Quantitative valence bond computations of curve crossing diagrams for a hydride-transfer model reaction: CH₄ + CH₃⁺ → CH₃⁺ + CH₄

Quantitative calculations of diabatic curve crossing diagrams are applied to a model hydride-transfer process, CH₄ + CH₃⁺ → CH₃⁺ + CH₄, by means of a multistructure valence bond (VB) technique. The energies of the various VB structures relevant to the reacting system are followed throughout the reaction coordinate, showing that the reaction is best described as an avoided crossing between the two covalent VB curves, respectively representing the bonding scheme of the reactant and product complexes. The ionic structure characterizing the direct hydride transfer plays a marginal role. It follows that the hydrogen being transferred has a predominant radical character. An alternative, more economical way of generating curve crossing diagrams is also presented, based on the fast computation of the weights of valence bond structures out of standard MO wavefunctions. This fast procedure is found to give results in qualitative agreement with rigorous VB calculations.