Comparison of canonical variational transition state theory rate constants for hydrogen atom association with alkyl radicals and with the (111) surface of diamond

A model potential energy function developed previously for H + CH[sub 3] [yields] CH[sub 4] association is extended, with transfer of parameters, to H atom association with other alkyl radicals and with the diamond (111) surface. Reaction path following calculations are performed to determine canonical variational transition state theory (CVTST) rate constants for these association reactions. The CVTST rate constants for H atom association with C[sub 2]H[sub 5], i-C[sub 3]H[sub 7], and t-C[sub 4]H[sub 9] agree with experimental and/or estimated rate constants to within a factor of 2. This finding indicates it is not a severe approximation to assume transferability of potential energy parameters for different H atom and alkyl radical association reactions. Differences between the CVTST rate constants for these associations are discussed in terms of moment of inertia ratios between the transition state and reactants and frequencies for the transitional bending modes. The CVTST rate constant for H atom association with the diamond (111) surface is approximately 2 times smaller than that for H + t-C[sub 4]H[sub 9] association, which results from a factor of 2 difference in reaction path degeneracies for these two associations and agrees with a kinetic model proposed previously. CVTST may overestimate the Hmore » + diamond (111) surface association rate constant, since it does not treat this energy-transfer process. 53 refs., 5 figs., 2 tabs.« less