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

Article abstract of DOI:10.1016/j.mencom.2008.11.005

Radical cations of trimethylene oxide under photobleaching at 77 K could form either distonic radical cations ·CH2CH2CH=OH⁺/·CH2CH2O⁺=CH2 or oxetan-2-yl radicals depending on the Freon matrix used.

Full text of DOI:10.1016/j.mencom.2008.11.005

Mendeleev
Communications
Mendeleev Commun., 2008, 18, 305–306
Photochemical reactions of trimethylene oxide
radical cations in Freon matrices at 77 K
Mikhail Ya. Mel’nikov,* Vladilen N. Belevskii, Anastasiya D. Kalugina,
Ol’ga L. Mel’nikova, 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.2008.11.005
+
Radical cations of trimethylene oxide under photobleaching at 77 K could form either distonic radical cations ·CH CH CH=OH /
2
2
+
·
CH CH O =CH or oxetan-2-yl radicals depending on the Freon matrix used.
2 2 2
Radical cations (RCs) are the primary products of one-electron
oxidation of organic molecules and act as intermediate particles
in various processes in solids. The method of RC stabilization in
(a)
(b)
1
low temperature Freon matrices has been used extensively for
the studies of the structure and reactivity of RCs in their ground
and excited electronic states. It is known2,3 that photolysis of
RC of dimethyl ether and tetrahydrofuran in a Freon 11 matrix
induces charge transfer to the matrix molecules, whereas photo-
(c)
(e)
(f)
chemical reactions of the same RC in SF and a Freon mixture
6
(
Freon 11/Freon 114B2) lead to their deprotonation.
The aim of this work was to study the mechanism and efficiency
of the photochemical reactions of the RC of trimethylene oxide
(
TMO) stabilized in different Freon matrices at 77 K.
(d)
We used CFCl3 (Freon 11, 99%, Aldrich), CFCl CF Cl
2
2
(
Freon 113, 99.9%), CF CCl (Freon 113a, 99%, Aldrich) as
3
3
matrices; in certain cases, Freons were additionally purified using
standard techniques. TMO (97%, Aldrich) was used without
additional purification.
The solutions of TMO in Freons (0.1–0.5 mol%) evacuated
at ~0.1 Pa were sealed in quartz and SK-4B glass tubes and
irradiated with 2–4 kGy doses at 77 K. A 5BXV6-W X-ray
generator (50 kV, 80 mA) was the emitting source.
0
5
10 mT
Figure 1 EPR spectra of the X-ray irradiated 0.1 mol% solutions of the
TMO in Freon 113a: (a) at 77 K, (b) simulation with the following
parameters: a(β-4H) = 6.5 mT, a(γ-2H) = 1.1 mT, (c) experimental spectrum
after photobleaching (l = 546 nm) at 77 K, (d) simulation of the spectrum
of the distonic RC with ·CH CH ~ fragment, (e) after consequent annealing to
2
2
1
40 K; and (f) X-ray irradiated 2.5 mol% solutions of the TMO in Freon 11
at 150 K.
The EPR spectra were recorded on a Varian E-3 X-band
spectrometer. The concentration of paramagnetic centres in a
sample was determined by comparison to the reference sample,
a single crystal of CuCl ·2H O with a known number of para-
7
8
of Coulomb, exchange and density-functional contributions.
Molecular geometries were fully optimized (tolerance on gradient:
10 a.u.). A very fine integration grid was used for the DFT
–
5
2
2
magnetic Cu2 ions. The error in the absolute value of the con-
centration of paramagnetic species determined in this way did
not exceed ±20%. Simulations of the EPR spectra were carried
out with the use of the PEST WinSim and WINEPR Simfonia
+
–8
exchange-correlation terms (accuracy of 10 a.u. per atom).
A high accuracy solution of SCF equations was used (tolerance
on density: 10–8 a.u.). The hyperfine splitting constants (hfcs)
were evaluated from the spin density based electric field gradient
tensors at nuclear positions using the point nucleus model. All
4
software. Optical spectra were recorded on a Specord M-40
9
spectrophotometer at 77 K; quartz tubes with 1 mm optical path
lengths were used for optical measurements. A DRSh-250 high-
pressure mercury lamp with a glass filter to select mercury
calculations were done using the PRIRODA-04 program package.
The EPR spectra of the X-ray irradiated 0.1 mol% solutions
of TMO in Freon 113a at 77 K have a spectral pattern as a
quintet of triplets [Figure 1(a)]. The central part of the EPR
spectrum is distorted by the signal from the matrix and oxetan-
2-yl radicals. The hfcs and g-tensor components were obtained
from the modeling of the experimental EPR spectra [a(β-4H) =
= 6.5 mT, a(γ-2H) = 1.15 mT, giso » 2.010] [Figure 1(b)]. These
values are in good agreement with previous data10 [a(β-4H) =
= 6.4 mT, a(γ-2H) = 1.1 mT, giso » 2.007], and with the results
of our quantum chemical calculations [a(β-4H) = 6.91 mT,
a(γ-2H) = 1.28 mT].
–1
spectral lines at l = 546 nm (Tmax = 35%, Δn1/2 = 1800 cm ) was
used as a light source. The absolute light intensity measured
–5
–3 –1
by actinometry with Reinike salt was 5.8×10 einstein cm s .
Quantum yields were calculated from the dependences of the
concentration of the RC and reaction products on the absorbed
light dose; the error of the absolute values of the quantum yields
determined in this way did not exceed ±25%.
Quantum chemical calculations were carried out using the
spin-unrestricted density functional theory (DFT) with non-
5
empirical hybrid functional PBE0. We used core correlation
The EPR spectra of the X-ray irradiated 0.1 mol% solutions
of TMO in Freon 11 at 77 K also consist of a quintet signal
with additional hyperfine structure. Angular dependence of
6
consistent triple zet basis set Λ22. To speed up the calculations,
resolution-of identity techniques are used for the evaluation
©
2008 Mendeleev Communications. All rights reserved.
– 305 –

Mendeleev Commun., 2008, 18, 305–306
the EPR spectra in this matrix observed at 77 K makes their
Table 1 Quantum yields (j) of photochemical reactions of the TMO RC
in Freon matrices.
interpretation more complicated. The hfcs obtained from the
modeling of the experimental ESR spectra in Freon 11 are almost
equal to the values given above for Freon 113a matrix.
The EPR spectra of the X-ray irradiated 0.1–0.5 mol% solu-
tions of the TMO in Freon 113 at 77 K are similar to those
observed in the Freon 11 and Freon 113a matrices. The larger
line width obtained in Freon 113 if compared to the line width
in Freon 11 and Freon 113a matrices, caused poor resolution
of the hyperfine structure originating from two γ-protons in the
EPR spectra of the TMO RC in Freon 113.
Photolysis (at 546 nm) of the TMO RC in Freon 11 and
Freon 113a matrices at 77 K leads to the formation of new
paramagnetic species [Figure 1(c)] with the conversion of more
than 90%. The hfcs obtained from modeling of the observed
EPR spectra [Figure 1(d)] are aiso(1H) = 2.08 mT, aiso(1H) =
Matrix
j
Matrix
j
Matrix
j
Freon-11
0.4
Freon-113a
0.5
Freon-113
0.07
110 K. The quantum-chemical calculated hfcs values [a(α-H) =
=
0.79 mT, a(β-2H) = 2.90 mT] are in good agreement with the
experimental data.
The quantum yields of the photochemical transformation of
the TMO RCs, obtained from the dependence of the quantity
of the reacted TMO RCs on the dose of light absorbed, are
summarised in Table 1. These quantum yields are several times
higher in the polycrystalline Freon 11 and Freon 113a matrices
than in the glassy Freon 113 matrix. Therefore, the reaction
leading to the formation of neutral radicals (1) is less effective
if compared to the formation of the distonic RCs (2).
=
2.42 mT and aiso(2H) = 0.72 mT. The species formed under
+
·
·
+
cyclo-C H O ® cyclo-C H O + H
(1)
(2)
photolysis of the TMO RC in Freon 11 and Freon 113a matrices,
after annealing of their frozen solutions at 140–150 K have
identical EPR spectra with the spectral pattern of triplet of
triplets [Figure 1(e)]. The temperature-dependent changes in
the EPR spectra are totally reversible. The spectral pattern of
the signal in the EPR spectra measured at 150 K allows us to
assign the products of the reaction to the distonic RC with the
fragment ·CH CH ~. The hfcs, measured from the experimental
3
6
3
5
cyclo-C3H6O+ ® ·CH2CH2CH=OH /·CH2CH2O =CH2
·
+
+
It is possible that difference in the reaction pathways and
efficiency of the RC transformations in different matrices could
be explained by the fact that, in the polycrystalline matrices
(
Freon 11 and Freon 113a), the processes of the relaxation of
14
the excited states are slower than in the glassy Freon 113
matrix. Therefore, in the polycrystalline matrices, RCs undergo
transformations in the non-relaxed vibrationally excited state.
2
2
EPR spectrum at 150 K, are a(α-2H) = 2.23 mT, a(2H) = 0.81 mT,
giso » 2.003 and are characteristic of this type of the RCs.11
The quantum-chemical calculations have shown that the hfcs
agree well with two types of species: conformers of the distonic
14
Different solvent surrounding RC in different Freon matrices
could also have some influence on the reaction pathways of the
RC transformations and, in particular, on the reactions of their
deprotonation.
+
RCs ·CH CH O =CH [a(α-2H) = 2.39 mT, a(β-2H) = 0.57 mT],
2
2
2
+
and ·CH CH CH=OH [a(α-2H) = 2.46 mT, a(β-2H) = 0.66 mT].
The formation of ·CH CH CH=OH occurs as a result of
2
2
+
2
2
This work was supported by the Russian Foundation for
Basic Research (grant no. 07-03-00105) and the Presidium of
the Russian Academy of Sciences (programme no. ChD-01).
1
,2-migration of β-H atom to oxygen with the subsequent
cleavage of the C–O bond. Note that the formation of these
RCs is 75 kJ mol–1 more energetically favourable in the ground
+
state than the formation of ·CH CH O =CH .
2
2
2
References
Photolysis (at 546 nm) of the X-ray irradiated 0.1 mol%
solutions of TMO in Freon 11 at 77 K results in disappearance
of two overlapping absorption bands with maxima at 450 and
1
2
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00 nm in their optical spectra. The absorption band at 300 nm
3
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–
could be attributed to the absorption of the ionic pairs CFCl ···Cl ,
2
for which the absorption maximum at 310 nm has been reported
previously.12 At the same time, absorption at 430–460 nm is
4
5
6
7
D. R. Duling, J. Magn. Reson., 1994, 104B, 105.
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2
,3
characteristic of the RC of the ethers. The extinction coeffi-
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at 450 nm and TMO RC concentration measured by EPR. The
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2
13, 514.
3
3
–1
–1
estimated value e » 3.2×10 dm mol cm is in good
8
9
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max
,3
2
agreement with data for the RC of the ethers. The oscillator
8
04 (Russ. Chem. Bull., Int. Ed., 2005, 54, 820).
strength for the electronic transition (f = 0.1) was obtained
from the formula13 f = 4.32×10 emaxΔn1/2, where emax is the
~
–9
10 F. Williams and X.-Z. Qin, Radiat. Phys. Chem., 1988, 32, 299.
1
1 D. V. Baskakov, O. L. Melnikova, V. I. Feldman and M. Ya. Melnikov,
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2002, 36, 103].
molar absorption coefficient in the maximum of the absorption
3
–1
–1
band, dm mol cm ; Δn is the half-width of the absorption
1
/2
–
1
band, cm .
12 M. Ya. Melnikov, D. V. Baskakov and V. I. Feldman, Khim. Vys. Energ.,
2002, 36, 346 [High Energy Chem. (Engl. Transl.), 2002, 36, 309].
Photolysis (at 546 nm) of the TMO RCs in the Freon 113
matrix at 77 K leads to the formation of TMO radicals. These
radicals have a triplet signal in the EPR spectra with hfcs
a(2H) = 2.9 mT. The formation of the same oxetan-2-yl radicals
in 2.5 mol% solutions in Freon 11 is happening during X-ray
irradiation at 77 K. Annealing of the X-ray irradiated 2.5 mol%
solution of TMO in Freon 11 to 150 K causes a change of
the EPR spectra to the triplet of doublets [a(α-H) = 0.9 mT,
a(β-2H) = 2.9 mT] [Figure 1(f)]. These temperature-induced
changes are reversible. Our experimentally obtained hfcs are
in good agreement with the ones [a(α-H) = 0.8 mT, a(β-2H) =
13 Einführung in die Photochemie, ed. G. O. Bekker, Deutscher Verlag
der Wissenshaften, Berlin, 1976.
14 V. I. Feldman and M. Ya. Mel’nikov, Khim. Vys. Energ., 2000, 34, 279
[High Energy Chem. (Engl. Transl.), 2000, 34, 236].
1
0
=
2.87 mT] measured earlier for oxetan-2-yl radicals, which
were formed after annealing of the TMO RCs in Freon 113 to
Received: 20th March 2008; Com. 08/3107
–
306 –