Quantitative valence bond computation of a curve crossing diagram for a model S_N2 reaction, H^- + CH₃H′ → HCH₃ + H′^-

A quantitative avoided crossing diagram for the model S_N2 reaction, H_l^- + CH₃H_r. → H_lCH₃ + H_r^-, is computed by using a VB method based on local fragment orbitals. The essential parameters of curves are quantified at the 6-31G*+D level. The vertical electron-transfer energy gap, G, is 174 kcal/mol. The height of the crossing point (relative to H^- + CH₄) is 42% of G. The quantum mechanical resonance energy of the transition state, i.e., the avoided crossing interaction, is 15.9 kcal/mol. The barrier of 56.8 kcal/mol is computed as the difference between the height of the crossing point and the avoided crossing interaction. All these parameters show insignificant basis set dependence and support previous estimations of these parameters which formed the crucial hypotheses for the use of the avoided crossing model to discuss reactivity trends.