Some Observations on the Thermochemistry and Kinetics of Peroxy Radicals

Recent studies from one laboratory over the past decade of the equilibrium reaction R • + O 2 RO 2 • for seven different radicals have led to discrepancies in six cases in Δ H 1 ° of from 3 to 7 kcal/mol when compared to estimates from group additivity. These studies have utilized flash photolysis to generate R • radicals and have measured the decay of [R • ] by mass spectrometry using near threshold photoionization. This decay was fitted to a double-exponential rate expression. Only one additional reaction was introduced in addition to 1 and −1, namely, the wall decay (w) of R • radicals, measured in the absence of O 2 . Reexamination of the differential equations has shown that they are not compatible with the reported data and in fact lead to negative values of K 1 and k 1 . It has not been possible using the equation given by the authors of the papers under consideration to derive K 1 from k w and the exponents of the double-exponential decay. It is shown that HO • 2 elimination is important for EtO 2 •, iPrO 2 •, and tBuO 2 • at the temperatures used. If this is the only additional reaction, it is further shown that the authors' values of Δ H 1 are still not compatible with their equations.