PRERESONANCE RAMAN-SPECTRA FOR IMIDAZOLE AND IMIDAZOLIUM ION - INTERPRETATION OF THE INTENSITY ENHANCEMENT FROM A PRECISE ASSIGNMENT OF NORMAL-MODES

The change of the intensity of Raman bands at resonance relative to that observed far from resonance for Im and ImH+ is interpreted from the change in their geometry in going from the ground to the excited state. Thus, the vibrational composition for each normal mode in the ground state is obtained from a normal coordinate analysis carried out on the basis of ab initio 3-21 G force fields. Then, the change in bond length in the excited state is deduced from the change in bond orders calculated for the lowest lying pi-pi* molecular orbital excitations. This has allowed the calculation of relative resonance Raman intensities which are discussed by comparison with the measured relative intensities for Raman bands at excitation varying from 514.5 to 200 nm. The satisfactory agreement obtained between observed and calculated data gives another criterion for the reliability of the force fields used.