Theoretical investigation of the photolysis mechanism of fluorinated Criegee intermediate HFCOO

Abstract

Hydrofluoroolefins (HFOs) have emerged as promising alternatives for ozone-depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) due to their drastically shorter atmospheric lifetimes (days to weeks vs years to decades for CFCs and HCFCs) and significantly lower global warming potential. While HFOs’ rapid degradation minimizes their direct environmental accumulation, the ecological risks posed by their reactive degradation intermediates—particularly hydrofluorocarbonyl oxide (HFCOO), a fluorinated Criegee intermediate generated via HFO-ozone reactions—require urgent mechanistic clarification. The atmospheric persistence and chemical reactivity of HFCOO are intrinsically governed by its excited-state dynamics, where competing photochemical pathways determine whether it undergoes ultrafast dissociation or survives to mediate secondary pollutant formation. In this paper, we examine the deactivation mechanism of HFCOO by employing high-level electronic structure calculations and on-the-fly surface hopping dynamic simulations. Our results reveal that the first excited singlet state (S1) of HFCOO is a dark state populated via nπ* transitions, while the second excited singlet state (S2), accessed through ππ* transitions, is crucial for O–O bond cleavage. We find that both syn- and anti-configurations of the S2 state exhibit rapid O–O bond dissociation, producing hydrofluorocarbonyl (HFCO) and excited oxygen atoms within 30 and 50 fs, respectively. Our study underscores the ultrafast photodissociation dynamics of HFCOO in the atmosphere, contributing valuable insights into the environmental safety assessment of HFOs and improving atmospheric models for predicting their ecological impacts.