Photochemical reactions of trimethylene sulfide radical cations in Freon matrices at 77 K

Article abstract of DOI:10.1016/j.mencom.2008.03.002

Photobleaching (λ = 436 nm) of trimethylene sulfide radical cations (λ_max ≈ 460 nm, ε_max ≈ 345 dm³ mol^-1 cm^-1) stabilised in different Freon matrices at 77 K results in the formation of distonic radical cations CH₂CHSHCH₂^·+.

Full text of DOI:10.1016/j.mencom.2008.03.002

Mendeleev
Communications
Mendeleev Commun., 2008, 18, 67–68
Photochemical reactions of trimethylene sulfide
radical cations in Freon matrices at 77 K
Mikhail Ya. Mel’nikov,* Vladilen N. Belevskii, Anastasiya D. Kalugina,
Vladimir I. Pergushov and Daniil A. Tyurin
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation.
Fax: +7 495 939 1814; e-mail: melnikov@excite.chem.msu.ru
DOI: 10.1016/j.mencom.2008.03.002
Photobleaching (l = 436 nm) of trimethylene sulfide radical cations (lmax » 460 nm, emax » 345 dm3 mol–1 cm–1) stabilised in
+
·
different Freon matrices at 77 K results in the formation of distonic radical cations CH2CHSHCH2 .
Radical cations (RCs) are primary products of the one-electron
oxidation of organic molecules and act as intermediate particles
in various radiation-chemical, electrochemical, photochemical
and catalytic solid-phase processes.
responding increase in the amount of the paramagnetic reaction
products as a function of the light dose absorbed; the error of
absolute measurements of quantum yields did not exceed 25%.
The calculations by the method of density functional theory
(DFT) with the Perdew–Burke–Ernzerhof (PBE) and PBE-1
exchange-correlation functional were carried out using the
PRIRODA program package.9
The investigation of chemical reactions of organic RCs (i.e.,
alkanes, ethers, esters, acetals, amides, carboxylic acids, etc.) in
ground and excited states gained notable development in recent
years.1–3 Though new methods of registration of RC in solu-
tions, such as quantum beats spectroscopy,4 have recently been
developed, the method of stabilization of RC in Freon matrices
at low temperatures5 is still the most widely spread one used
in the study of structure and activity of RC. It is known that
photobleaching of the RC of cyclobutane in Freon 11 at 77 K
leads to a transformation into the RC of but-1-ene.6
The aim of this work was to study the mechanism and
efficiency of photochemical reactions of RCs of cyclic sulfides
stabilised in different Freon matrices at 77 K taking the
trimethylene sulfide RC as a model.
We used CFCl3 (Freon 11, ~99%, Aldrich), CFCl2CF2Cl
(Freon 113, 99.9%), CF3CCl3 (Freon 113a, 99%, Aldrich) as
matrices; in certain cases Freons were additionally purified
using standard techniques. Trimethylene sulfide (97%, Aldrich)
was used without additional purification.
The X-ray irradiation of frozen (77 K) 0.1 mol% solutions of
trimethylene sulfide in Freon 11 results in the appearance of a
signal in the EPR spectrum, which is characterised by axially
anisotropic g-tensor (g|| > g^) and hyperfine interaction of four
magnetically equivalent protons and also one proton with a
noticeably smaller hyperfine splitting constant [Figure 1(a)].
The hyperfine couplings equal to a1H(4H ) = 3.2 mT, a2H(1H ) =
= 1.2 mT, and g = g|| – g^ = 0.022, giso » 2.019 [Figure 1(d)]
provide the best simulation of the experimental spectra of these
paramagnetic particles. As we have indicated, the annealing of
X-ray irradiated 0.1 mol% solutions of trimethylene sulfide in
Freon 11 to 150 K results in the disappearance of additional
-proton hyperfine coupling and appearance of an EPR signal
close to the one observed in the literature10 [Figure 1(b)]. Hence,
comparing our results and previously reported data10 allows us
to attribute the observed EPR spectrum to the trimethylene
sulfide RC. Note that we did not observe the formation of
dimeric trimethylene sulfide RCs [a(8H) = 0.9 mT11] in the range
of the trimethylene sulfide concentrations used in all of the
test Freons.
The solutions of test compounds in Freons (0.1 mol%)
evacuated at ~0.1 Pa were sealed in quartz and SK-4B glass
tubes and irradiated at 2–4 kGy and 77 K. A 5BKhV6-W X-ray
generator (50 kV, 80 mA) was the emitting source.
The EPR spectra of the generated paramagnetic particles
were recorded on a Varian E-3 X-band spectrometer. The error
of absolute measurements of concentration of paramagnetic
particles did not exceed 20%. Simulations of EPR spectra were
carried with the use of the PEST WinSim and WINEPR Simfonia
software.7 Optical spectra were recorded on a Specord M-40
spectrophotometer at 77 K; quartz tubes with 1 mm optical
path lengths were used for optical measurements. The oscil-
lator strengths for electronic transitions were obtained as8
Along with the signal from the RC of trimethylene sulfide,
additional signals were observed in the EPR spectrum from
other radical products formed from Freon and trimethylene
sulfide molecules under X-ray irradiation. Though these inter-
mediates have broadened components of hyperfine structure,
they still have an influence on the overall form of the EPR
spectra observed in various irradiated Freons.
The irradiation of trimethylene sulfide solutions in Freon 11
at 77 K generates coloured products, which are characterised by
the UV-VIS electronic absorption spectrum consisting of two
bands at 370 and 460 nm, which could be resolved by Gaussian
two-peak approximation. The consequent photobleaching (436 nm)
leads to the disappearance of this originally generated UV-Vis
absorption spectrum. It is known that dimethyl sulfide and tetra-
hydrothiophene RCs stabilised in 2-butyl chloride at 77 K12 have
absorption maxima at 375 and 465 nm. We estimate the molar
~
f = 4.32×10–9emax
n1/2, where emax is the molar absorption coef-
ficient in the maximum of the absorption band, dm3 mol–1 cm–1;
n
1/2 is the half-width of the absorption band, cm–1.
A DRSh-250 high-pressure mercury lamp with a glass filter
to select mercury spectral lines at l = 436 nm (Tmax = 27%,
n
1/2 = 2400 cm–1) was used as a light source. The absolute
light intensity measured by ferryoxalate actinometry (l = 436 nm)
was 1.2×10–5 Einstein cm–3 s–1. Quantum yields were calculated
from the decrease of the amount of radical cations and the cor-
absorption coefficient at 460 nm as emax
It was estimated by comparison of the values of concentration
»
3.4×102 dm3 mol–1 cm–1.
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© 2008 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2008, 18, 67–68
changes of trimethylene sulfide RC, measured by EPR method,
with the corresponding absorbances obtained. The oscillator
strength for this electron transition is f = 7.3×10–3. The molar
absorption coefficient and oscillator strength are rather low for
the RC of trimethylene sulfide due to low symmetry of this
electron transition.
Table 1 Quantum yields (j) of photochemical reactions of the tri-
methylene sulfide RC in Freon matrices.
a
Matrix
j436
j1/j2
Freon 11
Freon 113a
Freon 113
0.023 0.007
0.022 0.007
0.010 0.003
1/2
1/1
1/1
Photobleaching at 436 nm leads to the disappearance of the
trimethylene sulfide RC and formation of new paramagnetic
species [Figure 1(c)] in all of the Freon matrices. These species
have a triplet EPR spectrum with a binominal intensity distribu-
tion due to interaction with two magnetically equivalent protons.
The hyperfine splitting constant is a(2H) = 2.2 mT, giso » 2.003.
Some trimethylene sulfide RCs disappear after photobleaching
at 436 nm and 77 K in Freon matrices without formation of
paramagnetic species as the total quantity of paramagnetic species
in the frozen solution decreases. Thus, only 30% trimethylene
sulfide RCs react in a Freon 11 matrix, and about 50% in a
Freon 113 and Freon 113a. This indicates that the two processes,
namely, the photoinduced charge transfer, which leads to a
decrease of the integral intensity of the EPR spectra, and
the formation of the new paramagnetic species, have similar
quantum yields.
aj1 is the quantum yield of charge transfer reaction, j2 is the quantum yield
of formation of distonic RC.
pathway of trimethylene sulfide RC transformation is linked
+
·
with the formation of the distonic RC CH2CHSH CH2, where a
positive charge is localised on the sulfur atom, whereas spin
density is localised on the methylene group. DFT calculations
give the values of hyperfine splittings on two protons a(2H) =
= 2.1–2.4 mT, that is in accordance with the one observed
experimentally [Figure 1(c)]. Thus, the results of quantum
chemical calculations show that thioformaldehyde RC complexes
+
·
with ethylene molecules (2) or the distonic RC CH2CHSH CH2
could be formed (4), considering hyperfine splitting constants.
The quantum yields of trimethylene sulfide RC transforma-
tion in the Freon matrices are low (Table 1). The variation
in the nature of the Freon matrix has little influence on the
efficiency and products of the photochemical reactions of the
trimethylene sulfide RC.
The fact that we have not observed the formation of the
complexes of thioformaldehyde RC with ethylene molecules
from non-relaxed RC ( Ip » 3.2 eV) immediately after X-ray
irradiation and that we also have not observed any influence
of the Freon matrix nature on the quantum yields (j2) of the
photochemical reactions allowed us to consider reaction (4)
+
+
·
·
·
·
·
cyclo-C3H6S ® H2CS
(1)
(2)
(3)
(4)
+
+
·
cyclo-C3H6S ® [H2CS ···H2C=CH2]
+
+
·
cyclo-C3H6S ® [cyclo-C2H3S–Me]
+
+
·
cyclo-C3H6S ® CH2CHSH CH2
Several reaction paths for photobleaching of trimethylene
sulfide RC are feasible. The first is accompanied by the forma-
tion of thioformaldehyde RC and molecule of ethylene. In this
case, either RCs of thioformaldehyde (1) or their complexes with
ethylene molecules (2) could be stabilised. Quantum chemical
calculations by the DFT method give the hyperfine splitting
constant a(2H) = 5.4 mT for thioformaldehyde RC, which is not
consistent with one observed experimentally. The quantum-
chemical calculations show that such a structure of complex RC
of thioformaldehyde with ethylene molecules could be realised
for which aiso(2H) » 2.3–2.4 mT. Another path of trimethylene
sulfide RC transformation could be connected with an isomeri-
zation into propylene sulfide RC (3). However, the experi-
mental EPR spectrum of propylene sulfide RC [a(3H) = 1.6 mT,
aMe(H) £ 0.3 mT] [Figure 1(c)] differs from that observed after
photobleaching the trimethylene sulfide RC. The last possible
the most probable to occur [reaction (2) in contrast to reaction
¹
(4) has
V
³ 0 and its efficiency should depend on the free
volume, which differs for different matrices]. Furthermore, the
value of giso » 2.003 for the product of the reaction is near to
the value of giso for alkyl radicals and is in marked contrast
with the values characteristic of complexes of sulfur-containing
RCs with neutral molecules, for example, giso » 2.013 for
complexes with arenes.13
This work was supported by the Russian Foundation for
Basic Research (project no. 07-03-00105) and the Presidium of
the Russian Academy of Sciences (ChD-01).
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2.5 mT
Figure 1 EPR spectra of trimethylene sulfide in Freon 11 (a) X-ray
irradiated frozen 0.1 mol% solutions at 77 K, (b) solution (a) after annealing
to 150 K, (c) solution of trimethylene sulfide RC after photobleaching with
l = 436 nm at 77 K, (d) spectrum (a) simulated with the following parameters:
a(4H ) = 3.2 mT, a(1H ) = 1.2 mT, g|| > g^ and g = 0.022.
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Received: 7th September 2007; Com. 07/3010
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