Direct measurement of the fast, reversible addition of oxygen to cyclohexadienyl radicals in nonpolar solvents

The title reaction was measured directly by laser flash photolysis in several nonpolar solvents, using both the strong ultraviolet and weaker visible absorption bands of the cyclohexadienyl radical. Both the visible and ultraviolet transient absorptions are shown to have identical time dependence, confirming that both absorptions correspond to the same species. The cyclohexadienyl radical's spectra were observed in several nonpolar solvents and are reported. The rate constant of the title reaction is 1.2 +/- 0.4 x 10(9) M-1 s(-1) in cyclohexane solvent at room temperature (298 K). This reaction is diffusion-limited, which is consistent with previous literature reports, but 2 orders of magnitude faster than the rate constant measured recently in the gas phase. Reasons for the discrepancy are explored, with the most likely explanation being that the title reaction becomes equilibrated in both the liquid and gas phases. Calculations reveal that the weak solvation of the nonpolar solvents coupled with higher oxygen concentrations in solution are sufficient to make the equilibrium of cyclohexadienyl with its peroxyl products observable. In cyclohexane solvent, a multiexponential decay is observed at these conditions. The current data place upper and lower bounds on the equilibrium constant for the title reaction in cyclohexane. Possible mechanisms are discussed for explaining the slower decay of equilibrated cyclohexadienyl radicals found in both liquid and vapor phases.