SINGLET-TRIPLET SPLITTINGS AND 1,2-HYDROGEN SHIFT BARRIERS FOR METHYLPHENYLBORENIDE, METHYLPHENYLCARBENE, AND METHYLPHENYLNITRENIUM IN THE GAS-PHASE AND SOLUTION - WHAT A DIFFERENCE A CHARGE MAKES

In the isoelectronic series methylpnenylborenide, methylphenylcarbene, and methylphenylnitrenium, fundamental differences are-predicted for singlet state geometries, singlet-triplet state splittings, barriers t o singlet 1,2-hydrogen migration, and sensitivity of 1,2-hydrogen migr ation to solvent effects in n-heptane and acetonitrile. We conclude th at isoelectronic analogies are dangerous for systems having different formal charges, and that the interaction of the divalent center with a conjugating substituent is very sensitive to the electron donating or withdrawing nature (and power) of the hypovalent atom. Solvent effect s on the singlet-triplet splitting result from static polarity differe nces whereas the solvent effects on 1,2-hydrogen migration result prim arily from polarizability differences. For the experimentally characte rized carbene case, extensive comparison of calculated and measured re sults is provided.