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Mendeleev Communications
Mendeleev Commun., 2011, 21, 153­154

Reversible photochemical transformations of cis- and trans-2,3-dimethyloxirane radical cations in freonic matrices at 77 K
Ivan D. Sorokin, Vladimir I. Feldman, Ol'ga L. Mel'nikova, Vladimir I. Pergushov, Daniil A. Tyurin and Mikhail Ya. Mel'nikov*
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation. Fax: +7 495 939 1814; e-mail: melnikov46@mail.ru
DOI: 10.1016/j.mencom.2011.04.014

The reversible photoinduced transformations of 2,3-dimethyloxirane radical cations in freonic matrices at 77 K are related to the conversion between the cis- and trans-isomers of an open form and a cyclic form. The structure and photochemistry of radical cations (RCs) origin ating from small heterocycles are of considerable interest since these species exhibit high lability and reveal a variety of reaction pathways. It has been established1­4 that the RCs of oxiranes are stabilized in the ringopen form resulting from C­C bond cleavage in freonic matrices at 77 K. According to reported data,4 the action of visible light (400­600 nm) on the irradiated solutions of methyl oxirane and cis2,3dimethyloxirane in Freon 11 at 77 K leads to irreversible changes in the optical and EPR spectra, which were attributed to the cis­trans isomerization of the ringopen forms of the oxirane RC. The aim of this work was to identify paramagnetic species stabilized upon the irradiation of cis and trans2,3dimethyl oxirane in Freons at 77 K and to study their photochemically and thermally induced transformations. The nature of the paramagnetic centers formed upon the low temperature irradiation of cis and trans2,3dimethyloxiranes in Freons, as well as the pattern of their photochemical and thermal transformations, were found identical (as revealed by both EPR and optical spectroscopy). For this reason, we will not specify cis or transisomer in the further discussion of our results. The Xray irradiation of the frozen 0.3 mol% 2,3dimethyl oxirane/Freon 113a solutions at 77 K results in an EPR spectrum [Figure 1(a)], which can be fitted as a superposition of two signals characterized by nearly binomial patterns consisting of nine and eight equidistant lines, respectively. The former signal appears to be a result of an unpaired electron interacting with eight equivalent protons, and it can be charac
CFCl3 (Freon 11, ~99%, Aldrich), CF3CCl3 (Freon 113a, obtained syn thetically5 from Freon 113, 99%, Aldrich; the main product constituted more than 99% of the mixture) were used; in certain cases, Freons were purified additionally using a standard procedure. Commercial cis and trans2,3dimethyloxiranes (97%, Aldrich) were used as received. The dimethyloxirane/Freon solutions (0.3­0.5 mol%) were inserted into quartz or SK4B glass tubes, evacuated to ~0.1 Pa and irradiated with Xrays at 77 K to doses of 2­4 kGy. A 5BHV6W Xray source (33 kV, 80 mA) was used. The EPR spectra of paramagnetic species were recorded on a Varian E3 spectrometer. The absolute error in the concentration of paramagnetic species determined by EPR was within ±20%. The EPR spectra were simulated with the use of the PEST WinSim and Simfonia6 standard software packages. The optical absorption spectra were measured on a Specord M40 spec trophotometer at 77 K using flat quartz tubes with an optical path length of 0.1 cm. Oscillator strength values in the electron transitions were derived from the equation7 f » 4.32â10­9emaxDn1/2, where emax refers to the molar absorption coefficient at the absorption band maximum (dm3 mol­1 cm­1), and Dn1/2 is the halfwidth of the absorption band (cm­1).
© 2011 Mendeleev Communications. All rights reserved.


(a) (b)

(c) (d) 2 mT Figure 1 EPR spectra of the irradiated solutions of 2,3dimethyloxirane in Freon 113a: (a) measured immediately after Xray irradiation at 77 K, (b) after exposing the irradiated sample to light ( l = 578 nm) at 77 K, (c) after sub sequent action of light with l = 436 nm at 77 K, (d) after warming the sample to 111 K. Arrows show the components belonging to the spectrum of Mn2+ ions, used as an external standard.

terized by the hyperfine coupling (hfc) constant a(8H) = 1.9 mT, while the latter can be assigned to a species with seven equivalent protons and virtually identical hfc constant. The optical absorption spectrum recorded at 77 K shows the appearance of a radiation induced absorption band in the region of l > 380 nm. A standard photobleaching technique (photolysis with l > 690 nm light) was used to eliminate the ionic products of matrix irradiation.13 The subsequent action of light at l = 578 nm at 77 K led to a nearly complete transformation in the EPR spectra: the nineline spectrum was converted to an eightline spectrum, while the total amount of paramagnetic species remained virtually unchanged [Figure 1(b)]. At the same time, an absorption band at 575 nm in the optical spectra disappears, while the intensity of a band at 465 nm increases
A highpressure mercury lamp (DRSh250) supplied with glass filters and/or interference filters [l = 436 nm, peak transmission (Tmax) = 27%, Dn1/2 = 2400 cm­1; l = 578 nm, Tmax = 30%, Dn1/2 = 297 cm­1] was used as a light source. The absolute light intensities at the specified wavelengths were 1.28â10­8 and 2.2â10­9 Einstein cm­3 s­1, respectively. Quantum yields in photochemical reactions were derived from the increase in the number of resulting paramagnetic species or the decrease in the number of initial RC versus the light dose absorbed by the sample. The DFT quantum chemistry calculations were carried out using a PBE0 approximation.8,9 Expansion of the exchangecorrelation density on an auxiliary basis set was used to boost the computations.10 Valencycor relation L2 basis was used in the calculations.11 The accuracy of the self consistence amounted to 10­7 atomic units (a.u.), the accuracy of the exchangecorrelation density integration amounted to 10­9 a.u. per atom, while the geometries were optimized up to the 10­5 a.u. gradient norm. The PRIRODA software package was employed for computations.12

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Mendeleev Commun., 2011, 21, 153­154
Me O H Me O Me Me H H

1.5 1.0 0.5 0.0 ­0.5 ­1.0 ­1.5 400 D

H Me

O

H Me

1

H

2 500 l/nm 600 700

Figure 2 Difference optical absorption spectra measured after exposing the irradiated solutions of 2,3dimethyloxirane in Freon 113a to light with dif ferent wavelengths at 77 K: (1) l = 578 nm and (2) l = 436 nm.

(Figure 2). The subsequent action of light with l = 436 nm at 77 K results in a reverse process, as confirmed by the changes in both EPR [Figure 1(c)] and optical absorption spectra (Figure 2). These mutual transformations can be carried out repeatedly since the loss in the integral intensity of the EPR spectra and the corre sponding decrease of the absorption band intensity in the optical spectra do not exceed 10% within a reaction cycle. A comparison of the variations in the EPR and optical spectra made it possible to estimate the absorption coefficients for the species involved in the transformations: e578 » 5.3â104 dm3 mol­1 cm­1 ( f » 0.56) and e436 » 2.0â104 dm3 mol­1 cm­1 ( f » 0.30), respectively. On the basis of experimental and published4 data, we believe that the nineline EPR spectrum [a(8H) = 1.9 mT] and the 575 nm absorption band are associated with the open configuration of the 2,3dimethyloxirane RC, which is isoelectronic to the allyl radical (characterized by a delocalized electron structure). This species (MeCHO· +CHMe) resulting from the C­C bond cleavage may exist in both cis and transforms. On the other hand, the eightline EPR spectrum [a(7H) = 1.9 mT] can be attributed to the cyclic form of the 2,3dimethyloxirane RC. This interpretation is supported by the results of quantum chemical calculations, which show that such hfc constants are characteristic of the cyclic transisomer of the RC possessing an elongated C­C bond (~0.178 nm). The feasibility of a similar cyclic RC formation (including the notion of an elongated C­C bond) had been discussed earlier.14 Furthermore, the experi mental absorption spectrum of the species revealed in our studies resembles that of the trimethylene oxide RC (detected in freonic matrices at 77 K). The latter RC exhibits an absorption maximum at 450 nm15 (although the absorption coefficient is substantially lower, e = 3.2â103 dm3 mol­1 cm­1, possibly, due to the transition being symmetry forbidden in this case). Proposed scheme:
cis,trans[MeC(H)OC(H)Me]· + ® cyclo[MeC(H)OC(H)Me]· +.

eightline EPR spectrum [a(7H) = 1.9 mT] into the nineline spectrum [a(8H) = 1.7 mT] [Figure 1(d)], while the optical spec trum remains unchanged. These transformations are reversible upon subsequent cooling (77 K), which can be explained by intra molecular motion dynamics in one of the RC transisomers (the one with the elongated C­C bond). These dynamics result in an increase in the hfc constant at one of the methine groups. Note that, in the case of the sample containing the open 2,3dimethyl oxirane RC, warming (from 77 to 111 K) results in transforma tion of the initial nineline EPR spectrum [a(8H) = 1.9 mT, Figure 1(c)] to another nineline spectrum [a(8H) = 1.7 mT, Figure 1(d)]. This conversion is accompanied by a change in the optical absorption spectrum (the band at 575 nm disappears while the intensity of the band at 465 nm increases). Cooling the sample down to 77 K results in the above transition (nineline to eightline signal) in the EPR spectra. Therefore, a slight increase in the temperature allows the cyclization of the 2,3dimethyl oxirane RC open form to occur in the matrix of Freon 113a. This result seems unexpected. Indeed, we may note that photocycliza tion is known for allyl radicals.16,17 Meanwhile, the thermal nature of the process for the isoelectronic RC revealed in the present study can only be explained by specific solvatation of the RC cyclic form (in contrast to the open form), which is probably determined by variations in charge localization for the two types of structures. A similar situation occurs for the intermediates produced in irradiated 2,3dimethyloxirane in a Freon 11 matrix (some devia tions can be ascribed to variations in the dynamics of paramag netic particles in those matrices). Thus, we can conclude that the photochemical transformations of 2,3dimethyloxirane radical cations (reported in ref. 4) can be attributed to the conversion between the cis, transisomers of the open form and the cyclic form rather than to the cis­trans photoisomerization proposed earlier. This study was supported by the Russian Foundation for Basic Research (project no. 100300195). References
1 L. D. Snow and Ff. Williams, Chem. Phys. Lett., 1988, 143, 521. 2 X.Zh. Qin, L. D. Snow and Ff. Williams, J. Phys. Chem., 1985, 89, 3602. 3 J. Rideout, M. C. R. Symons and B. Wr. Wren, J. Chem. Soc., Faraday Trans., 1986, 82, 167. 4 K. Ushida, T. Shida and K. Shimokoshi, J. Phys. Chem., 1989, 93, 5388. 5 W. T. Miller, E. W. Fager and P. H. Griswald, J. Am. Chem. Soc., 1950, 72, 705. 6 D. R. Duling, J. Magn. Reson., 1994, 104B, 105. 7 EinfÝhrung in die Photochemie, ed. G. O. Bekker, Deutscher Verlag der Wissenshaften, Berlin, 1976. 8 C. Adamo and V. Barone, Chem. Phys. Lett., 1998, 298, 113. 9 C. Adamo and V. Barone, J. Chem. Phys., 1999, 110, 6158. 10 D. N. Laikov, Chem. Phys. Lett., 1997, 281, 151. 11 D. N. Laikov, Chem. Phys. Lett., 2005, 416, 115. 12 D. N. Laikov and Yu. A. Ustynyuk, Izv. Akad. Nauk, Ser. Khim., 2005, 804 (Russ. Chem. Bull., Int. Ed., 2005, 54, 820). 13 M. Ya. Mel'nikov, D. V. Baskakov and V. I. Feldman, Khim. Vys. Energ., 2002, 36, 346 [High Energy Chem. (Engl. Transl.), 2002, 36, 309]. 14 T. Clark, J. Chem. Soc., Chem. Commun., 1984, 666. 15 (a) M. Ya. Mel'nikov, V. N. Belevskii, A. D. Kalugina, O. L. Mel'nikova, V. I. Pergushov and D. A. Tyurin, Mendeleev Commun., 2008, 18, 305; (b) M. Ya. Mel'nikov, A. D. Kalugina, O. L. Mel'nikova, V. I. Pergushov and D. A. Tyurin, Khim. Vys. Energ., 2009, 43, 355 [High Energy Chem. (Engl. Transl.), 2009, 43, 303]. 16 K. Holtzhauer, C. ComettaMorini and J. E. M. Oth, J. Phys. Org. Chem., 1990, 3, 219. 17 V. A. Radzig, L. Yu. Ustynyuk, N. Yu. Osokina, V. I. Pergushov and M. Ya. Mel'nikov, J. Phys. Chem., 1998, 102/27, 5220.

The quantum yields of the photochemical transformations of the RC at 77 K are j1 » 0.39 in the case of longwave light ( l = 578 nm) and j2 » 0.07 in the case of shortwave light ( l = 436 nm). According to our quantum chemical computations and data reported previously,4 the cis and transisomers of the 2,3di methyloxirane RC should give similar optical spectra and magnetic resonance parameters. This is in apparent conflict with the experi mental data. For this reason, we cannot ascribe the observed photo chemical reactions of the 2,3dimethyloxirane RC to the cis­trans transitions between the isomers of the RC open form. Warming the sample containing the cyclic 2,3dimethyloxirane RC (stabilized at 77 K) up to 111 K leads to the conversion of the

Received: 27th September 2010; Com. 10/3600

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