Unlike chlorofluorocarbons, hydrofluoroolefins (HFOs) do not deplete ozone and, when unreacted, contribute little to global warming. However, recent experiments by McGillen reveal that HFO ozonolysis can produce CHF3, which has a high global warming potential. We used computation to study the ozonolysis of three HFOs also studied by McGillen: trans-CHF=CHCF3 (I), CH2=CHCF3 (II), and cis-CHCF3=CHCF3 (III). We used the ωB97X-D/cc-pVTZ model chemistry to predict the energies, geometries, and frequencies of all species, and we performed statistical rate theory calculations in MultiWell to predict branching fractions for the reaction pathways of chemically activated species. The ozonolysis of all three HFOs produces Criegee intermediates (CIs) with a CF3 substituent either syn (9a in Scheme 1) or anti (9b, not shown) to the peroxy bond. The CI isomerizes to a dioxirane (17) and then to a diradical (19) that decomposes to CHF3 + CO2 either directly or via trifluoroacetic acid (24a and 24b). The overall yield of CHF3 from a given HFO depends on the yields of 9a and 9b and on the extent of their chemical activation. For I, the transition states (TSs) leading to 9a and 9b are 5-8 kcal mol-1 lower in energy than the TSs leading to CIs with a F substituent, and the formation of 9a and 9b is exothermic by -64 to -66 kcal mol-1. In contrast, for II, the TSs leading to 9a and 9b are 5-8 kcal mol-1 higher in energy than the TSs leading to the parent CI, and the formation of 9a and 9b is exothermic by only -44 to -45 kcal mol-1. Simulations based on these data predict that the ozonolysis of I has a CHF3 yield of 0.708 at 298 K and 1 atm, while the CHF3 yield from the ozonolysis of II is only 0.041 at 298 K and 1 atm. The ozonolysis of III has a 100% yield of CIs 9a and 9b, but lower chemical activation leads to a CHF3 yield of only 0.313 at 298 K and 1 atm. Yield predictions deviate significantly from experiment, but the predicted trend in I vs. II does agree with experiment.

Computational modeling of greenhouse gas production in hydrofluoroolefin ozonolysis

ACS Fall 2021

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