C–C bond fission pathways of chloroalkenyl alkoxy radicals

Abstract

Density-functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energetics of the chloroalkenyl alkoxy radicals arising from Cl-initiated reactions of isoprene as well as the transition states and products of their decomposition reactions. Geometry optimizations of the various species were performed at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-31G(d,p) level, and single-point energies were computed using second-order Møller–Plesset and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations. The activation and reaction energies of C–C bond scission of the alkoxy radicals are in the ranges of 12–25 and −3–22 kcal mol−1, respectively. Using the obtained activation barriers and transition state structures, we have calculated the high-pressure limit decomposition rates of the chloroalkenyl alkoxy radicals using transition state theory, ranging from 1×10−5 to 2×104 s−1. The results indicate that C–C bond decomposition of the chloroalkenyl alkoxy radicals is rather slow and likely plays a minor role in the Cl-isoprene reactions. Implications of the present results on the formation yields of methyl vinyl ketone, methacrolein, and 1-chloro-3-methyl-3-buten-2-one are discussed.