关键词: N-甲基吡咯-2-甲醛, 激发态结构动力学, 共振拉曼光谱, 溶剂效应

Abstract: Resonance Raman spectra of N-Methylpyrrole-2-carboxaldehyde (NMPCA) were obtained and seven excitations covered the A- and B-band electronic absorptions. The electronic excitations and the Franck-Condon region structural dynamics of NMPCA were studied by resonance Raman spectroscopy and time-dependent density functional theory (TD-DFT) calculations. The A- and B-band electronic absorptions were assigned to π →π^* transitions on the basis of the TD-B3LYP/6-311 ++ G(d,p) level of theory. The resonance Raman spectra showed Raman intensity in the fundamentals, the overtones and the combination bands for about 11-13 vibrational modes (A-band excitation) or 7-11 vibrational modes (B-band excitation). These were predominately due to the C＝O stretch mode ν₇, the ring deformation+N₁－ C₆ stretch ν₁₇, the ring deformation mode ν₂₁ and the C₆－N₁－C₂/C₂－C₃－C₄ anti-symmetry stretch mode ν₁₄. This indicates that the Franck-Condon region S_π structural dynamics of NMPCA mainly occurs along the C＝O stretch, the ring deformation, and the N₁－C₆ stretch reaction coordinates. In a certain solvent and under different excitation wavelengths the relative intensity of the C＝O stretch mode ν₇ versus the C₆－N₁－C₂/C₂－C₃ －C₄ anti-symmetry stretch mode ν₁₄ shows an intense to weak to intense change as the excitation wavelengths decrease. This intensity variation directly reflects the S_n/S_π state-mixing or crossing of the potential energy surfaces in the Franck-Condon region. Solvents can efficiently tune the Franck- Condon region S_n/S_π state-mixing or crossing processes.

Key words: N-Methylpyrrole-2-carboxaldehyde, Excited state structural dynamics, Resonance Raman spectrum, Solvent effect