Raman Scattering Intensities of the Characteristic Vibrations of Some Aromatic Nitro Compounds

Abstract

The Raman scattering intensities of the 1600 cm-1 band of ortho-alkyl-substituted nitrobenzenes, and of the 1600 cm-1, the 1120 cm-1 and the 860 cm-1 bands of para-substituted nitrobenzenes were examined experimentally in view of analytical applications. The latter two bands are very intense and characteristic of para-substituted nitrobenzenes, though the assignment of them is not fully determined. From the similarity of the intensity behaviour between the 1600 cm-1 band, which is assigned to the ring stretqhing mode, and the NO2 symmetric stretching band, it was inferred that the intensity change among the samples can be attributed not to the difference in the local electron density in the vibrating moieties but to the difference in the spatial extent of electron clouds in relation to the intramolecular charge transfer. This suggests that resonance Raman effect contributes to the intensities. In view of this, the excitation wavelength dependnce of the intensities (the excitation profiles) were examined for para-substituted nitrobenzenes. It was shown that the intensities of the 1600 and the 1120 cm-1 bands are enhanced by 1.2 to 3. 0 times when the excitation wavelength changes from 514.5 to 457. 9 nm, while those of the 860 cm-1 band are enhanced by 1.0 to 1.8.9 times. The difference in the spectral patterns among the samples due to the reversal of the relative intensities between the 1120 and the 860 cm-1 bands was discussed in terms of the difference in the excitation profiles. © 1984, The Chemical Society of Japan. All rights reserved.