Kinetics of one-electron oxidation by the cyanate radical

The reaction of cyanate ions with hydroxyl radicals was studied by pulse radiolysis and found to lead to formation of a transient absorbing at 330 nm (∈ = 970 M^-1 cm^-1). The rate of formation of this transient is slower than the rate of reaction of NCO^- with OH, in agreement with previous results. This indicates that the initial OH adduct, NC(OH)O^-, which is known from ESR experiments to rearrange to NHCO₂^-, reacts with another NCO^- to form the observed transient. Rate constants for oxidation by this cyanate radical were determined for several compounds, including ascorbate, urate, and several phenols and anilines. They were found to vary from <10⁶ to 3 × 10⁸ M^-1 s^-1. Comparing these rate constants with those for other oxidizing radicals, and taking into consideration the redox potentials of the radicals and the substrates, we conclude that the cyanate radical has a one-electron oxidation potential of about 0.6-0.7 V vs. NHE. Such a weak oxidant cannot be assigned the structure (NCO)₂^•- and we suggest, therefore, the alternate structure (^-O₂CNHNCO^-)^•.