Vibrational spectroscopic studies of internal rotation of symmetrical groups—IV. Toluene and some deuterated derivatives in the vapour phase

Abstract

The vapour phase infrared spectra of C 6H 5CH 3, C 6D 5CH 3 and C 6H 5CD 3 have been measured in detail in the wavenumber regions associated with the “perpendicular” methyl (or methyl- d 3) bond-stretching, antisymmetrical bending and rocking vibrations. Although the spacings are irregular, fine-structure associated with internal rotation is clearly observed within the absorption band contours from the perpendicular stretching modes of the CH 3 and CD 3 groups. The mean features of these bands may be explained qualitatively in terms of almost-free internal rotation of the methyl and methyl- d 3 groups, together with effects associated with the splitting of the degeneracy of the “perpendicular” modes. Similar internal-rotation fine structure could not be observed with any certainty on the bands from the antisymmetrical bending or methyl rocking modes. These observations can reasonably be accounted for in terms of substantial splitting of the in-plane and out-of-plane vibrational frequencies of these modes together with overlapping from other absorption bands associated with the phenyl or phenyl- d 5 groups.