Ab initio study of the influence of resonance stabilization on intramolecular ring closure reactions of hydrocarbon radicals.

The intramolecular ring closure reactions of unsaturated hydrocarbon radicals potentially play an important role for the formation of molecular weight growth species, especially during the pyrolysis and oxidation of alkenes under low to intermediate temperatures. In this work we investigated a series of intramolecular cycloaddition reactions of both allylic- and alkyl-type dienyl radicals. In the first set of reactions, a resonant linear radical is converted into a non-resonant cyclic radical. In the second set, a non-resonant linear alkenyl radical isomerizes to either a resonant cyclic radical or a cyclic carbinyl radical. In both cases, three different reaction schemes are examined based on the location of the partially-formed resonance structure in the cyclic transition state. For each reaction scheme, both the endo- and exo-pathways were investigated. High pressure rate parameters are obtained from the results of CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. The results are discussed in the context of a Benson-type model to examine the impact of the partially-formed resonance stabilization on both the activation energies and pre-exponential factors. The results are compared to previously reported rate parameters for cycloaddition reactions of alkenyl radicals. The differences in the activation energies are primarily due to the bimolecular component of the activation energy. However, in some cases, the presence of the partial resonance structure significantly increases the strain energy for the ring that is formed in the transition state. The pre-exponential factors are also impacted by the formation of a partial resonance structure in the transition state. Lastly, the C6H9 potential energy surface is examined to show how the trends that are outlined here can be used to estimate rate parameters, which are needed to analyze pressure-dependent reaction systems.