High yields of H-atoms and OH-radicals were produced by pulse radiolysis of Ar/H2O mixtures, and in the presence of C6H6, the addition reactions (1a) OH + C6H6 → HO−C6H6 and (2) H + C6H6 → H−C6H6 have been studied by transient ultraviolet absorption spectroscopy of the short-lived radicals OH, HO−C6H6, and H−C6H6. The ultraviolet absorption spectra of HO−C6H6 and H−C6H6 have been recorded in the range 250−350 nm with a maximum value of σ(HO−C6H6) = (4.6 ± 0.7) × 10-18 cm2 molecule-1 observed at 280 nm. Studies of OH-decay and the simultaneous formation of HO−C6H6 have been used to determine an overall rate constant of k(OH + C6H6) = (1.2 ± 0.2) × 10-12 cm3 molecule-1 s-1 at T = 298 K. Phenol was identified as a primary product with a relative yield of 25 ± 5% derived from the amplitude of the strong absorption band at 275 nm. Different reaction mechanisms have been considered, but only the direct displacement reaction (1b) OH + C6H6 → H + C6H5OH seems to account for the experimental results. The kinetics of the reaction (3) HO−C6H6 + NO2 → products has been studied, and a rate constant of (1.1 ± 0.2) × 10-11 cm3 molecule-1 s-1 has been determined at T = 338 K. Attempts to identify the reaction products by UV spectroscopy were unsuccessful. Transient absorption signals observed in the presence of oxygen have been assigned to peroxy radicals produced in the reaction HO−C6H6 + O2 → HO−C6H6-O2. A rate constant of (5.0 ± 1.0) × 10-13 cm3 molecule-1 s-1 has been derived from the observed formation kinetics.

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