UV SPECTRUM AND KINETICS OF HYDROXYCYCLOHEXADIENYL RADICALS

High yields of H-atoms and OH-radicals were produced by pulse radiolys is of Ar/H2O mixtures, and in the presence of C6H6, the addition react ions (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 spe ctra of HO-C6H6 and H-C6H6 have been recorded in the range 250-350 nm with a maximum value of sigma(HO-C6H6) = (4.6 +/- 0.7) x 10(-18) cm(2) molecule(-1) observed at 280 nm. Studies of OH-decay and the simultan eous formation of HO-C6H6 have been used to determine an overall rate constant of k(OH + C6H6) = (1.2 +/- 0.2) x 10(-12) cm(3) 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 abs orption band at 275 nm. Different reaction mechanisms have been consid ered, 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 r ate constant of (1.1 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1) has b een determined at T = 338 K. Attempts to identify the reaction product s by UV spectroscopy were unsuccessful. Transient absorption signals o bserved in the presence of oxygen have been assigned to peroxy radical s produced in the reaction HO-C6H6 + O-2 --> HO-C6H6-O-2. A rate const ant of (5.0 +/- 1.0) x 10(-13) cm(3) molecule(-1) s(-1) has been deriv ed from the observed formation kinetics.