An ab initio molecular orbital study of the p-benzoquinone radical anion and its lithium ketyl: electronic spectra and intramolecular cation transfer in ketyl ion pairs

Abstract

STO-3G SCF (unrestricted Hartree–Fock) molecular orbital calculations are described for the p-benzoquinone (PBQ) radical anion and the corresponding lithium ketyl. At equilibrium the lithium atom occupies one of two equivalent positions collinear with the carbonyl axes and 2.843 a.u. from the nearer oxygen atom. This planar structure contrasts with earlier predictions that the most stable position of the cation in ketyl ion pairs is above the plane of the anion; its significance in relation to the electronic spectra of ketyl ion pairs is examined. The potential surface generated by displacing Li with respect to PBQ is found and is used for discussion of the possible mechanism of intramolecular transfer of Li⁺ between the two equivalent equilibrium sites. It is suggested that the rate-determining factor in this process is the formation of a solvent-shared ion pair and not (as has previously been assumed) the passage of the Li atom through the lowest saddle point on the contact ion pair potential surface.