Resonance Raman studies of imidazole, imidazolium, and their derivatives: The effect of deuterium substitution

Resonance Raman spectra of imidazole, imidazolium cation, 4-methylimidazole, histidine, and their cations are presented for the proto and N-deutero forms. N-Deuteration greatly simplifies the resonance Raman spectra for all of these species. The deutero cations have only one strongly enhanced Raman band. This change in number of active vibrations is interpreted in terms of a change in the form of the ground electronic state normal modes of motion. The ground-state equilibrium geometry and vibrational force field are calculated at the 6-31++G** Hartree-Fock level for imidazole and imidazolium. The low-lying excited electronic states are calculated at the ground-state equilibrium geometry with configuration interaction involving singly excited states. A single state is expected to dominate the absorption and Raman spectral intensities. The equilibrium geometry of this state for imidazole and imidazolium at the 6-31+G/CIS level is calculated and expressed in terms of the displacement along each of the ground-state modes for each isotopic species. This ab initio procedure correctly predicts the intensity of the strongly enhanced normal modes of the neutral and cationic species including the large change in intensity observed with isotope exchange. It is found that the effect of replacement of N-H by N-D in the cation is to leave one of the two strong modes of the proto species the same in the d₂ species while the other active mode of the proto form becomes distributed among several modes in the deuterated species such that none has a significant displacement upon excitation and thus has a low Raman intensity. A quantitative comparison is made between theory and observations for imidazolium in its three N-proto isotopic forms.