Mendeleev Communications Electronic Version, Issue 1, 1998 (pp. 1–42) Photochemical reactions of dimethyl ether radical cations in freon matrices and SF6 at 77 K Michail Ya. Mel’nikov,* Dmitrii V. Baskakov, Irina A. Baranova, Vladilen N. Belevskii and Ol’ga L. Mel’nikova Department of Chemistry, M. V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 939 1814; e-mail: melnikov@melnik.chem.msu.su It has been shown for the first time that under the action of light within the absorption band of dimethyl ether radical cations in freon matrices [lmax @ 436 nm, emax @ (2.5±0.5)×103 M–1 cm–1], the radical cations decay due to charge transfer to freon molecules, whereas in an SF6 matrix they undergo deprotonation with quantum yields F@(4–15)×10–2 and F@(2–6)×10–4, respectively, at 77 K.Radical cations are among the most important intermediates in many photochemical, radiation and oxidation processes. However, scant data are available on the reactivity of electronically excited radical cations.1 Recently, the quantum yields of phototransformations of some radical cations in various freon matrices were determined2–5 and the previously stated opinion on the significance of charge transfer processes from organic radical cations to freon molecules6 was confirmed.The SF6 matrix was used previously, e.g. for the stabilisation of some radical cations,7 but the phototransformations in this matrix have not been studied. However, the large difference between the ionisation potentials of SF6 and those of the majority of organic compounds (D � 5 eV) permits studies of the phototransformations of radical cations to be carried out in this matrix in the absence of charge transfer to matrix molecules.The purpose of this study was to obtain data on the mechanism and efficiency of dimethyl ether (DME) radical cations stabilised in freon and SF6 matrices at 77 K. In the experiments, DME solutions (0.1–0.5 vol.%) in a freon mixture containing 1:1, v/v, of CFCl3 (freon-11) and CF2BrCF2Br (freon-114B2), whose glass transition temperature is 77 K, as well as in freon-11 (0.5 vol.%), freon-114B2 (0.5 vol.%) and SF6 (0.02–4 vol.%) were evacuated to 10–4 Torr and irradiated with X-rays (E = 50 kV); the total absorbed dose was 0.5–2.0 kGr.EPR and optical absorption spectra of the intermediates formed were recorded on an E-3 Varian radiofrequency spectrometer and a Specord M-40 spectrophotometer (optical path 0.3 cm) using the same samples.The absolute error in the determination of the concentration of paramagnetic centres by EPR under the conditions used did not exceed ±20%. A high-pressure mercury lamp with a narrow-band glass filter (l = 436 nm, Dn1/2 @ 3000 cm–1) was used as the light source.The absolute intensity of light was determined by ferric oxalate actinometry (l = 436 nm); the light intensity was 1.6×10–4 einstein cm–3 s–1. The volume of each sample was 0.08–0.13 cm3. Since all of the matrices used in our experiments, except freon mixture, were polycrystalline, we used the monomolecular photochemical reaction of di-p-cresylnitroxyl (DCN), which was carried out in 10–4 M solutions in the same matrices, as a special standard for the evaluation of the effective optical path in these matrices.Assuming that the quantum yields of DCN phototransformation in various frozen freons and SF6 are nearly the same, we found that the effective optical path in various polycrystalline matrices is 1.5–4.0 times longer than that in glassy samples.The data obtained were in good agreement with the previous estimates3 made using the photochemical reaction of diphenyldiazomethane as the standard. The extinction coefficients and the quantum yields reported in the present study were obtained in 4 to 6 successive experiments; the error values are given for a confidence limit of 0.95.Upon exposure of DME solutions in individual freons and in their mixtures to X-ray irradiation at 77 K, their EPR spectra displayed a characteristic signal due to DME radical cations [a(6H) ª 43.0 G],9 which had the best resolution in freon-11. In freon-11 and freon-114B2, the DME radical cations account for 80% of the overall concentration of paramagnetic centres produced by irradiation.In the optical absorption spectra, the irradiation of DME solutions in freon mixtures at 77 K results in the appearance of absorption bands with lmax @ 370 and 590 nm, which can be assigned to radical cations of freons,8 and an absorption band with lmax @ 435 nm. The EPR spectra of irradiated DME solutions in freon mixtures contain a signal due to the DME radical cations and an overlapping signal which appears upon irradiation of pure freon mixture.The intensities of both absorption and EPR spectra of freon radical cations and radicals were comparable to those of DME radical cations. Because in this case the most high-field components of the EPR spectrum of DME radical cations were not distorted by any other overlapping signals, the determination of the concentration of radical cations was carried out using the shape factor of these components obtained in irradiated solutions in freon-114B2 (the shape of the EPR spectrum lines of DME radical cations is most similar to that observed for freon mixtures).When irradiated solutions of DME in freon mixtures are exposed to light with l = 436 nm, changes in intensity of the (a) (b) (c) 50 G Figure 1 EPR spectra of irradiated solutions of DME in freon-11 (a) and SF6 (0.02 vol.%) (b), (c), before (a), (b) and after the action of light with l = 436 nm, at 77 K.aThe proportion of radical cation with a relatively high reactivity F1. Table 1 Quantum yields of photochemical reactions of DME radical cations in freon-11, freon-114B2 and SF6 at 77 K. Matrix F1 F2 ba Freon-11 0.15±0.03 0.06±0.01 0.4±0.1 Freon-114B2 0.04±0.01 — — SF6 (6.1±0.4)×10–4 (3.4±0.4)×10–4 0.3±0.05Mendeleev Communications Electronic Version, Issue 1, 1998 (pp. 1–42) absorption band with lmax @ 435 nm correlate with changes in the concentration of DME radical cations determined by EPR. This allowed us to assign this absorption band to DME radical cations and to determine their extinction coefficient [emax @ (2.5±0.5)×103 M–1 cm–1] and the oscillator strength in the corresponding electron transition (f ª 0.07) (Figure 2).In all freon matrices used, the action of light with l = 436 nm at 77 K results in the decay of DME radical cations without the formation of any paramagnetic particles. This process has the same spectral dependence as the absorption spectrum of DME radical cations.Since the energy of a photon with l = 436 nm is higher than the difference between the ionisation potentials of freons and DME, it is natural to relate the changes observed to the photo-induced charge transfer from DME radical cations to matrix molecules. The dependence of the photo-induced decay kinetics of DME radical cations in a freon-11 matrix on the absorbed light dose has a bimodal shape, as in the cases reported previously;2,4,5 this may suggest a kinetic nonequivalence of reacting particles in the solid phase (Table 1).To eliminate the possibility of charge transfer to matrix molecules, we studied the photo-transformation of DME radical cations in an SF6 matrix. The EPR spectrum of irradiated solutions of DME in SF6 at 77 K displays a superposition of a well resolved signal of DME radical cations [a(6H) ª 43.0 G] and a signal due to ·CH2OCH3 radicals [a(2H) ª 18.0 G] [Figure 2(b)].† Computer simulation of experimental EPR spectra shows that a 200-fold increase in the concentration of DME in SF6 results in an increase in the relative yield of neutral radicals from just 0.4 to 0.6.This implies that at the concentration of DME in SF6 used for photochemical experiments (0.02 vol.%), the accumulation of ·CH2OCH3 radicals under X-ray irradiation is due to the decomposition of the DME radicacations which have not undergone relaxation, rather than to ion-molecular reactions in associates.The action of light with l = 436 nm on irradiated DME solutions (0.02 vol.%) results in a decrease in intensity of the EPR signal of radical cations and a synchronous increase in the signal of ·CH2OCH3 radicals, while the total concentration of paramagnetic particles remains unchanged [Figure 1(c)].The dependence of the photo-transformation kinetics of DME radical cations in SF6 on the absorbed light dose also has a bimodal shape (Table 1). The quantitative conversion of DME radical cations to ·CH2OCH3 radicals under the action of light could be interpreted with reasonable reliability as a result of photo-induced deprotonation of DME radical cations: † At high amplifications, the EPR spectra of irradiated Me2O solutions at 77 K display components of the SF6 – radical spectrum;10 on increasing the temperature of the samples to 135 K, the individual spectrum of SF5 · radicals is observed.11 The small quantum yield of this process explains why we were unable to detect it in freon matrices where it cannot compete with the highly efficient charge transfer to matrix molecules.Along with the conversion of DME radical cations, we observe that the intensity of the EPR signal assigned to SF6 – changes. We assume that these changes may be due to the reaction: Unfortunately, the wide extent of the EPR spectrum of SF6 – prevents us from making a quantitative comparison of DME and SF6 radical cations.It is important to note that the action of light with l = 436 nm on irradiated, pure SF6 does not cause such changes in the EPR spectra. The study was carried out with financial support from the Russian Foundation for Basic Research (RFBR) (grant no. 95-03-08110) and RFBR–INTAS (grant no. 95-0008). References 1 T. Bally, in Radical Ionic Systems, eds. A. Lund and M. Shiotani, Kluwer Academic Publishers, Dordrecht, 1991, p. 3. 2 M. Ya. Mel’nikov, E. N. Seropegina, V. N. Belevskii, S. I. Belopushkin and D. V. Baskakov, Mendeleev Commun., 1996, 183. 3 M. Ya. Mel’nikov, E. N. Seropegina, V.N. Belevskii, S. I. Belopushkin and O. L. Mel’nikova, Khim. Vys. Energ., 1997, 31, 281 [High Energy Chem. (Engl. Transl.), 1997, 31, 250]. 4 M. Ya.Mel’nikov, V. N. Belevskii, S. I. Belopushkin and O. L. Mel’nikova, Izv. Akad. Nauk, Ser. Khim., 1997, 1302 (Russ. Chem. Bull., 1997, 46, 1245). 5 M. Ya.Mel’nikov, O. L.Mel’nikova, V. N. Belevskii and S. I. Belopushkin, Khim. Vys. Energ., 1998, 32, 57 [High Energy Chem. (Engl. Transl.), 1998, 32, in press]. 6 N. Shida and Y. Takemura, Radiat. Phys. Chem., 1983, 21, 157. 7 K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891. 8 R. Mehnert, in Radical Ionic Systems, eds. A. Lund and M. Shiotani, Kluwer Academic Publishers, Dordrecht, 1991, p. 231. 9 M. S. R. Symons and B. W. Wren, J. Chem. Soc., Perkin Trans. 2, 1984, 511. 10 R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 1966, 45, 1845. 11 A. Hasegawa and F. Williams, Chem. Phys. Lett., 1977, 45, 275. 2500 2000 1500 1000 500 0 375 400 425 450 475 500 525 l/nm e/M–1 cm–1 Figure 2 Absorption spectrum of DME radical cations in irradiated solutions of DME (0.4 vol.%) in freon mixtures at 77 K. CH3OCH3 +· ·CH2OCH3 + H+ H+ + SF6 – HF + SF5 Received: Moscow, 23rd September 1997 Cambridge, 21st November 1997; Com. 7/07578B