Document B6bVO3Z00nE9E7qVgQ5oarZE

Russian Chemical Bulletin, International Edition, Vol. 50, No. 12, pp. 23812383, December, 2001 2381 Formation and decay of trifluoromethyl radicals during photodecomposition of the long-lived perfluoro-2,4-dimethyl-3-ethylpent-3-yl radical S. R. Allayarov, D. A. Gordon, T. E. Chernysheva, and I. M. Barkalov Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax: +7 (096) 515 3588. E-mail: sadush@icp.ac.ru UV irradiation of the long-lived radical [(CF3)2CF]2CC2F5 (1) in a hexafluoropropylene trimer (HFPT) glassy matrix at 77 K and in a HFPT solution at 300 K leads to its decomposition to the CF3 radical and perfluoroolefin molecule. About 90% of the CF3 radicals formed recombine at 300 K. The remaining radicals add to the HFPT molecules generating the long-lived radicals [(CF3)2CF]3C. Unlike the CF3 radicals produced by the photodecomposition of radicals 1, the CF3 radicals formed during radiolysis of HFPT are not stabilized in the glassy HFPT matrix at 77 K. Key words: long-lived perfluoroalkyl radical, photodecomposition, CF3 radical, ESR spectroscopy. Numerous publications on ESR studies of the CF3 radicals formed upon low-temperature radiolysis of fluoroorganic compounds are available in the literature.1 Meanwhile, the kinetics of their accumulation in solid matrices is poorly studied. This is associated with difficult separation of a complicated ESR spectrum of the CF3 radicals from the spectrum of irradiated fluoroorganic compounds and with a very low concentration of the stabilized CF3 radicals. The products of radical photodecomposition of the [(CF3)2CF]2CC2F5 radicals (1) have previously2 been studied in a solid HFPT matrix (77 ). Radicals 1 were shown to decompose to form the CF3 radicals. The absorption spectra of HFPT and radicals 1 and the excitation spectrum during the photodecomposition of radical 1 have been studied.3 The optical absorption spectrum of liquid HFPT exhibits two absorption bands: at max < 185 nm and at max < 250 nm (250 = 6 L mol1 cm1). The maximum of the optical absorption band of radicals 1 appears at max < 220 nm (250 = 4.9 L mol1 cm1). The study of the action spectrum showed that the photodecomposition of radical 1 begins with irradiation at < 350 nm. When photolysis is conducted in quartz ampules with the full light from a high-pressure mercury lamp for 400 min, an almost complete conversion of radicals 1 can be achieved in the glassy HFPT matrix at 77 K and an individual ESR spectrum of the CF3 radicals can be observed. In this work, we studied the specific features of accumulation and decay of the CF3 radicals during photodecomposition of radicals 1 in the glassy (77 ) and liquid (300 ) HFPT matrices. Experimental A solution containing 31020 g1 of radicals 1 in HFPT was prepared according to a described procedure.2 To prepare samples with a lower concentration of radicals 1, this solution was diluted with the starting HFPT. HFPT samples contained two isomers: [(CF3)2CF]2C=CFCF3 (47%) and (CF3)2CFC(C2F5)=C(CF3)2 (53%). ESR spectra were recorded on an EPR-21 radiospectrometer (Institute of Problems of Chemical Physics, RAS). An error of determination of the absolute concentration of paramagnetic species was 2030%, and that for determination of the relative concentration was 510%. Samples were UV irradiated in vacuo at 77 K in quartz ampules 45 mm in diameter. A DRSh-1000 high-pressure mercury lamp with a sufficiently intense continuous emission spectrum in the 210750 nm range was used as UV source. Samples photolyzed at 77 K were heated by cooled nitrogen vapor and stored at each temperature of experiment for 10 min. Results and Discussion The ESR spectrum of the sample photolyzed at 77 K is the superposition of the spectra of radicals 1 and CF3. The accumulation kinetics of the latter was monitored by a change in the intensities of broad lines2 with splittings at 75.5 and 27.3 mT. As can be seen in Fig. 1, after photolysis for 400 min an increase in the concentration of the stabilized CF3 radicals virtually stops. The limiting concentration of the CF3 radicals depends on the concentration of radicals 1 in the sample. When the concentration of radicals 1 increases from 61017 to 31020 g1, the limiting concentration of the stabilized CF3 radicals 100-fold increases (see Fig. 1). The photodecomposition rate of 1 is [R]/dt = I0(1 10D), where [R] is the current concentration of radicals 1, I0 is the intensity of the incident light, is the quantum yield of the reaction, t is the time of irradiation, and D is absorbance. The absorbance of the samples containing radicals 1 in concentrations of 61017 g1 and 31020 g1 at 250 nm (250 = 49 m2 mol1) is 0.245 and 122.5, Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 22732275, December, 2001. 1066-5285/01/5012-2381 $25.00 2001 Plenum Publishing Corporation 2382 Russ.Chem.Bull., Int.Ed., Vol. 50, No. 12, December, 2001 Allayarov et al. [R]10a/g1 12 8 1 4 2 0 200 400 600 800 t/min Fig. 1. Kinetics of accumulation of the CF3 radicals during photolysis (77 ) of samples containing 31020 (1, = 18) and 61017 g1 (2, = 16) radicals 1 in the HFPT matrix. respectively. At low absorbances (0.245), the k value equal to (1847)104 s1 can be determined from the equation log([R]0/[R]) = kt (k = I0l). At higher D values (122.5), the photodecomposition kinetics of radi- cals 1 is more complicated but a decrease in the decay rate of the radicals in the samples with 31020 g1 we attribute to a sharp increase in D. In the case when all photochemically generated CF3 radicals are stabilized in the sample, the overall concentration of radicals 1 and CF3 during photolysis should remain unchanged. The change in the overall concentration of these radicals during photolysis at 77 K is presented in Fig. 2. It is seen that when photolysis is carried out for 400 min the concentration of the radicals decreases, and further the concentration remains unchanged up to the time of irradiation of 1000 min. A reason for the decrease in the overall concentration of radicals CF3 and 1 is radical decay during photolysis at 77 K. Quantum-chemical calculations showed that radicals 1 cannot recombine between each other or with the CF3 radicals because of electronegative and sterically bulky CF3 groups. Therefore, it is reasonable to attribute the decrease in the overall concentration of radicals 1 and CF3 to the decay of the [R]/[R]0 (%) 100 1 80 2 60 400 800 t/min Fig. 2. Plots of the total concentration of radicals 1 and CF3 detected in the ESR spectra of samples containing 31020 (1) and 61017 g1 (2) of radicals 1 vs. time of UV irradiation at 77 K. [R]/[R]0 (%) 100 80 60 40 1 2 20 3 80 100 120 140 T/K Fig. 3. Temperature plots of the concentration of the CF3 radicals in samples containing 1.21019 (1), 1.21018 (2), and 1.21017 g1 (3) ofCF3 radicals in the HFPT matrix at 77 . CF3 radicals. The decay of the CF3 radicals due to F atom abstraction from the HFPT molecule or bond cleavage is thermodynamically unfavorable. Therefore, the CF3 radicals recombine during photolysis at 77 K. We studied the thermal stability of the CF3 radicals in the HFPT matrix. As can be seen in Fig. 3, on heating the CF3 radicals stabilized in the solid matrix of glassy HFPT, radical decay begins at temperatures much lower than the vitrification temperature of the HFPT matrix. The beginning of their decay shifts to- ward lower temperatures with an increase in the concentration of the CF3 radicals in the sample. This is related, most likely, to the fact that at high concentra- tions of radicals 1 due to high absorbances of the samples, the CF3 radicals are mainly formed in the narrow near-boundary layer, and the average distance between them is much shorter than that in other cases considered. When the sample is devitrified (150 ), the CF3 radicals decay completely. The CF3 radicals generated during photolysis at 300 K cannot be stabi- lized in the liquid and decay. Unlike photolysis in the solid HFPT matrix, in the liquid some CF3 radicals add to the [(CF3)2CF]2C=CFCF3 molecule to form the long-lived [(CF3)2CF]3C radical (2). Quantum-chemical calculations showed that due to steric factors the addition of the CF3 radical to the second isomer of the HFPT molecule is impossible. The ESR spectrum of radical 2 at 300 exhibits a multiplet with splitting H 0.27 mT, whose each line is subsplit to a quadru- plet with H 0.03 mT. On cooling to 77 K, the ESR spectrum of radical 2 reversibly transforms into a singlet with a width of 2.2 mT. The formation of radical 2 has previously4 been observed due to the addition of the CF3 radical, appeared during the thermal decomposition of radical 1, at the double bond of the HFPT molecule. Thus, the [(CF3)2CF]2C=CFCF3 isomer of the HFPT molecule can be used as a spin trap for the CF3 radicals. Formation and decay of trifluoromethyl radicals Russ.Chem.Bull., Int.Ed., Vol. 50, No. 12, December, 2001 2383 [R]1017/g1 5 4 1 2 3 3 2 1 0 40 80 120 160 t/min Fig. 4. Plots of the concentration of radicals 2 formed during photolysis of samples containing 61017 (1), 31018 (2) and 61018 g1 (3) of radicals 1 vs. time of UV irradiation at 300 K. The accumulation curves of radicals 2 (Fig. 4) in the samples containing before irradiation at 300 K 61017, 31018, and 61018 g1 of radicals 1 have a maximum after 15 min of irradiation. Therefore, during photoly- sis the decay of radicals 2 occurs along with their accumulation. Since the CF3 radicals cannot recombine with radicals 2 due to steric hindrances, the decay of radicals 2 is related to their photochemical transfor- mations. Unlike the CF3 radicals generated during the pho- todecomposition of radicals 1, the CF3 radicals formed by the radiolysis of HFPT are not stabilized in it at 77 . No spectrum of the CF3 radicals is observed in the ESR spectra of the HFPT samples exposed to -60 radiation (a dose of 610319 MGy) at 77 K. Our analysis of the HFPT products shows that CF4 composes a considerable fraction of molecular products. During radiolysis of perfluororganic compounds, the CF4 molecule can be formed by the recombination of the CF3 radicals with the F atoms, which explains, most likely, the absence of a signal from the CF3 radicals in the ESR spectrum of HFPT -irradiated at 77 K. This work was financially supported by the Russian Foundation for Basic Research (Project No. 01-0397006). References 1. S. Ya. Pshezhetskii, A. G. Kotov, V. K. Milinchuk, V. A. Roginskii, and V. M. Tupikov, EPR svobodnykh radikalov v radiatsionnoi khimii [ESR of Free Radicals in Radiation Chemistry], Khimiya, Moscow, 1972, p. 202 (in Russian). 2. S. R. Allayarov and A. I. Mikhailov, Izv. Akad. Nauk, Ser. Khim., 2001, 1142 [Russ. Chem. Bull., Int. Ed., 2001, 50, 1198]. 3. D. A. Gordon, S. R. Allayarov, S. I. Kuzina, I. M. Barkalov, and A. I. Mikhailov, Izv. Akad. Nauk SSSR, Ser. Khim., 1989, 2203 [Bull. Acad. Sci. USSR, Div. Chem. Sci, 1989, 38, 170 (Engl. Transl.)]. 4. K. V. Sherer, T. Ono, . Yamanouchi, R. Femandez, P. Henderson, and H. Goldwhite, J. Am. Chem. Soc., 1985, 107, 718. Received May 14, 2001; in revised form May 31, 2001