Intramolecular aromatic 1,5-hydrogen transfer in free radical systems

Molecular orbital calculations of the hydrogen-atom transfer between the ortho positions of the rings of the o-benzophenone predict an activation free energy of 8.5 kcal/mol at 298 K. Using transition-state theory, the predicted rate of H-transfer is 7.9 x 10(6) mol(-1), or 4.0 x 10(-6) mol(-1), per ortho hydrogen, in excellent agreement with experimental reports. A similar H-transfer in the divinyl ketone radical is predicted to have a higher barrier by 1.4 kcal/mol despite the fact that the latter can more easily accommodate a wider C-H-C (142.3 degrees vs 135.2 degrees) angle in the transition state. Critical stabilization it the TS for the benzophenone radical H-transfer appears to be the result of C-H ... O H-bonding interactions between the oxygen and the ortho H's that are not involved in the H-transfer. These results are confirmed by the H ... O distances and the overlap populations between the H and O orbitals.