Theoretical study on the reaction of the phenoxy radical with O ₂, OH, and NO ₂

Unlike the chemistry underlying the self-coupling of phenoxy (C ₆H ₅O) radicals, there are very limited kinetics data at elevated temperatures for the reaction of the phenoxy radical with other species. In this study, we investigate the addition reactions of O _2, OH, and NO ₂ to the phenoxy radical. The formation of a phenoxy-peroxy is found to be very slow with a rate constant fitted to k = 1.31 × 10 ^-20T ^2.49 exp (-9300/T) cm ³/mol/s in the temperature range of (298-2,000 K) where the addition occurs predominantly at the ortho site. Our rate constant is in line with the consensus of opinions in the literature pointing to the observation of no discernible reaction between the oxygen molecule and the resonance-stabilized phenoxy radical. Addition of OH at the ortho and para sites of the phenoxy radical is found to afford adducts with sizable well depths of 59.8 and 56.0 kcal/mol, respectively. The phenoxy-NO ₂ bonds are found to be among the weakest known phenoxy-radical bonds (1.7-8.7 kcal/mol). OH- and O ₂-initiated mechanisms for the degradation of atmospheric phenoxy appear to be negligible and the fate of atmospheric phenoxy is found to be controlled by its reaction with NO ₂.