Vibrational spectra of 4-methylimidazole: assignment of modes and calculation of Raman and resonance Raman intensities at the ab initio 6-31G level

Abstract

Fourier transform infrared (FT-IR) and Raman (FT-R) spectra of 4-methylimidazole and its N-deuterated analogue were obtained for the solid and the aqueous solution. Assignments of the observed band wavenumbers are discussed by comparison with normal mode wavenumbers and IR and Raman intensities calculated from ab initio 6-31G force fields and optimized geometries for both 4- and 5-methylimidazole. The latter molecules are considered to be similar to two tautomers which are assumed to be present in aqueous solution in particular. These assignments were applied to the calculation of the resonance Raman relative intensities for the excited states corresponding to the lowest lying π → π∗ orbital excitations, obtained from the change in bond order and the L −1 matrix. Compared with the observed band intensities at resonance in Raman spectra recorded with excitation at 218 nm, the agreement appears to be excellent for several modes.