Some observations on the thermochemistry and kinetics of peroxy radicals

Recent studies from one laboratory over the past decade of the equilibrium reaction [GRAPHICS] for seven different radicals have led to discrepancies in six cases in Delta H-1 degrees 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 HO2. 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 Delta H-1 are still not compatible with their equations.