Intensities of Raman vibration bands. I

Abstract The intensities of some Raman vibrational bands of key groups such as C = O and C = N, present in aliphatic and aromatic molecules, have been measured relative to the carbon tetrachloride 458 cm -1 b and as standard. In using solutions of some of these compounds in organic solvents, small variations of band intensity have been noticed which seem to be related to the refractive index of the medium , although the usual corrections have been applied for this factor in determining the intensities. In aliphatic nitriles, small structural changes appear to have little effect on the intensity of C = N vibrations, unless the group as conjugated or attached to an aromatic nucleus. With substituted ethyl benzoates, acetophenones, propiophenones or benzonitriles, the plots of C = O or C = N band intensity against Hammett constants σH show a minimum for the parent compound. There is, however, a smooth regularity between band intensity and Taft resonance factor σR. The changes of intensity are also related to the shifts in position of the ultra-violet absorption bands caused by the substituents. The results may be explained, however, in terms of the resonance Raman effect, taking into account the effect of substituents in bringing an electronic level nearer to the exciting frequency.