The Journal of Physical Chemistry, Vol. 83, No. 16, 7979
Product Formations Mechanism in Ketene Photolysis
during the interpulse interval. References and Notes Herron, J. T.; Huie, R. E. J. fhys. Chem. Ref. Data 1973, 2, 467-518. Huie, R. E.; Herron, J. T. frog. React. Kinet. 1975, 8 , 1-80. Cvetanovic, R. J. Adv. fhotochem. 1963, 7, 115-182. Sloane, T. M. J . Chem. Phys. 1977, 67, 2267-2274. G!eeves, J. T.; McDonald, J. D. J. Chem. fhys. 1975, 62, 1582-1583. Umstead, M. E.; Shortridge, R. G.; Lin, M. C. Chem. fhys. 1977, 20, 271-276. Lin, M. C.; Shortridge, R. G.; Umstead, M. E. Chem. fhys. Lett. 1976, 37, 279-284. Hsu, D. S. Y.; Coicord, L. J.; Lin, M. C. J. Phys. Chem. 1978, 82,
121-124. Nakamura, K.; Koda, S. Int. J . Chem. Kinet. 1977, 9 , 67-81. Nakamura, K.; Koda, S.; Akita, K. Bull. Chem. SOC.Jpn. 1978, 57,
1665- 1670. Koda, S. Chem. Phys. Lett. 1978, 55, 353-357. Heicklen, J. A&. fhotochem. 1969, 7, 57-148, and references cited therein. Atkinson, R.; Cvetanovic, R. J. J. Chem. fhys. 1971, 55, 659-663. Singleton, D. L.; Cvetanovic, R. J. J . Am. Chem. SOC.1976, 98,
6812-681 9. Belias, M. G.; Rousseau, V.; Strauz, 0. P.; Gunning, H. E. J. Chem. fhys. 1964, 41, 768-774. Heicklen, J.; Knight, V.; Greene, S.A. J. Chem. fhys. 1965, 42,
22 1-227. Gilbert, J. R.; Slagle, I. R.; Graham, R. E.; Gutman, D. J. fhys. Chem. 1976, 80, 14-18. JANAF Thermochemical Tables. Natl. Stand. Ref. Data Ser. 1969, No. 26. Ibid. 1971, No. 37. Mathews, C. W. Can. J. Phys. 1967, 45, 2355-2374. Quanch-Tat-Trung; Durocher, G.; Sauvageau, P.; Sandorfy, C. Chem. Phys. Lett. 1977, 47, 404-407. Ishiguro, T.; Hamada, Y.; Tsuboi, M. Japanese Annual Meeting of Molecular Structure; Bunshikozo Toronkai, 1978; 1E02. Sam. C. L.: Yardiev. J. T. Chem. Phvs. Lett. 1979. 61. 509-512. Masmanidis, C. A.1 Jaffe, H. H.; Ellis: R. L. J. fhys. Chem. 1975, 79, 2052-2061. Yonezawa, Y.; Fueno, T. Bull. Chem. SOC.J m . 1975, 48, 22-25. Herzberg, G. "Electronic Spectra and Electronic Structure of Polyatomic Molecules", Van Nostrand-Reinhold: New York, 1966; p 616. Miliigan, D.E.; Jacox, M. E.; Bass, A. M.; Comeford, J. J.; Mann, D. E. J. Chem. Phys. 1965, 42, 3187-3195. Hansen. D. A.: Atkinson. R.: Pitts. Jr., J. N. J . Photochem. 1977. 7 , 379-404. Sheinson, R. S.;Toby, F. S.;Toby, S.J . Am. Chem. SOC.1975, 9 7 , 6593-6595. (29) Craig, N. C. Spectrochim. Acta, Part A 1972, 28, 1195-1201.
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(30) Berney, C. V. Spectrochim. Acta, Part A 1971, 27, 663-672. (31) Hackett, P. A.; Phillips, D. J . fhys. Chem. 1974, 78, 665-686. (32) Dodd, J. N.; Sandle, W. J.; Williams, 0. M. J . fhys. 8,1970, 3 , 256-270. (33) Deech, J. S.;Baylis, W. E. Can. J. Phys. 1971, 49, 90. (34) Caivert, J. G.; Pitts, Jr., J. N. "Photochemistry"; Wiley: New York. 1967. (35) The quenching cross sections of N,O, NO, C,F, and C2H4for Hg(3P) are 21.2 X 28.3 X IO-", 7.9 X IO-'', and 39.6 X lo-'' cmd. See ref 34 and Michael and Suess. Michael, J. V.; Suess, G. D. J . fhys. Chem. 1974, 78, 482-487. (36) DeMore, W. 6. Chem. Phys. Lett. 1972, 16, 608-610. (37) Johnston, T.; Heicklen, J. J . Phys. Chem. 1967, 47, 475-477. (38) Hsu, D. S.Y.; Lin, M. C. Chem. fhys. 1977, 21, 235-238. (39) The maximum [O$P)] is calculated to be (2 X IO', molecule ~ m - ~ s-')(0.6 X s) = 1.2 X 10' molecule ~ m - Assuming ~ . that the rate constant for the reaction between O(3P)and 'CF, is 1.O X lo-' cm3 molecule-' s-', the consumption rate of 3CF, by OpPJ is less than (1.0 X lo-' cm3 molecule" s-lX1.2 X lo9 molecule cm )[%F,] = (1.2 s-')[~CF,]. On the other hand, the quenching rate of by C,F, of 10 mtorr is (3.9 X cm3 molecule-' s-') (3.3 X 10" molecule C~-~)[%F,]= (1.3 X 10, s-')[~CF,]. The quenching rate constant is from Table 111. Thus it is clear that the reactions of %F2 with O$P) or 3CF, can be neglected compared with process 9, even if the reactions are very fast. (40) Baulch, D. L.; Drysdale, D. D.; Horne, D. G.; Lloyd, A. C. "High Temperature Reaction Rate Data", Voi. 2; Butterworths: London,
1973. (41) Fontijn, A.; Meyer, C. 6.; Shiff, H. I.J. Chem. Phys. 1964, 40, 64-70. (42) Paulsen, D. E.; Sheriin, W. F.; Huffman, R. E. J. Chem. fhys. 1970, 53, 647-658. (43) Mitchell, R. C.; Simons, J. P. J . Chem. SOC.B 1968, 1005-1007. (44) Tyerman, W. J. R. Trans. Faraday SOC.196g7 65, 163-174 (45) I f C2F40t is assumed to be a vibrationai-excited tetrafiuorooxirane which possesses an excess energy of 80 kcal mol-', the first-order
(46) (47) (48) (49) (50) (51) (52)
rate constant of its unimolecular decom ositlon to yield CF,(X) and cF,o is estimated to be about 2 x 10'2 spin terms of rate parameters of the thermal decomposition (Lenzi, M.; Mele, A. J . Chem. fhys, 1965, 43, 1974-1977) according to RRK theory. Moss, A. Z.; Yardley, J. T. J. Chem. fhys. 1974, 61, 2883-2889. Wampler, F. 6.; Otsuka, K.; Calvert, J. G.; Damon, E. K. Int. J. Chem. Kinet. 1973, 5 , 669-690. Wampier, F. B. Int. J . Chem. Kinet. 1976, 8 , 687-694. Simons, J. P. J. Chem. SOC.1965, 5406-5413. Staemmler, V. Theor. Chim. Acta 1974, 35, 309-327. Bauschlicher, Jr., C. W.; Schaefer, 111, H. F,; Bagus, P. S. J . Am. Chem. SOC.1977. 99. 7106-7110. NOTEADDEDIN PROOF:We have heard very recently that Professor C. W. Mathews indentified the emission completely differently (private communication). I f his interpretation is right, the following results and discussions should be revised.
Mechanism of Product Formation during the Photolysis of Ketene Robert L. Russell and F. S. Rowland" Department of Chemistry, University of California, Irvine, California 927 17 (Received August 17, 1978) Publication costs assisted by the Office of Basic Energy Sciences, Department of Energy
The photolysis of ketene has been studied with both 14CHzC0and CHTCO, and a new mechanism is suggested for the reactions of triplet methylene in pure ketene. The isotopic experiments are consistent with the set of reactions, (6) plus (8) to (ll),,CH2 + 3CHz CzHz+ 2H (8), H + CH2C0 CH3 + CO (9), 3CHz+ CH3 CzH4+ H (10,11),CH, + CH3 C2Hs (6); and not with the earlier proposal of (3) to (6), 3CHz+ CHzCO CH, + CHCO (3),CHCO + CHCO CzHz+ 2CO (4), CH3 + CHCO CzH4+ CO ( 5 ) . The formation of atomic hydrogen rather than molecular hydrogen in (8) has important consequences for interpretation of the flash photolysis of ketene.
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Introduction The gas-phase photolysis of ketene to form CH2and CO (eq 1) is one of the most thoroughly studied systems in photochemistry both because of its intrinsic interest and CH2=C=O + UV CH2 + CO (1) because of its importance as a source for studying the +
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reactions of methylene.lJ There is now general agreement that the absorption by ketene of UV light in the 27003400-A wavelength range leads to the formation of both singlet CH2(lA1) and triplet CHz(3B1). However, the percentage of the two spin states varies with the wavelength of incident light, and perhaps with other parameters such as the nature and quantity of other molecules present. 0 1979 American Chemical
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The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
The quantum yield for decomposition is pressure dependent over most of this wavelength range although absorption of a quantum of 2800-A light always leads to decomposition of ketene at pressures of 1 atm or less. In the absence of molecules other than ketene itself, the quantum yield for formation of CO is essentially two per ketene molecule directly photolyzed. The chief additional product in such systems is ethylene with lesser quantities of acetylene and ethane and traces of C3 and C4 molecule^.^ With O2 also present during photolysis the yield of ethylene is reduced while the yields of acetylene and ethane are eliminated. During ketene photolysis at 3130 A with hydrocarbon substrates also present, about 25-30% of the reacting methylene is diverted into other reaction paths by reaction with 0,. The accepted explanation for this scavenging effect of O2 is that roughly 70% of the methylene is present as CH2(lA1)and 30% as CH,("J, which are abbreviated hereafter as lCHz and 3CH2,respectively. Triplet 3CH2reacts with great selectivity toward 02,and hence is diverted into oxidized carbon pathways (e.g., CO, COP)by trace concentrations of OZa4Singlet lCH,, on the other hand, reacts with comparable efficiency (
