Effect of halogen substitution on p-phenylenebiscarbene

Abstract

The effect of halogen (F, Cl, and Br) perturbation on the geometries and relative energies of five electronic states (one quintet, two triplets, and two singlets) of p-phenylenebiscarbene (1-H) was examined computationally using MCSCF/6-31G(d) wavefunctions. In all four cases (1-X, X=H, F, Cl, Br) the ground state is a singlet diradical of A′ symmetry. Halogen substitution is found to have a minimal effect on the geometries and on the relative energies of electronic states of the same symmetry. However, it has a pronounced stabilizing effect on the energies of the A″ states relative to the A′ ones. Among the halogens the stabilization is strongest for the fluorine derivative resulting in a small T(A″)-S(A′) splitting of 8 kcal/mol. The hydroxy analog (1-OH) has almost degenerate singlet A′ and triplet A″ states, whereas the aminosubstituted one (1-NH2) is computed to be a ground-state (A″) triplet. Several of the findings can be accommodated by a qualitative valence-bond model, which also relates the T(A″)-S(A′) splittings in 1-X and the corresponding phenylcarbene (PhCX). © 2005 Wiley Periodicals, Inc.