Tropospheric Oxidation of Dichlorotetrafluoropropene (CF<sub>3</sub>─CF═CCl<sub>2</sub>) Initiated by OH Radical: Reaction Mechanisms, Kinetic Studies, and Atmospheric Implications
Rabu Ranjan Changmai, Manabendra Sarma
Abstract
A detailed degradation study of the halogenated alkenes initiated by oxidants present in the atmosphere is of utmost importance because of its ability to release free halogen, which affects the environment to a large extent. In this work, we have studied the hydroxyl radical (·OH) initiated degradation of a halogenated alkene, 1,1-dichloro-2,3,3,3-tetrafluoroprop-1-ene, CF3CF═CCl2 (CFP) in the atmosphere. We have proposed that the reaction proceeds via two ways: (i) addition reactions to either side of the double bond, and (ii) Cl-abstraction reactions. We have used both the Density Functional Theory (DFT) and post-Hartree–Fock methods with triple-ζ correlation consistent basis sets, cc-pVTZ for geometry optimization, and frequency calculation. Thermodynamic and kinetic parameters are studied using the above-mentioned theoretical accuracies. The potential energy surface (PES) showed that both the addition reactions are exothermic in nature, whereas Cl-abstraction reactions are endothermic. The kinetic and branching ratio calculation results revealed that the addition of an ·OH radical to double bonded carbon atoms (C1 and C2) is more dominant than the Cl-abstraction reactions. The overall rate constant calculated at 298 K using the M06-2X functional is found to be in closer approximation with the experimental results than the post-Hartree–Fock, MP2 method. The atmospheric lifetime and ozone depletion potential (ODP) of CFP are calculated as 2.48 days and 7.60 × 10–5, respectively. Finally, the photochemical ozone creation potential (POCP) of CFP is estimated to be 6.40.