meta and para substitution effects on the electronic state energies and ring-expansion reactivities of phenylnitrenes

The electronic structures of the triplet ground states and first three excited singlet states for phenylnitrene, 14 meta-, and 17 para-substituted congeners have been characterized using density functional theory and multireference second-order perturbation theory (CASPT2). Ring expansion pathways to form didehydroazepines have activation enthalpies of about 9 kcal · mol^-1 and are fairly insensitive to substitution - in the case of the strongest para donor, MeNH-, this barrier increases to about 13 kcal · mol^-1. The trends in state energies as a function of substitution are rationalized using a (2,2) configuration interaction theory and qualitative molecular orbital theory. Analysis of spin-orbit coupling in the nitrenes using the same model in conjunction with explicit calculation of spin-orbit coupling matrix elements rationalizes why electron donating substituents increase rates of intersystem crossing.