Infrared studies on rotational isomerism. I. Ethylene glycol

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Abstract
The problem of the equilibrium configuration of the ethylene glycol molecule was investigated by measuring the infrared spectra of the vapor (up to 125 °C), liquid, and solid phases between 100 and 3 700 cm−1. The two isotopic molecules (CD2OH)2and (CH2OD)2were likewise studied. Most of the fundamental frequencies could be observed and assigned. These assignments, as well as the phase and temperature behavior of the spectra, confirm that in all cases the molecules exist only as gauche isomers, which are more stable than the trans as a result of intramolecular hydrogen bonds. Because these bonds are far from linear, the OH stretching vibrations are little affected, but the bending vibrations are strongly shifted towards higher frequencies.Comparison of the calculated thermodynamic functions with the calorimetric data shows that restriction of all three internal rotations in the glycol molecule lowers the entropy by about 10 e.u. at normal temperature. From the observed torsional frequencies, the average barrier heights are estimated to be of the order of 10 kcal/mole for rotation about the C—C bond, and 3 to 4 kcal/mole for rotation about the C—O bonds, thereby indicating a fairly rigid structure.