Understanding the Atmospheric Oxidation of HFE-7500 (C3F7CF(OC2H5)CF(CF3)2) Initiated by Cl Atom and NO3 Radical from Theory

Abstract

Kinetics and mechanistic pathways for atmospheric oxidation of HFE-7500 ( n-C3F7CF(OCH2CH3)CF(CF3)2) initiated by Cl atom and NO3 radical have been studied using density functional theory. Oxidative degradation pathways facilitated by H-abstraction from the -OCH2 or -CH3 groups in HFE-7500 have been considered. It has been shown that H-abstraction from the α-site (-OCH2) is favored over other reaction pathways. The rate constants were computed employing transition-state theory and canonical variation transition-state theory incorporating small curvature tunnelling correction, over the temperature range of 250-450 K at atmospheric pressure. Calculated rate constants at 298 K and 1 atm compare well with earlier experiments. Temperature dependence of the rate constants and branching ratios for these pathways contributing to overall reaction are described. It has been shown that the rate constants over the studied temperature range was found to fit well to the modified Arrhenius equation (in cm3 molecule-1 s-1) kCl = 1.10 × 10-14 T0.04 exp(-69.87 ± 1.41/T) and kNO3 = 7.66 × 10-26 T3.30 exp(596.40 ± 1.22/T). Standard enthalpies of formation for the reactant (C3F7CF(OCH2CH3)CF(CF3)2) and the products [C3F7CF(OC•HCH3)CF(CF3)2 and C3F7CF(OCH2C•H2)CF(CF3)2] during H-abstraction are derived using the isodesmic approach. Atmospheric implications of the titled molecule are presented.