Strong Raman activities of low frequency vibrational modes in alkylbenzenes: conformation specific σ–π interactions between alkyl chain and benzene ring

Non-resonant Raman intensities for alkylbenzenes were studied based on FT-Raman spectra and density functional calculations. Strong bands were found for a bending mode and a torsion mode in a range of 19–217 cm−1, where no strong bands were observed for benzene and alkanes. In a staggered form of ethylbenzene and a gauche form of n-propylbenzene, non-totally symmetric torsion modes of 19 and 38 cm−1 gave remarkably enhanced Raman intensities much stronger than ring-breathing modes. Strong Raman activities of low frequency modes in alkylbenzenes can be ascribed to stereo-specific σ–π interactions between alkyl chain and benzene ring.