COMPARISON OF CANONICAL VARIATIONAL TRANSITION-STATE THEORY RATE CONSTANTS FOR H ATOM ASSOCIATION WITH ALKYL RADICALS AND WITH THE (111) SURFACE OF DIAMOND

A model potential energy function developed previously for H + CH3 --> CH4 association is extended, with transfer of parameters, to H atom a ssociation with other alkyl radicals and with the diamond (111) surfac e. Reaction path following calculations are performed to determine can onical variational transition state theory (CVTST) rate constants for these association reactions. The CVTST rate constants for H atom assoc iation with C2H5, i-C3H7, and t-C4H9 agree with experimental and/or es timated mte constants to within a factor of 2. This finding indicates it is not a severe approximation to assume transferability of potentia l energy parameters for different H atom and alkyl radical association reactions. Differences between the CVTST rate constants for these ass ociations are discussed in terms of moment of inertia ratios between t he transition state and reactants and frequencies for the transitional bending modes. The CVTST rate constant for H atom association with th e diamond (111) surface is approximately 2 times smaller than that for H + t-C4H9 association, which results from a factor of 2 difference i n reaction path degeneracies for these two associations and agrees wit h a kinetic model proposed previously [J. Phys. Chem. 1993,97,23]. The H + diamond (111) surface association rate constant is weakly sensiti ve to both the nonbonded potential between the associating H atom and H atoms attached to the surface and the lattice potential. The lattice partition function changes less than 10% in forming the association t ransition state. In contrast to these CVTST results, a recent trajecto ry study shows that the H atom + diamond (111) surface association rat e constant is sensitive to the lattice potential. This is because the transfer of the H atom relative translational energy to lattice vibrat ion, which is necessary for association to occur, is sensitive to the lattice potential. Thus, CVTST may overestimate the H + diamond (111) surface association rate constant, since it does not treat this energy -transfer process.