Quantitative valence-bond computations of curve crossing diagrams for a gas-phase SN2 reaction: F–+ CH3F → FCH3+ F–

Abstract

A quantitative curve crossing diagram for the identity S_N2 reaction, F^–+ CH₃F → FCH₃+ F^–, is computed with the multistructure VB method, at the PS-31 + G* level with a basis set of double-zeta plus polarization quality with pseudo-potentials for the core electrons and additional diffuse functions. The VB value for the central barrier is ca. 11.3 kcal mol^–1, in agreement with a recent estimate based on experimental data (reference 9) and with most modern MO computations followed by correlation corrections (reference 7b,d). Because of the low energy of the triple ion structure, F^–CH₃+ F^–, the variational curves can be generated all the way to the crossing point, but beyond it the variational procedure does not terminate in the charge-transfer state despite the natural correlation of the VB structures to this state. The relationship between the natural correlation and the variational VB procedure is discussed. It is shown that this feature of the variational procedure does not affect its ability to provide the key curve crossing quantities at their variational values. These quantities are, in turn, found to follow the trends of the qualitative model. The vertical charge-transfer energy gap is large (241.8 kcal mol^–1). The height of the crossing point relative to the ion–dipole cluster is 37.25 kcal mol^–1(ca. 15% of the gap). The resonance energy of the transition state is 25.9 kcal mol^–1. The relationship between the barrier and the curve crossing quantities is discussed by comparison to the hydride exchange reaction, H^–+ CH₃H → HCH₃+ H^–. It is shown that the relative barriers reflect the interplay between the charge-transfer energy gap and the curvature which determines the height of the crossing point relative to the gap. This behaviour is in good agreement with the predictions of the qualitative curve crossing model. The significance of the curvature factor is shown to be related to the characteristic structure–reactivity coefficients of the reaction (e.g., the Brønsted coefficient).