RESONANCE RAMAN STUDIES OF IMIDAZOLE, IMIDAZOLIUM, AND THEIR DERIVATIVES - THE EFFECT OF DEUTERIUM SUBSTITUTION

Resonance Raman spectra of imidazole, imidazolium cation, 4-methylimid azole, histidine, and their cations are presented for the proto and N- deutero forms. N-Deuteration greatly simplifies the resonance Raman sp ectra for all of these species. The deutero cations have only one stro ngly enhanced Raman band. This change in number of active vibrations i s interpreted in terms of a change in the form of the ground electroni c state normal modes of motion. The ground-state equilibrium geometry and vibrational force field are calculated at the 6-31++G* Hartree-Fo ck level for imidazole and imidazolium. The low-lying excited electron ic states are calculated at the ground-state equilibrium geometry with configuration interaction involving singly excited states. A single s tate is expected to dominate the absorption and Raman spectral intensi ties. The equilibrium geometry of this state for imidazole and imidazo lium at the 6-3 1 +G/CIS level is calculated and expressed in terms of the displacement along each of the ground-state modes for each isotop ic species. This ab initio procedure correctly predicts the intensity of the strongly enhanced normal modes of the neutral and cationic spec ies including the large change in intensity observed with isotope exch ange. 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 th e same in the d2 species while the other active mode of the proto form becomes distributed among several modes in the deuterated species suc h that none has a significant displacement upon excitation and thus ha s a low Raman intensity. A quantitative comparison is made between the ory and observations for imidazolium in its three N-proto isotopic for ms.