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 CF 3 CF 2 CFHO formed in the atmospheric degradation of HFC-236cb (CF 3 CF 2 CFH 2 ). Experiments were performed over the temperature range 228−296 K at 7.8−1000 Torr total pressure. Two reaction pathways are possible for CF 3 CF 2 CFHO radicals: reaction with oxygen, CF 3 CF 2 CFHO + O 2 → CF 3 CF 2 C(O)F + HO 2 ( k O2 ) and decomposition via C−C bond scission, CF 3 CF 2 CFHO → CF 3 CF 2 + HC(O)F ( k d ). CF 3 CF 2 CFHO radicals were produced by two reactions: the CF 3 CF 2 CFHO 2 self-reaction and the CF 3 CF 2 CFHO 2 + NO reaction. In the absence of NO at 800 Torr total pressure the rate constant ratio k d / k O2 was determined to be ( ) × 10 25 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) × 10 19 molecules cm -3 where k d,0 and k d, ∞ are the second- and first-order rate constants for decomposition in the low- and high-pressure limits, respectively. CF 3 CF 2 CFHO radicals formed in the CF 3 CF 2 CFHO 2 + NO reaction undergo more C−C bond scission than those generated in the CF 3 CF 2 CFHO 2 self-reaction. This is consistent with a significant fraction ( %) of the alkoxy radicals being formed with sufficient internal energy to undergo prompt decomposition. Overall, we calculate that less than 1% of the CF 3 CF 2 CFH 2 (HFC-236cb) released to the atmosphere degrades to form CF 3 CF 2 C(O)F while >99% gives CF 3 CF 2 radicals and HC(O)F.