Thermochemistry and Kinetics of the OH- and Cl-Initiated Degradation Pathways of the HCFC-132b Atmospheric Pollutant

Abstract

The gas-phase hydrogen abstraction reaction of 1,2-dichloro-1,1-difluoroethane (CH2ClCClF2) with OH and Cl radicals is theoretically investigated by employing density functional theory methods combined with coupled-cluster-based composite schemes. The mechanism and kinetics of the degradation entrance channels of CH2ClCClF2, recently detected with increasing concentration in the atmosphere, is elucidated by using cost-effective ab initio triple-slash dual-level direct dynamics, jChS//B2PLYP-D3(BJ)/jun-cc-pV(T+d)Z///M06-2X-D3/jun-cc-pV(T+d)Z. Thermal rate constants are calculated over the temperature range of 200–1000 K by adopting both canonical- (CVT) and microcanonical (μVT)-variational transition state theory also including the microcanonically optimized multidimensional tunneling transmission coefficient (μOMT). The theoretical rate coefficient for the H-abstraction reaction initiated by the OH radical is computed to be kOHCVT/μOMT = 1.72 × 10–14 cm3 molecule–1 s–1 and kOHμVT/μOMT = 1.57 × 10–14 cm3 molecule–1 s–1 at 298 K, in excellent agreement with the available experimental data. The rate constant for the H-abstraction reaction by the Cl atom, here obtained for the first time, is predicted to be kClCVT/μOMT = 5.96 × 10–14 cm3 molecule–1 s–1 and kClμVT/μOMT = 5.65 × 10–14 cm3 molecule–1 s–1 at 298.15 K, thus showing increased efficiency with respect to the OH-prompted reaction.