Bimolecular sinks of Criegee intermediates derived from hydrofluoroolefins – a computational analysis

Abstract

A novel range of stabilised Criegee intermediate (sCI) species with halogenated substituent groups have been identified as products to the reaction between with gaseous ozone and hydrofluoroolefins (HFOs), a series of recently-developed and increasingly prevalent haloalkene refrigerants. The bimolecular chemistry of this group of hydrofluoroolefin-derived sCIs (HFO-sCIs) has yet to be explored in any significant detail so this work evaluates the reaction chemistry of common tropospheric gaseous species with the following group of HFO-sCIs: syn- & anti-CF₃CHOO & syn- & anti-CF₃CFOO. Using high-level theoretical calculations (DF-HF/DF-LCCSD(T)-F12a//B3LYP/aug-cc-pVTZ), this study demonstrates that HFO-sCIs will deplete many pollutants (e.g. HCHO, SO₂ & H₂S) but also act as a source of other atmospheric contaminants (e.g. SO₃ & TFA). The bimolecular reactivity of the HFO-sCIs were compared against CH₂OO, the most frequently studied sCI, for which the general reactivity trend has been identified: k_THEO (syn-CF₃CHOO) < k_THEO (anti-CF₃CHOO) ≈ k_THEO (CH₂OO) ≪ k_THEO (anti-CF₃CFOO) < k_THEO (syn-CF₃CFOO). In general syn & anti-CF₃ substituents reduce overall sCI reactivity compared to similar non-halogenated sCI species, whereas both syn & anti-F substituents significantly increase HFO-sCI reactivity. While HFO-sCI reactivity is largely dictated by the identity and location of the sCI substituent groups, there are co-reactants that alter these observed trends in reactivity, for example HCl reacts more rapidly with CH₂OO than it does with syn- & anti-CF₃CFOO.