Modeling the Ground- and Excited-State Unimolecular Decay of the Simplest Fluorinated Criegee Intermediate, HFCOO, Formed from the Ozonolysis of Hydrofluoroolefin Refrigerants

Hydrofluoroolefins (HFO) are fourth-generation refrigerantsdesignedto function as efficient refrigerants with no ozone depletion potentialand zero global warming potential. Despite extensive studies on theirchemical and physical properties, the ground- and excited-state chemistryof their atmospheric oxidation products is less well understood. Thisstudy focuses on the ground- and excited-state chemistry of the simplestfluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO),which is the simplest fluorinated CI formed from the ozonolysis ofHFOs. HFCOO contains syn- and anti-conformers, which have Boltzmannpopulations of, respectively, 87 and 13% at 298 K. For both conformers,the calculated ground-state reaction energy profiles associated withcyclization to form fluorodioxirane is lower than the equivalent unimoleculardecay path in the simplest CI, H2COO, with anti-HFCOO returninga barrier height more than half of that of H2COO. The excited-statedynamics reveal that photoexcitation to the bright S-2 stateof syn-HFCOO and anti-HFCOO is expected to undergo a prompt O-Ofission with the former conformer expected to dissociate withan almost unity quantum yield and to form both O (D-1) +HFCO (S-0) and O (P-3) + HFCO (T-1)products. In contrast, photoexcitation of anti-HFCOO is expected toundergo an O-O bond fission with a non-unity quantum yield.The fraction of photoexcited anti-HFCOO that dissociates is predictedto exclusively form O (D-1) + HFCO (S-0) products,which is in sharp contrast to H2COO. The wider implicationsof our results are discussed from both physical and atmospheric chemistryperspectives.

Automated summary

This study focuses on the ground- and excited-state chemistry of the simplest fluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO), formed from the ozonolysis of HFOs. The results show that HFCOO is more prone to undergo cyclization reactions in the ground state compared to the simplest CI. In the excited state, HFCOO undergoes O-O bond fission to produce various products. The implications of these findings are discussed in terms of both physical and atmospheric chemistry.