Intensities of Raman vibration bands, II. Effects of solvent and solute structure

Abstract

The Raman band intensity of the carbonyl group in methyl benzoate has been measured in five solvents, for a wide range of concentration. After ‘correction’ the values still appear to vary with concentration, and from all the data a smooth variation has been found with refractive index of the mixture. Similar measurements have been made with the vibrational chromophores C = O , C ≡ N and C ≡ C in many media, at the same concentration, and this relation with refractive index has been confirmed. Extrapolation of the curves to low values of the refractive index suggests values for the band intensities in the vapour phase. This dependence of band intensity upon refractive index cannot yet be convincingly explained, but it is suggested that a further examination is required of the corrections usually made for reflexion losses in the Raman measurements. Marked changes of intensity of the C=O Raman band in methyl benzoate occur when substituents are introduced into the aromatic ring. These have been correlated with inductive and mesomeric effects of the substituents, and with the shifts in the electronic absorption bands in relation to the Raman exciting frequency. The results suggest that this latter factor, through the resonance Raman effect, may be the main cause of the variation of band intensity.