Kinetics of the Self-Reaction of C2H5 Radicals

Abstract

The kinetics of the self-reaction of ethyl radicals was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 301−800 K and at bath gas (mostly helium, balance radical precursors) densities of (3.00−12.0) × 1016 molecules cm-3. Ethyl radicals were produced in well-characterized concentrations by a combination of the 193-nm photolysis of oxalyl chloride ((CClO)2) with the subsequent fast reaction of Cl atoms with ethane. The observed overall C2H5 + C2H5 rate constants demonstrate a negative temperature dependence. Master equation modeling of collisional effects indicates that the reaction is in the high-pressure limit under all experimental conditions except for those used at the highest temperature, 800 K, where a minor falloff correction (8%) was applied to obtain the high-pressure-limit rate constant value. The following expression for the high-pressure-limit rate constant of reaction 1 was obtained: k1∞ = (2.29 × 10-6) T -1.66 exp(−552 K/T) cm3 molecule-1 s-1. The disproportionation to recombination branching ratio was determined at 297 and 400 K; the results are in agreement with the well-established value of 0.14.