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 Moller-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 1x10(-5) to 2x10(4) 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. (C) 2003 American Institute of Physics.