole-2-thione isolated in a low-temperature matrix
2
generation of trace amounts of CS , was also detected in the
spectrum recorded after UV irradiation.
occurring upon exposure to the UV (l > 295 nm) light are
presented in Fig. 7. The extracted spectrum of new bands that
appeared simultaneously upon UV irradiation with decrease of
the bands, belonging to the initial spectrum of the thione form IV,
is presented in Fig. 8. This spectrum was compared with the
theoretical spectrum predicted for the thiol isomer Va. Good
Molecules of 5-methylthio-1,3,4-thiadiazole-2-thione isolated
in a low-temperature Ar matrix were also irradiated with UV
(l > 295 nm) light. Figure 9 illustrates spectral changes occurring
3
upon 2 /
4
h exposure of the matrix to UV radiation. All bands
belonging to the initial spectrum of the thione tautomer VI
decreased in intensity while a set of new bands emerged. The
Figure 6. (A) Infrared spectrum of the photoproduct(s) generated upon UV (l > 320 nm) irradiation of 5-mercapto-1,3,4-thiadiazole-2-thione isolated in
an Ar matrix; (B) spectrum of the dithiol isomer IIIa of the compound predicted at the DFT(B3LYP)/6-311þþG(2d,p) level. The calculated wavenumbers
were scaled by the factor of 0.98.
J. Phys. Org. Chem. 2010, 23 56–66
Copyright ß 2009 John Wiley & Sons, Ltd.
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H. ROSTKOWSKA, L. LAPINSKI AND M. J. NOWAK
1
Figure 7. Infrared spectrum of 5-methyl-1,3,4-thiadiazole-2-thione isolated in an Ar matrix recorded (A) after deposition of the matrix and (B) after 2 /
4
h
of UV (l > 295 nm) irradiation.
most conspicuous features that appeared upon UV irradiation
were the absorption bands in the wavenumber range,
matches well the experimental spectrum of the photoproduct of
5-methylthio-1,3,4-thiadiazole-2-thione. This confirms the assign-
ment of structure VIIa to the product photogenerated from the
initial thione tautomer VI of the compound. The bands observed
in the experimental spectrum of the photoproduct are listed in
Table S12 in the Supporting Information, alongside the
wavenumbers, absolute intensities and PEDs calculated for the
thiol isomer VIIa.
characteristic of the stretching and bending motions of SH
ꢂ1
groups. Bands at 2594 and at 899 cm
were identified as
resulting from n SH and b SH vibrations, respectively. In those
spectral regions, presence of only one band due to n SH and one
band due to b SH indicates the generation of a sole thiol
conformer. The emerging spectrum of the photoproduct is
juxtaposed (in Fig. 10) with the spectrum calculated at the
DFT(B3LYP)/6-311þþG(2d,p) level for the most plausible photo-
product structure, the thiol conformer VIIa. Such a structure of
the photoproduct results from a simple proton shift from NH
It can be seen in Fig. 10 that some bands present in the
spectrum obtained after UV (l > 295 nm) irradiation have no
counterparts in the spectrum predicted for form VIIa. Among
them, the most pronounced are the bands at 1528 and
ꢂ1
—
group to the sulfur atom of the C—S moiety (see Scheme 5c) not
1273 cm – the wavenumbers characteristic of the IR spectra
[
31,33]
accompanied by any rotation of C-SH and/or C-SCH groups. The
of CS and CS molecules.
This indicates that part of the
3
2
theoretical spectrum, predicted for the thiol rotamer VIIa,
5-methylthio-1,3,4-thiadiazole-2-thione molecules decomposed
Figure 8. (A) Infrared spectrum of the photoproduct(s) generated upon UV (l > 295 nm) irradiation of 5-methyl-1,3,4-thiadiazole-2-thione isolated in an
Ar matrix; (B) spectrum of the thiol isomer Va of the compound predicted at the DFT(B3LYP)/6-311þþG(2d,p) level. The calculated wavenumbers were
scaled by the factor of 0.98.
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 56–66

PHOTOTRANSFORMATIONS OF 1,3,4-THIADIAZOLE-2-THIONES
Figure 9. Infrared spectrum of 5-methylthio-1,3,4-thiadiazole-2-thione isolated in an Ar matrix recorded (A) after deposition of the matrix and (B) after
3
/
2
4
h of UV (l > 295 nm) irradiation.
Figure 10. (A) Infrared spectrum of the photoproduct(s) generated upon UV (l > 295 nm) irradiation of 5-methylthio-1,3,4-thiadiazole-2-thione isolated
in an Ar matrix; (B) spectrum of the thiol isomer VIIa of the compound predicted at the DFT(B3LYP)/6-311þþG(2d,p) level. The calculated wavenumbers
were scaled by the factor of 0.98.
during UV irradiation. The bands of the photodecomposition
products are much broader than the bands due to isomer VII.
Most probably, the reason for such broadening is the interaction
of the photodecomposition products which are trapped in the
same matrix cage.
tautomeric forms generated via proton transfer from NH group to
—
the sulfur atom of the vicinal C—S group (Scheme 5). Thereby, it
was experimentally proven that the photoinduced thione ! thiol
reaction occurs for 1,3,4-thiadiazole-2-thiones, in spite of larger
NHꢀꢀꢀS distance in comparison to other, previously studied
phototautomerizing compounds.
A light-driven hydrogen atom transfer from endocyclic nitro-
gen atom to exocyclic sulfur atom must involve dissociation of the
NH bond as the key step of the reaction. UV-induced detachment of
hydrogen atoms from NH bonds was recently observed for such
CONCLUDING REMARKS
When exposed to UV light, the matrix-isolated molecules of
[
34,35]
[36]
[37,38]
1,3,4-thiadiazole-2-thiones, studied in the present work, under-
compounds as pyrrole,
imidazole,
indole,
and
[
39,40]
went phototransformations that could be monitored spectro-
scopically. Scrutiny of the IR matrix spectra, recorded before and
after UV irradiation, allowed to identify the photoproducts as the
adenine.
For the molecules of these compounds seeded
in supersonic jet expansion, fast hydrogen atoms detaching upon
UV-excitation from the NH groups were experimentally detected.
J. Phys. Org. Chem. 2010, 23 56–66
Copyright ß 2009 John Wiley & Sons, Ltd.
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H. ROSTKOWSKA, L. LAPINSKI AND M. J. NOWAK
In the molecules of pyrrole, imidazole, indole, and adenine, the
hydrogen atom dissociated from the endocyclic nitrogen atom of
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[
41]
a five-membered ring.
theoretical calculations
These experiments were inspired by
[
42–45]
predicting that the photoinduced
1620.
H-atom detachment should be driven by a repulsive ps* state. It
seems quite likely that hydrogen atom detachment on the
[20] A. J. Barnes, J. Mol. Struct. 1984, 113, 161.
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could be trapped by a neighboring sulfur atom. In the latter
case, the molecule would change its tautomeric form. A
Photo-Induced Dissociation Association (PIDA), mechanism of
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[
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[
[
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 56–66