Structural dynamics of 4-formaldehyde imidazole and imidazole in light absorbing S2(ππ*) state

The short-time structural dynamics of 4-formaldehyde imidazole and imidazole in light absorbing S2(ππ*) state were studied by using resonance Raman spectroscopy and quantum mechanical calculations. The vibrational spectra and ultraviolet absorption spectra of 4-formaldehyde imidazole were assigned. The resonance Raman spectra of imidazole and 4-formaldehyde imidazole were obtained in methanol and acetonitrile with excitation wavelengths in resonance with the first intense absorption band to probe the short-time structural dynamics. complete active space self-consistent field calculations were carried out to determine the minimal singlet excitation energies and structures of S1(nπ*), S2(ππ*), and conical intersection point S1(nπ*)/S2(ππ*). The results show that the A-band structural dynamics of imidazole is predominantly along the N1H/C4H/C5H/C2H in-plane bending reaction coordinate, which suggests that excited state proton or hydrogen transfer reaction takes place somewhere nearby the Franck–Condon region. The significant difference in the short-time structural dynamics between 4-formaldehyde imidazole and imidazole is observed, and the underlying mechanism is interpreted in term of excited state charge redistribution. Copyright © 2015 John Wiley & Sons, Ltd.