Effect of halogen substitution on o-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 o-phenylenebiscarbene (1-H) is examined computationally using MCSCF/6-31G(d) wavefunctions. In all four cases (1-X, X=H, F, Cl, Br) a singlet diradical of A′ symmetry is the ground state. Halogen substitution is found to have a minimal effect on the geometry 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 those of the A′ states. The stabilization is strongest for the fluorine derivative resulting in a small T(A″)-S(A′) splitting of around 7 kcal mol-1. A qualitative valence-bond model can accommodate many of these findings. © 2005 Taylor & Francis Group Ltd.