Theoretical Study of the Alkoxy Radicals Derived from Isoprene:  Pressure- and Temperature-Dependent Decomposition Rates

Pressure- and temperature-dependent thermal decomposition rates for the primary alkoxy radicals derived from OH-initiated oxidation of isoprene have been calculated. The master-equation calculations are based on ab initio energetics and structures previously reported. We also provide analytical descriptions of the theoretical falloff curves using the semiempirical Troe formalism. Additional calculations are also provided for two isomers not previously reported and to further characterize basis set effects on the energetics. The results indicate that the decomposition rates for these radicals should not be considered to be in the high-pressure limit at 760 Torr. Under atmospheric conditions the alkoxy radicals will be activated, and we find that prompt reaction for several isomers may be significant. The results are compared to recent estimates of overall alkoxy radical decomposition rates derived from OH cycling studies.