Valence bond calculations on the transition state for the E2 reaction of fluoride ion with ethyl fluoride — Implications for the More O'Ferrall – Jencks diagram

Abstract

A valence bond self-consistent field (VBSCF) calculation has been carried out on the bimolecular elimination (E2) reaction of fluoride ion with ethyl fluoride. The transition state was first optimized at HF/6-31+G and the resulting orbitals were used in the VBSCF calculation. A transition-state hybrid consisting of four structures (F–H-CH2-CH2-F, F-HCH2=CH2F–, F-H–CH2-CH2-F, and F–H-CH2-CH2+F–) failed to give a reasonable energy. Incorporation of four charge-separated structures (F–H+–CH2-CH2-F, F–H+–CH2-CH2+F–, F–H+CH2=CH2F–, and F-H–CH2-CH2+F–) for a total of eight resulted in an energy slightly below Hartree–Fock. A full-space VBSCF calculation utilizing 50 structures showed that the eight structures give a sufficiently accurate wave function. The transition state is dominated by charge-separated structures, leading to a greater degree of charge separation than in either the reactant or product complex. Therefore, the transition state does not evolve synchronously from the reactants, and explicit allowance should be made for this fact when applying descriptions of the reaction path, such as that in the More O'Ferrall – Jencks diagram. Key words: elimination reaction, E2, transition state, VBSCF calculation, XMVB programs.