Thermal Rate Coefficients via Variational Transition State Theory for the Unimolecular Decomposition/Isomerization of 1-Pentyl Radical: Ab Initio and Direct Dynamics Calculations

Abstract

Calculations of thermal rates for the reactions of the isomeric pentyl radicals involving (1,2), (1,3), (1,4), and (1,5) intramolecular H-atom transfer, C−C bond scission, and H-atom elimination have been carried out. Potential energy surfaces and associated properties for these reactions have been used for direct dynamics studies within conventional and variational transition state theory formalism including nonclassical effects, using the dual-level technique (PUMP-SAC2/6-311G**///AM1). We found that for C−C scission, the barrier is broad, and a significant tightening of the loose transition state reduces the rate coefficients across a wide temperature range. Converse behavior is predicted for the isomerization reactions where the optimal combination of a low effective mass with a narrow barrier opens the best tunneling paths. High-pressure limiting rate coefficients and kinetic parameters obtained in this study show good agreement with experimental measurements and previous theoretical work.