Effect of halogen substitution on p‐ phenylenebiscarbene

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 amino‐substituted one (1‐NH 2 ) 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. Int J Quantum Chem, 2005