Atmospheric chemistry of HFC-236cb: Fate of the alkoxy radical CF₃CF₂CFHO

An FTIR/environmental chamber technique was used to study the fate of the alkoxy radical CF₃CF₂CFHO formed in the atmospheric degradation of HFC-236cb (CF₃CF₂CFH₂). Experiments were performed over the temperature range 228-296 K at 7.8-1000 Torr total pressure. Two reaction pathways are possible for CF₃CF₂CFHO radicals: reaction with oxygen, CF₃CF₂CFHO + O₂ → CF₃CF₂C(O)F + HO₂ (k_O2) and decomposition via C-C bond scission, CF₃CF₂CFHO → CF3CF₂ + HC(O)F (k_d). CF₃CF₂CFHO radicals were produced by two reactions: the CF₃CF₂CFHO₂ self-reaction and the CF₃CF₂CFHO₂ + NO reaction. In the absence of NO at 800 Torr total pressure the rate constant ratio k_d/k_O2 was determined to be (6.6^+16.3_-4.7) × 10²⁵ exp(-(3560 ± 295)/T) molecules cm^-3. The pressure dependence of k_d/k_O2 was studied at 238 K and was well described by a Troe type expression using k_d,0/k_O2 = 30.8 ± 6.9 and k_d,∞/k_O2 = (2.31 ± 0.12) × 10¹⁹ molecules cm^-3 where k_d,∞ and k_d,∞ are the second- and first-order rate constants for decomposition in the low- and high-pressure limits, respectively. CF₃CF₂CFHO radicals formed in the CF₃CF₂CFHO₂ + NO reaction undergo more C-C bond scission than those generated in the CF₃CF₂CFHO₂ self-reaction. This is consistent with a significant fraction (67⁺¹⁹_-22%) of the alkoxy radicals being formed with sufficient internal energy to undergo prompt decomposition. Overall, we calculate that less than 1% of the CF₃CF₂CFH₂ (HFC-236cb) released to the atmosphere degrades to form CF₃CF₂C(O)F while >99% gives CF₃CF₂ radicals and HC(O)F.