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M. Pagacz-Kostrzewa et al. / Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 37–44
of MT was deposited with a large excess of argon onto a cold
CsI window. The cold substrate was maintained at a temperature
15 K/10 K for deposition/measurement by means of a closed cycle
helium refrigerator APD Cryogenics (ARS-2HW). Infrared spectra
were recorded between 4000–400 cm−1 in a transmission mode
with a resolution of 0.5 cm−1 by means of a Bruker 66 FTIR spec-
trometer equipped with a liquid N2 cooled MCT detector.
2.2. Photolysis experiments
After the infrared spectra of the initially deposited matrices
were recorded the samples were irradiated with the tunable UV
light provided by the frequency doubled signal beam of a pulsed
(7 ns) optical parametric oscillator Vibrant 355 (Opotek Inc.) (rep-
etition rate 10 Hz, average pulse energy ∼4.0 mJ) pumped with a
pulsed Nd:YAG laser (Quantel). The xCryo traverse system for the
precise movement and the repeatable position of the cryostat in
the course of irradiations and measurements was supplied by EMD-
tech.
Fig. 1. B3LYP/6-311++G(2d,2p) optimized structures of the 1H-MT, 2H-MT min-
ima and the transition state joining them together with the B3LYP/6-311++G(2d,2p)
zero-point vibrational energy corrected relative energies (kJ mol−1). The imaginary
frequency of TS is given in cm−1
.
equilibrium in the gas phase (prior to deposition) between 1H-MT
below, both 1H and 2H tautomeric forms are detected in argon
matrices. Moreover, they are present with the population close to
Thus, MT is another example [18,42] where, in spite of the large
value of the energy barrier for the tautomerization, both species in
question are experimentally detected. The mechanism of this phe-
nomenon has been addressed by Yang and Rodgers for the case of
cytosine [43]. These authors explained the coexistence in the gas
phase of several tautomers by a significant lowering of the tau-
tomerization barriers in hydrogen bonded aggregates present in
the gas phase before condensation as compared to the unimolecular
tautomerization. Following this approach two MT dimer structures
containing either 1H or 2H subunits and the transition state joining
them (TSD) were optimized at the B3LYP/6-311++G(2d,2p) level.
The key results of these calculations are presented in Figure S1 in
Supplementary data where the results of the intrinsic reaction coor-
dinate (IRC) analysis performed for TSD is shown together with the
relevant structures and energetics and is compared to the same
results obtained for the monomeric species. A comparison of the
calculated energy barriers indicates a significant lowering of the
barrier height for the bimolecular tautomerization comparing to
the unimolecular reaction (231.49 versus 90.19 kJ mol−1). These
results suggest that at a certain stage of the vaporization of the
crystalline sample the MT dimers may be formed facilitating a
transformation of the 1H form, present in the solid sample [39]
into 2H form.
2.3. Computational details
All calculations were performed with the Gaussian 09-D.01 pro-
at the B3LYP/6-311++G(2d,2p) level [30]. Anharmonic frequencies
and intensities were calculated at the same level of theory. The
vertical excitation energies were calculated using the time depend-
ent functional theory TD-DFT [31,32]. The vibrational spectra were
simulated using SYNSPEC program [33].
3. Results and discussion
3.1. Structures and energetics of the MT minima
the predominating form adopted by the majority of 5-substituted
tetrazoles in the gas phase [24,34]. In turn, various physicochem-
ical methods revealed that in solutions and in the solid state the
more polar 1H tautomer prevails [35,36] and 5-methyltetrazole
follows this general trend. The recent quantum chemical calcu-
lations performed for this molecule at different levels of theory
showed the 8–17 kJ mol−1 electronic energy difference between
the two tautomers with the 2H form being more stable [37,38].
Our B3LYP/6-311++G(2d,2p) results show the same stability order
with ꢀEZPE equal to 7.35 kJ mol−1. The X-ray diffraction method
revealed the presence of the 1H tautomer in the MT crystal [39].
Transition state connecting the two minima was located at
206.69 kJ mol−1, respectively for the forward and reverse reac-
tions and are in the range reported for this type of transformation
[18,24,40,41]. The optimized structures of the species of interest
are presented in Fig. 1.
Assuming that the equilibrium between the MT tautomers is
achieved in the gas phase, the relative populations of both species
may be estimated. Using the B3LYP/6-311++G(2d,2p) free energy
values and ꢀG = −RTln K equation, where ꢀG means the difference
between Gibbs free energy for two tautomers and K is the equilib-
rium constant for these species, the population at the sublimation
temperature (ca. 305 K) was found to be 4 and 96%, for 1H and 2H
forms, respectively. These values are close to those reported earlier
[24].
3.2. Photolysis of 5-methyltetrazole (MT) isolated in argon
matrices
The experimental spectrum of the MT/Ar sample recorded
immediately after deposition at 15 K (recorded at 10 K) showed that
both 1H-MT and 2H-MT tautomers were present in the matrix with
the apparent predominance of the latter form (see Figure S2 and
Table S1 in Supplementary data). Such matrices were subjected
to narrow-band irradiations and the obtained results will be dis-
cussed in the following paragraphs. Several series of irradiations of
the MT/Ar matrices were carried out using UV radiation from the
tunable OPO laser system. After each irradiation, an infrared spec-
trum of the matrix was taken to monitor possible transformations
of the precursor molecules.
First, the screening measurements have been performed by
gradual decreasing wavelength in steps ranging from 1 to 5 nm
to cover the 330–225 nm range. An onset of the photolysis began
The large values of the energy barriers for the unimolecu-
lar 1H-MT ↔ 2H-MT tautomerization suggest that the tautomeric