A theoretical investigation of nitrooxyalkyl peroxy radicals from NO3-initiated oxidation of isoprene

Density functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energetics of the nitrooxyalkyl peroxy radicals arising from the NO3-initiated oxidation of isoprene. Geometry optimizations of the peroxy radicals are performed using density functional theory at the B3LYP/6-31G(d,p) level and single-point energies are computed using second-order Møller-Plesset perturbation theory and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). The zero-point corrected energies of the nitrooxyalkyl peroxy radicals are 37–43 kcal mol−1 more stable than the separated NO3, O2 and isoprene reactants at the CCSD(T)/6-31G(d)+CF level. The rate constants for the addition of O2 to the NO3–isoprene adducts are calculated using the canonical variational transition state theory (CVTST), with an overall rate constant of 3.8×10−12 cm3 molecule−1 s−1. The results provide the isomeric branching ratios between eight nitrooxyalkyl peroxy radicals.