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

Abstract

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.