Hydroxyl radical as a probe of the charge distribution in aromatics: phenol

Initial radiation chemical yields of 1.48 (2), 0.24 (2), and 2.01 molecules per 100 eV of absorbed energy are reported for addition of *OH radical to each of the ortho, meta, and para positions of phenol. These yields represent 91% of the yield of 5.96 expected for *OH addition to 5 mM phenol and are in general agreement with other previous measurements. Pulse radiolysis experiments show that phenoxy radical is produced in a yield of approximately 0.42 as a result of addition of *OH at phenol's ipso position. The total of these yields (5.84) accounts for the addition of virtually all of the expected *OH radicals. The relative yields for addition to the ortho, meta, and para positions provide a measure of the charge distribution in phenol that correlates quite well with the unpaired spin distribution in phenoxyl radical. This correlation indicates that the OH substituent similarly affects the charge distribution on the aromatic ring of phenol and the unpaired spin distribution in the phenoxyl radical.