Raman spectra of gases. XI. Methyl torsional overtones in the ethyl halides

A series of peaks in the Raman spectra of gaseous CH3CH3, CH3CH2Cl, CH3CH2Br, CH3CD2Br, CD3CH2Br, CD3CD2Br, CH3CH2I, CH3CD2I, and CD3CH2I have been observed and assigned to the torsional overtones, (Δν = 2) of the methyl group. From these spectroscopic data, barriers to internal rotation have been calculated for the ethyl halides. For the compounds where the barriers are accurately known from previous work, the values obtained in this study agree quite well. The value for the internal rotational barrier in ethyl iodide is 3.67 ± 0.01 kcal/mole, which is considerably higher than the value previously reported from microwave studies. It is shown that Raman spectroscopy can be a convenient technique for obtaining torsional barrier heights when one uses the Δν = 2 quantum jumps of the torsional motion.