Unified Reaction Valley Approach Mechanism of the Reaction CH₃+ H₂→ CH₄+ H

A unified reaction valley analysis (URVA) is presented to investigate the mechanism of the reaction CH 3 + H 2 → CH 4 + 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 normal modes ω μ ( s ), reaction path vector η ( s ), and curvature vector κ ( s ) in terms of generalized adiabatic modes a n g ( s ) associated with internal parameters that are used to describe the reaction complex. In addition, URVA combines the investigation of the harmonic reaction valley with the analysis of attractive and repulsive forces exerted on the nuclei by analyzing the electron density distribution ρ( r, s ). It is shown that changes in ρ( r, s ) reflect changes in the reaction valley and vice versa. Five reaction phases can be distinguished (reactant, reactant preparation, transition state (TS), product preparation, and product phase), of which the chemically relevant phases are indicated by small (reorganization of electron structure) and large curvature peaks (bond breaking or forming). Relatively large peaks of the adiabatic force constants indicate those positions at which the reaction is accelerated by appropriate electronic structure changes. Position and height of the curvature peaks in the TS region reflect the energetics of the reaction and the nature of the TS in the sense of the Hammond postulate: The reaction is exothermic with an early TS that is shifted by Δ s = 0.3 amu 1/2 a 0 into the entrance channel.