The equilibrium conformation with the CiC70 plane bisecting the phenyl ring, with equivalent ortho and meta hydrogens, cannot cause the observed line broadening.(18) This relatively low barrier is surprising. Infrared19 and nmr20 studies indicate a barrier for benzaldehyde of the order of 7 kcal/mol. In two related radicals, -hydroxybenzyl8 and the benzaldehyde radical anion,21 the barrier must also be very high since these radicals possess distinct ortho and meta hydrogens. The lower barrier in the benzoyl radical may be attributed to the relative stability of the bisected form where the loss of resonance interaction with the carbonyl group is partially compensated by the greater delocalization of the unpaired electron.
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