No abstract

The gas-phase photobromination of ethyl chloride has been investigated over an extended temperature range in the presence and absence of ethane as external competitor. The rate constant for a-hydrogen abstraction in CH3CH2C1 was redetermined relative to C2H6 between 40 and 150 "C. The abstraction of @-hydrogen was measured in the internal competition in the range 70-150 "C by use of electron capture detection. The relative rates were combined with the known rate parameters for the bromination of C2H6 [log (A/cm3 mol-' S-I) = 14.14 f 0.06, E = 13.66 f 0.14 kcal mol-I] to obtain the absolute rate constants (cm3 mol-ls-') k(a) = 10'2.71*0.06 exp[-(9.95 f 0.14)/RT] and k ( @ ) = 1012.6M0.07 exp[-(14.14 f 0.15)/RT], where the activation energies are expressed in kcal mol-'. From an assessment of thermochemical and kinetic data the following radical heats of formation and C-H bond dissociation eneFgies (kcal mol-I) in C2H5C1 have been obtained: MH,0(CH3CHC1) = 17.6 f 1, Do(CH3CHCI-H) = 96.5 f 1 and AHfo(CH2CH2CI) = 21.8 f 1, Do(CH2CICH2-H) = 100.7 f 1. These results compare satisfactorily with recent determinations by the monoenergetic electron impact method.

The gas-phase photobromination of fluoroethane was investigated in the temperature range 80—150 °C in both the presence and absence of chloroethane as an external competitor. The rate constant for α-hydrogen abstraction in CH3CH2F was redetermined relative to that in CH3CH2Cl, and the abstraction of β-hydrogen was measured in the internal competition. The relative rates were combined with the known rate parameters for the bromination of C2H6 to obtain the absolute rate constants (cm3 mol−1 s−1): k(α) = (6.50 ± 3.58) × 1012 exp [−(10360 ± 370)/RT] and k(β) = (3.42 ± 1.95) × 1012 exp [−(12950 ± 390)/RT]. Using a justifiable approximation concerning the magnitude of the activation-energy difference for the reverse reactions between any two competitors with similar complexity the following thermochemical quantities (kcal mol−1) have been derived: ΔH°f(CH3CHF) = −16.8 ± 2, ΔH°f(CH2CH2F) = −14.2 ± 2, D°(CH3CHF–H) = 98.2 ± 2, D°(CH2FCH2–H) = 100.8 ± 2, and D°(CH2CH2–F) = 45.6 ± 2. The influence of halogen substitution on the rate parameters of hydrogen abstraction from monohaloethanes is discussed.

Pure and fine powder of zinc oxide was prepared by hydrolysis of bis(acetylacetonato)zinc(II) in methanol–water mixture. A significant difference in the particle shape was observed by TEM between ZnO powders prepared at 25 and 80 °C.

21) Mashimo, S.; Kuwabara, S.; Yagihara, S.; Higasi, K. J. Chem. Phys. (22) Agmon, N.; Huppert, D.; Masad, A.; Pines, E. J. Chem. Phys. 1991, (23) Conway, B. E.; Bockris, J. OM.; Linton, H. J. Chem. Phys. 1956, (24) Kolodney, E.; Huppert, D. Chem. Phys. 1981.63, 401. (25) See: "Spectrmcopy and Dynamics of the Elementary Proton Transfer (26) Brucker, G. A.; Swinney, T. C.; Kelley, D. F. J . Phys. Chem. 1991, (27) Barbara, P. F.; Walsh, P. K.; Brus, L. E. J. Phys. Chem. 1989, 93, (28) Flom, S. R.; Barbara, P. F. J. Phys. Chem. 1985, 89, 4489. (29) Konijnenberg, J.; Huizer, A. H.; C.A.G.O. Varma, J . Chem. Soc., (30) Cramer, H. Mathematical Meihods of Siatistics; Uppsala: Almquist 1989, 90, 3295. 95, 3190. 95, 10407. 29. 24, 834. in Polyatomic Systems". Chem. Phys. 1989, 136. Faraday Trans. 2 1989, 85. 1539 and references therein. and Wiksells, 1945. The gas-phase photobromination of CHClF, (1) in the presence of CHICl (2) as competitor has been studied in the temperature range 80-150 OC at halomethane pressures of -35 Torr and a Br2 pressure of -2.3 Torr. The temperature dependence of the rate constant ratio is found to obey the Arrhenius expression In ( k l / k z ) = (4.0885 f 0.0580) -(1 144 f 20)/T. This result is combined with an earlier relative study of CH3Cl (2) v8 C ! & (3) and a recent direct determination by kinetic s p a m m p y of the rate constant for the bromination of ethane (kJ to obtain absolute rate parameters for the reaction CHClF, + Br -CClF, + HBr. Using a justifiable approximation concerning the magnitude of the activation energy difference for the reverse reactions between any two competitors of similar complexity, and other thermochemical data from the literature, the following quantities have been derived: AH~0298(CC1F2) = -66.7 f 2 kcal mol-I and Do(CClF2-H) = 100.7 f 2 kcal mol-' where the uncertainties are conservative estimates. On the basis of the new value of k3, activation energies for the bromination of CH4 and other halomethanes are reported.