UNIFIED REACTION VALLEY APPROACH MECHANISM OF THE REACTION CH3-2-]CH4+H(H)

A unified reaction valley analysis (URVA) is presented to investigate the mechanism of the reaction CH3 + H-2 --> CH4 + H at the UMP2/6-31G( d,p) level of theory. URVA is based on the reaction path Hamiltonian, the intrinsic reaction coordinate s, and the characterization of norma l modes omega(mu)(s), reaction path vector eta(s), and curvature vecto r kappa(s) in terms of generalized adiabatic modes a(n)(g)(s) associat ed with internal parameters that are used to describe the reaction com plex. In addition, URVA combines the investigation of the harmonic rea ction valley with the analysis of attractive and repulsive forces exer ted on the nuclei by analyzing the electron density distribution rho(r ,s). It is shown that changes in rho(r,s) reflect changes in the react ion valley and vice versa. Five reaction phases can be distinguished ( reactant, reactant preparation, transition state (TS), product prepara tion, and product phase), of which the chemically relevant phases are indicated by small (reorganization of electron structure) and large cu rvature peaks (bond breaking or forming). Relatively large peaks of th e adiabatic force constants indicate those positions at which the reac tion is accelerated by appropriate electronic structure changes. Posit ion and height of the curvature peaks in the TS region reflect the ene rgetics of the reaction and the nature of the TS in the sense of the H ammond postulate: The reaction is exothermic with an early TS that is shifted by Delta s = 0.3 amu(1/2) a(0) into the entrance channel.