BOND-DISSOCIATION ENERGIES AND RADICAL HEATS OF FORMATION IN CH3CL, CH2CL2, CH3BR, CH2BR2, CH2FCL, AND CHFCL2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH4-nXn (X=F,Cl,Br; n = 1-3), the two chlorofluoromethanes, CH2FCl and CHFCl2, and CH4, shows that the recently reported heats of formation of the radicals CH2Cl, CHCl2, CHBr2, and CFCl2, and the CH bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH4 and the other halomethanes, the following revised H degrees f,298 (R) values have been derived: H degrees f(CH2Cl)=29.1 +/- 1.0, H degrees f(CHCl2)=23.5 +/- 1.2, Hf(CH2Br)=40.4 +/- 1.0, H degrees f(CHBr2)=45.0 +/- 2.2, and H degrees f(CFCl2) = -21.3 +/- 2.4 kcal mol-1. The previously unavailable radical heat of formation, H degrees f(CHFCl)=-14.5 +/- 2.4 kcal mol-1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate CH and CX bond dissociation energies in CH3Cl, CH2Cl(2), CH3Br, CH2Br(2), CH2FCl, and CHFCl2.