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

ABSTRACT

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.