Production of HF and CHF3 in Hydrofluoroolefin Ozonolysis: A Quantum Chemical and Statistical Rate Theory Study

Hydrofluoroolefins (HFOs) are refrigerants designed to have low global warming potential (GWP), but, depending on their structure, HFOs can react with O3 to produce CHF3, a greenhouse gas with a GWP of 15,500 with a 100-year time horizon. In particular, the most recent mass spectrometric chamber study of the ozonolysis of the common refrigerant trans-CHF═CHCF3 (HFO-1234ze(E)) reports a CHF3 yield of 0.079 at 298 K and 1 atm. A recent quantum chemical (ωB97X-D/cc-pVTZ) and RRKM/master equation (ME) study quantified CHF3 formation due to the isomerization of the chemically activated CF3CHOO Criegee intermediate (CI) to trifluoroacetic acid (TFA), which then decomposes to CHF3 and CO2. The predicted CHF3 yield of 0.636 at 298 K and 1 atm was far higher than the experimental result. We have revised the theoretical model of HFO ozonolysis by using the DLPNO-CCSD(T1)/cc-pVTZ quantum chemical method and incorporating other CI reaction pathways. We find that, relative to the 0-K energy of the more stable CI conformer, the transition structure (TS) for the decomposition of TFA to HF and an α-lactone has an energy of -66.5 kcal mol-1, while the TS for the decomposition of TFA to CHF3 and CO2 is -41.8 kcal mol-1. RRKM/ME simulations of trans-CHF═CHCF3 ozonolysis based on our new chemical mechanism predict a CHF3 yield of 0.076 at 298 K and 1 atm. Revised predictions of CHF3 yield from the ozonolysis of CH2═CHCF3 (HFO-1243zf) and cis-CHCF3═CHCF3 (HFO-1336mzz(Z)) differ substantially from experimental measurements, underscoring the need for both more accurate modeling and additional experimental work.