|Michael Sullivan Online|
meta and para Substitution Effects on the Electronic State Energies and Ring-Expansion Reactivities of Phenylnitrenes
William T. G. Johnson, Michael B. Sullivan, and Christopher J. Cramer
Int. J. Quantum Chem., 2001, 85, 492-508.
Contribution from the Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431
Received February 25, 2001
Revised Manuscript Received March 26, 2001
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 and are fairly insensitive to substitution - in the case of the strongest para donor, MeNH-, this barrier increases to about 13 kcal/mol. 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.