Theoretical study of chloroalkenylperoxy radicals
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DFT and ab initio molecular orbital calculations have been performed to investigate the structures and energetics of the Cl-O2-isoprene peroxy radicals arising from the Cl-initiated oxidation of isoprene. Geometry optimizations of the chloroalkenylperoxy radicals were performed using density function theory (B3LYP), and the energies were computed with the single-point calculation using different levels of theory for electron correlation and basis set effects. At the CCSD(T)/6-31G(d) level of theory corrected with zero-point energy (ZPE), the chloroalkenylperoxy radicals are about 39 to 43 kcal mol-1 more stable than the separated reactants (i.e., O2 + Cl + isoprene). We find no evidence for an energetic barrier to O2 addition and have calculated rate constants for the O2 addition step using canonical variational transition state theory (CVTST) based on Morse potentials to describe the reaction coordinate. The results provide the isomeric branching between the six Cl-O2-isoprene epoxy radicals, indicating that the two -chloroalkenylperoxy radicals with initial Cl addition at C1 and C4 positions and subsequent O2 addition at C2 and C3 positions, respectively, play an important role in determining the reaction pathways and final product distributions of the Cl-isoprene reaction system.