Spectroscopic characterization and photochemistry of the Criegee intermediate CFC(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 (CHOO) and the alkyl-substituted derivatives, the knowledge about the fluorinated analogue CFC(H)OO is scarce. By carefully heating the triplet carbene CFCH in an O-doped Ar-matrix to 35 K, the elusive carbonyl oxide CFC(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 O-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 CFC(H)OO decompose to give trifluoroacetaldehyde CFC(H)O and simultaneously rearrange to the isomeric dioxirane, cyclic-CFCH(OO), which undergoes isomerization to the lowest-energy carboxylic acid CFC(O)OH upon UV-light excitation at 365 nm. The O-oxidation of CFCH via the intermediacy of CFC(H)OO and cyclic-CFCH(OO) might provide new insight into the mechanism for the degradation of hydro-chlorofluorocarbon CFCHCl (HCFC-123) in the atmosphere.