Atmospheric Chemistry of Chloroprene Initiated by OH Radicals: Combined Ab Initio/DFT Calculations and Kinetics Analysis.

Abstract

Chloroprene (CP; CH2═C(Cl)-CH═CH2) is a significant toxic airborne pollutant, often originating from anthropogenic activities. However, the environmental fate of CP is incompletely understood. High level CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculations combined with kinetic modeling were employed here to glean new insight into the reaction mechanism, energies, and kinetics of the reaction of CP with OH radical (•OH). We report the energies of four different addition pathways and six different abstraction pathways. The •OH attack on the terminal C1 atom of the =CH2 group (which is directly attached to the =CCl moiety), leading to the formation of HOCH2-•C(Cl)-CH═CH2, was found to be a major path. The barrier height for the formation of the corresponding transition state was found to be -1.9 kcal mol-1 below that of the starting CP + •OH reactants. Rate coefficients were calculated for addition and abstraction pathways involving the CP + •OH reaction under pre-equilibrium approximation conditions, employing a combination of canonical variational transition state theory and small curvature tunneling. The overall rate coefficient for the reaction of CP + •OH at 298 K was found to be 1.4 × 10-10 cm3 molecule-1 s-1. The thermochemistry of the possible channels and atmospheric implications are provided. In addition, the fate of HOCH2-•C(Cl)-CH═CH2 in the presence of 3O2 was investigated. We found the reaction of the CP-derived peroxy radical adduct with HO2 and NO to make contributions to the formation of products such as formaldehyde, HO2 radical, Cl atom, HOCH2C(OOH)(Cl)CH═CH2, HOCH2C(O)Cl, ClC(O)CH═CH2, HOCH2C(O)CH═CH2, and HC(O) radical.