VARIATIONAL TRANSITION-STATE THEORY CALCULATION OF PROTON-TRANSFER DYNAMICS IN (H3CH...CH3)-

The energetics of proton transfer in the (H3CH...CH3)- COMPlex, which is one stage in the gas-phase reaction CH4 + CH3- --> CH3- + CH4, have been investigated with ab initio calculations employing a 4-31G basis. The minimum-energy reaction path was determined by a steepest-descent technique in mass-weighted coordinates from the symmetric saddle point (1.35 kcal/mol above separated CH4 + CH3-) to the reactant/product association well (9.25 kcal/mol below separated CH4 + CH3-). Motion along this path is found to occur in two relatively distinct phases: motion of the proton between two fixed carbons followed by separation of the two hydrocarbon fragments. Rate constants computed by variational transition state theory, including an adiabatic approximation for the vibrational modes transverse to the reaction coordinate, demonstrate the importance of including the contributions from tunneling at energies below the top of the 7.88 kcal/mol vibrationally adiabatic barrier for low to moderate temperatures. In addition, incorporating the effects of reaction-path curvature in the tunneling calculation is found to be important, especially at low temperature. However, due to the dual nature of the reaction path, various model barriers fit-to the saddle point information yield rate constants that are much too large at low temperatures, thus underscoring the importance of computing the reaction path explicitly.