Reaction of atomic bromine with difluorochloromethane: the heat of formation of the CClF2 radical and the D.degree.(CClF2-H) bond dissociation energy

Abstract

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.

Extension of Structure–Reactivity Correlations for the Hydrogen Abstraction Reaction by Bromine Atom and Comparison to Chlorine Atom and Hydroxyl Radical

“…The database of experimental gas-phase rate constants k Br is less extensive and less internally consistent than those for k Cl or k OH , nor do we have the advantage of a compilation of evaluated and recommended values as we had for k Cl and k OH . In addition, because of the lower reactivity of Br•, Arrhenius parameters have typically been obtained at higher temperature and therefore require longer, and questionable, extrapolations to 298 K. A compilation is given in Table . − All formulas have the carbon from which hydrogen abstraction occurs at the beginning followed by the substituents attached, that is, CH 3 X, CH 2 XY, or CHXYZ. For compounds with multiple potential abstraction sites, the experimental rate constant is typically assumed to be dominantly that of the most reactive site, a relatively safe assumption (see below) given the known high selectivity of Br•, compared with, for example, Cl•.…”

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

“…The accepted enthalpies of formation of CF 3 CH 2 F, [34][35][36][37][38][39] CH 2 FCH 2 F [34][35][36][37][38][39] and CH 2 ClCH 2 Cl [40] were combined with the enthalpy of reaction of 65.3 kJ mol À1 calculated at the B3PW91/6-31G(d',p') level to obtain DH8 f,298 (CF 2 ClCH 2 Cl) = À531.0 kJ mol À1 . Reaction (8) also was used to estimate the DH8 f,298 (CF 2 ClCH 2 Cl).…”

Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory

“…186 The computed QCISD barriers ranges from 16 to 18 kcal mol -1 with different basis sets and agree well with experimental values. 250 The differences in the activation energies for the H-abstrac- tions from methane (17.5 kcal mol -1 ), 250,251 ethane (13.0 kcal mol -1 ), 251,252 propane (9.5 kcal mol -1 ), 247 and isobutane (6.9 kcal mol -1 ) render H-abstractions with Br • from methyl groups insignificant when 3°C -H are present in the alkane. 253,254 As the TSs for H-abstraction with the Br radical are more productlike than for the other halogen atoms, the selectivities of radical brominations correlate to some extent with the stabilities of the resulting alkyl radicals.…”