Raman Spectrometric Investigation of the Molecular Structure of Alcohols; Conformational Dynamics of Ethanol

The Raman scattering profiles of the OH stretching vibrations of methanol, ethanol, and n‐propanol in the gas phase and of ethanol in alkane solution are measured and analyzed with regard to the conformational behaviour of the molecules. — The fine structure of the profiles is largely resolved because rotational motions do not appreciably contribute to the band width as the scattering tensor is almost isotropic. In the case of ethanol, the assignment of the two sub‐bands to the trans and gauche conformers follows from the intensity ratio. An assignment to the five possible conformers is proposed for the complex profile structure of propanol. A weak satellite on the high frequency wing of the methanol profile can be assigned to a vibrational transition in the first excited torsional state. — Quantitative information on the conformational dynamics can be gained from the temperature dependence of the ethanol profile observed in solution. By applying a theoretical consideration used in NMR spectrometry to explain the dependence of the spectra on molecular dynamics to the present problem on vibrational spectrometry, the following values of the trans ⇌ gauche transition rate are determined: (3.6 ± 0.6) · 1010 s−1 at 20°C and (9.0 ± 1.5) · 1010 s−1 at 137°C.