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

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.