meta and para Substitution Effects on the Electronic State Energies and Ring-Expansion Reactivities of Phenylnitrenes

Johnson, W. T. G.; Sullivan, M. B.; Cramer, C. J.
Int. J. Quantum Chem. 2001, 85, 492.

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.

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