Quantum mechanical computational studies of chemical reactions : II. Isotopic exchange reactions for the collinear H+H2system

Abstract

A computational study of the kinetic isotope effects of various isotopic reactions for the H+H₂ system was made on the level of collinear reactions using fully converged quantum results in the total energy range of 8–25 kcal/mole. The Porter-Karplus surface of H₃ was used. No substantial amount of reaction probability was found for total energy below the potential energy barrier, V ₁(s ^‡) = 9·13 kcal/mole. It was shown that quantum corrections for the ‘tunnelling effect’ should be based on an effective potential, namely [V ₁(s) + ε₀(s)], by including the zero-point vibrational energy (ε₀(s)) of ρ-motion perpendicular to the reaction path s and the dynamic effects due to the curvature of the reaction coordinate. The transmission coefficients for these isotopic reactions were calculated in the temperature range of 200–1300 K. It was found that their magnitudes were large and approximately the same for the isotropic reactions investigated here, reflecting the similar quantal threshold behaviours of the reaction probability. The major source of the isotope effect in the (collinear) rate constant, is thus the easily understood difference due to changes in the reduced masses and the zero-point energies. Previously reported resonances in reactive molecular collisions for the H+H₂ system are demonstrated by the presence of resonance circles in the complex plane representation of the transition probability amplitudes. The nature of the dynamic potentials supporting the threshold resonance was explored in a study of one-state approximations.