Atmospheric Oxidation Mechanism and Kinetics of Hydrofluoroethers, CH3OCF3, CH3OCHF2, and CHF2OCH2CF3, by OH Radical: A Theoretical Study.

Abstract

In the present work, the reaction mechanism of two segregated hydrofluoroethers (HFEs), CH3OCF3 (HFE-143a) and CH3OCHF2 (HFE-152a), and a nonsegregated HFE, CHF2OCH2CF3 (HFE-245fa2), with OH radical is studied using electronic structure calculations. The initial reaction between HFE and OH radical is studied by considering two (three for CHF2OCH2CF3) pathways, H-atom abstraction and C-O bond breaking, OH addition reaction and C-C bond breaking, and OH addition reaction, which leads to the formation of alkyl radical intermediate. The dominant atmospheric fate of initially formed alkyl radical intermediate is its reaction with O2. The peroxy radicals thus formed exit through the reaction with HO2 radical and NO radical resulting in the formation of products, carbonyl fluoride (COF2), trifluoromethylformate, trifluoro(hydroperoxymethoxy)methane, difluoro(hydroperoxy methoxy)methane, difluoromethylformate, 2-(difluoromethoxy)-1,1,1-trifluoro-2-hydroperoxyethane, and difluoromethyl ester. The rate constant is calculated for the initial H-atom abstraction reaction using canonical variational transition state theory with small curvature tunnelling corrections over the temperature range 272-350 K. The atmospheric lifetime and global warming potential of HFEs are obtained from the calculated reaction potential energy surface and rate constant. The results are discussed with respect to the atmospheric implications of CH3OCF3 (HFE-143a), CH3OCHF2 (HFE-152a), and CHF2OCH2CF3 (HFE-245fa2).