Direct measurements of formation rate constants of allylic radical from hot olefins formed by internal conversion. I

Abstract

The photofragmentation of 2,3,3-trimethyl-1-butene (TMB), 2,3-dimethyl-2-pentene (DMP), and tetramethylethylene (TME) has been studied in the gas phase by ArF (193 nm) laser flash photolysis. For the first time, the formation rate constants to the common product of trimethylallyl (TMA) radical have been measured directly under collision free conditions. The C–C bond (the C–H bond for TME) at the position β to the double bond dissociates and TMA radical is formed. The formation rate constants of the TMA radical are found to be 5.6×107 s−1 for TMB, 2.7×107 s−1 for DMP, and 1.7×107 s−1 for TME. These rate constant values can be explained in terms of the RRKM theory by assuming that the absorbed photon energy is redistributed in all the vibrational modes. Formation of the TMA radical is reduced by adding a foreign gas, however, a small amount of the radical was still detected under a high pressure. The unquenched processes are interpreted in terms of predissociation.