Spectroscopic characterization and photochemistry of the Criegee intermediate CF3C(H)OO

Criegee intermediates (CIs), also known as carbonyl oxide, are reactive intermediates that play an important role in the atmospheric chemistry. Investigation on the structures and reactivity of CIs is of fundamental importance in understanding the underlying mechanism of their atmospheric reactions. In sharp contrast to the intensively studied parent molecule (CH₂OO) and the alkyl-substituted derivatives, the knowledge about the fluorinated analogue CF₃C(H)OO is scarce. By carefully heating the triplet carbene CF₃CH in an O₂-doped Ar-matrix to 35 K, the elusive carbonyl oxide CF₃C(H)OO in syn- and anti-conformations has been generated and characterized with infrared (IR) and ultraviolet-visible (UV-vis) spectroscopy. The spectroscopic identification is supported by ¹⁸O-labeling experiments and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) and MP2/6-311++G(2d,2p) levels. Upon the long-wavelength irradiation (λ > 680 nm), both conformers of CF₃C(H)OO decompose to give trifluoroacetaldehyde CF₃C(H)O and simultaneously rearrange to the isomeric dioxirane, cyclic-CF₃CH(OO), which undergoes isomerization to the lowest-energy carboxylic acid CF₃C(O)OH upon UV-light excitation at 365 nm. The O₂-oxidation of CF₃CH via the intermediacy of CF₃C(H)OO and cyclic-CF₃CH(OO) might provide new insight into the mechanism for the degradation of hydro-chlorofluorocarbon CF₃CHCl₂ (HCFC-123) in the atmosphere.