Infrared spectrum and structure of the aci-nitro form of 2-nitrophenol in a low-temperature argon matrix

Matrix-isolation infrared spectroscopy was used to identify a reaction intermediate of 2-nitrophenol with an aid of density-functional-theory (DFT) calculations. The observed FTIR bands of a product from 2-nitrophenol upon irradiation of 248 nm were assigned to an aci-nitro isomer among 2 × 2 possible conformations, cis/trans around the CN+(O−)OH and CN+ (O−)OH bond axes, by comparison with calculated spectral patterns. A reverse reaction from aci-nitro isomer to 2-nitrophenol was observed upon irradiation with λ > 450 nm. The mechanism of this reversible photoreaction is discussed in terms of the vertical transition wavelengths and the oscillator strengths obtained by time-dependent DFT calculation.