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

Molecular orbital calculations of the hydrogen-atom transfer between the or tho 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 predic ted 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 simila r H-transfer in the divinyl ketone radical is predicted to have a higher ba rrier by 1.4 kcal/mol despite the fact that the latter can more easily acco mmodate a wider C-H-C (142.3 degrees vs 135.2 degrees) angle in the transit ion state. Critical stabilization it the TS for the benzophenone radical H- transfer appears to be the result of C-H ... O H-bonding interactions betwe en the oxygen and the ortho H's that are not involved in the H-transfer. Th ese results are confirmed by the H ... O distances and the overlap populati ons between the H and O orbitals.