Kinetics of Intramolecular Hydrogen Transfer in Alkyl Radicals from Ab Initio Calculations

摘要

Isomerization of fuel radicals is an important issue in the development of combustion models for real fuels. In this study the barrier heights and corresponding reaction rate constants for all unique intramolecular hydrogen transfer reactions for alkyl radical species with two (ethyl) to ten (decyl) carbon atoms have been determined by means of ab initio molecular orbital calculations. Fully optimized geometries for reactants, transition states, and products, are computed at the PM3 and Hartree-Fock (HF) levels of theory using several basis sets. To account for the effects of electron correlation, single-point energy calculations were made at the MP2 level of theory at the optimized HF geometries. From the ab initio data the reaction exothermicities have been computed and compared to the known experimental values. Rate constants were computed using conventional Transition State Theory (TST). The effect of tunneling on the computed rate constant is estimated using the method of Wigner. It will be shown that the barrier heights of these reactions tend to follow certain trends regardless of the length of the carbon chain. The implications of the rate constants predicted in this study on the simulation of real fuels will be discussed.