Structure and Electron Configuration of Imidazole-2-carboxaldehyde and Its Excited Triplet: Resonance Raman and Transient Absorption Spectroscopy and DFT Calculation Investigations

Imidazole-2-carboxaldehyde (IC) can be generated in atmospheric waters and absorbs solar radiation in the near UV region to produce its excited triplet state (³IC), which contributes to the formation of a secondary organic aerosol (SOA). The photoreactivity of IC is significantly influenced by its surroundings, such as water and acidic environment, because IC is capable of transforming into gem-diol under above conditions. Meanwhile, the electron configuration of ³IC is critical in elucidating the reaction mechanism of ³IC with other anthropogenic and biogenic volatile organic compounds (VOCs). In this study, steady-state and time-resolved resonance Raman as well as transient absorption spectroscopic experiments were conducted to provide vibrational and kinetic information on IC and ³IC in the presence of water and acid conditions. Using density functional theory (DFT) calculations, the H-bonding at the carbonyl O was confirmed and the hydrated structure of IC and ³IC was determined. 1,4-Cyclohexadiene is a good hydrogen donor, and it has a second-order rate constant of ∼10⁷ M^-1 s^-1 toward ³IC. The results of CASSCF calculations suggest that the hydrogen abstraction may involve the transition from the ππ* to nπ* triplet state via the surface-crossing point.