Hydrogen Shifts in Aryl Radicals and Diradicals: The Role of m-Benzynes

Density functional and coupled cluster results are presented for hydrogen shifts in radicals derived from polycyclic aromatic hydrocarbons (PAHs) and for rearrangement mechanisms for several phenylenes. RCCSD(T)/cc-pVDZ//UBLYP/cc-pVDZ free energy barriers for 1,4-H shifts at 298 K are consistently predicted to be ca. 25 kcal/mol, whereas barriers for 1,5- and 1,6-shifts range from 6 to 28 kcal/mol. The barriers correlate reasonably well with the distance from the radical center to the shifting hydrogen in the reactant. Proposed mechanisms (via diradical intermediates) of known rearrangements of linear [3]phenylene, benzo[b]biphenylene, and angular [4]phenylene have BD(T)/cc-pVDZ//(U)BLYP/cc-pVDZ computed barriers of 74-82 kcal/mol, consistent with pyrolysis temperatures of 900 to 1100 °C. Hydrogen shift reactions in most of the aryl diradicals arising from phenylenes produce m-benzyne intermediates which, despite being 8-15 kcal/mol more stable than other diradicals involved in the pathways, do not significantly lower the computed overall free energies of activation.