Atmospheric chemistry of HFE-7000 (i-C₃F₇OCH₃) and isofluoro-propyl formate (i-C₃F₇OC(O)H): reactions with OH radicals, atmospheric lifetime and fate of alkoxy radical (i-C₃F₇OCH₂O^•) – a DFT study

<p>The mechanism of hydrogen abstraction reaction between HFE-7000 (i-C₃F₇OCH₃) and OH radicals using M06-2X functional in conjunction with 6-31+G(d,p) basis set is investigated. The pre-reactive and post-reactive complexes from intrinsic reaction coordinate calculations are validated at entrance and exit channels, respectively. The standard enthalpies of formation for the species and bond dissociation energy for C–H bond are also estimated. The rate constants of the titled reactions over the temperature range of 250–450 K are reported. The OH-driven atmospheric life time of i-HFE-7000 is computed to be 3.19 years. The atmospheric fate of the alkoxy radical (i-C₃F₇OCH₂O^•) is also explored here for the first time. Three prominent plausible decomposition channels including oxidation are considered in detail. The thermochemical data reveal that reaction with O₂ is the dominant path for the decomposition of i-C₃F₇OCH₂O^• radical. Moreover, rate constant for the OH-initiated hydrogen abstraction of isofluoro-propyl formate (i-C₃F₇OC(O)H) is also reported.</p

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

Kinetics and mechanistic pathways for atmospheric oxidation of HFE-7500 (<i>n</i>-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) <i>k</i>_Cl = 1.10 × 10^–14 <i>T</i>^0.04 exp­(−69.87 ± 1.41/T) and <i>k</i>_NO₃ = 7.66 × 10^–26 <i>T</i>^3.30 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