Hydrofluoroolefins (HFOs) have become popular as refrigerants because of their putative low global warming potential (GWP). The low GWP is predicated on the presence of a carbon-carbon double bond in the HFO that makes it susceptible to reaction with OH, thus shortening its lifetime. However, the double bond makes HFOs susceptible to reaction with O₃ as well, and recent work by McGillen et al. revealed that the reaction of HFO-1234zeE (trans-1,3,3,3-tetrafluoro-propen-1-ene) with O₃ produces CHF₃, a highly potent greenhouse gas, in ~3% yield at 298 K and 1 atm. McGillen et al. attributed CHF₃formation to the isomerization and decomposition of Criegee intermediates (CIs) with geminal trifluoromethyl (CF₃) and H substituents. Density functional theory (ωB97X-D/cc-pVTZ) and master equation simulations of the same reaction drastically overpredicted the CHF₃ yield, with a 298 K and 1 atm value of ~60%. In the present work, we improve the accuracy of the computational model in two ways: (1) We compute energies using W1BD theory and, for open-shell singlet structures, EOM-SF-CCSD(dT) theory. (2) We include other mechanistic pathways for trifluoromethyl CIs, such as ester formation and decomposition into trifluoroacetyl and OH radicals. These changes in our computational modeling, along with updated measurements, bring predicted and experimental CHF₃ yields into better agreement with each other. We also provide computational models for the formation of CF₄, another molecule with a very high GWP, from the reactions of HFO-1225yeE and HFO-1225yeZ (trans- and cis-1,2,3,3,3-pentafluoro-propen-1-ene) with O₃.

Connor Martin

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