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 + O-2 --> CF3CF2C(O)F + HO2 (k(O2)) and decomposition via C-C bond scission, CF3CF2CFHO --> CF3CF2 + HC(O)F (kd). 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(similar to 4.7)(+16.3)3) x 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 Tree type expression using k(d,0)/k(O2) = 30.8 +/- 6.9 and k(d,infinity)/k(O2) = (2.31 +/- 0.12) x 10(19) molecules cm(-3) where k(d,0) and k(d,infinity) 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(-22)(+19)%) 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.