The anti conformer of ethanol is lower in energy than the symmetric and antisymmetric combinations of the gauche conformer by 39.49 and 42.77 cm^–1, respectively, as determined for the main isotopolog by
(1) J. C. Pearson, C. S. Brauer, and B. J. Drouin, 2008, J. Mol. Spectrosc. 251, 394.
These conformers are caused by the large amplitude motion of the OH-group. Even though there is extensive internal rotation interaction between the anti and the gauche conformers, the energy differences permit these interactions to be neglected for transitions with low quantum numbers; in practice, for transitions with J + 2K_a ≤ 32; in the case of the methyl monodeuterated isotopolog, the value may be slightly larger, at least at low values of K_a. The analysis was restricted to such transitions because these are the ones most likely to be observed by radioastronomical means. The internal rotation of the methyl group is actually frozen out for isotopologs with at least one D in the methyl group. This leads to two distinguishable conformers for the CH₂D isotopolog. Data of both conformers appear in separate entries. The data have been summaried by
(2) A. Walters, M. Schäfer, M. Ordu, F. Lewen, S. Schlemmer, and H. S. P. Müller, 2015, J. Mol. Spectrosc. 314, 6.
Additional microwave frequencies have been reported by
(3) J. P. Culot, 1969, Ann. Soc. Sci. Bruxelles 83, 65.
Predictions with J + 2K_a > 32 should be viewed with caution. The predictions have been truncated at J = 60 and K_a = 10.
Note: The partition function does not take into account the gauche-conformers. Therefore, scaled partition function values are available. The scaling factors were taken from the main isotopolog and refer to the ground vibrational state (anti plus gauche conformers versus only the anti conformer). The correction should be quite good. The first excited methyl and OH torsional modes are both about 250 cm^–1 above ground for the main isotolog, and this value may differ somewhat for the deuterated isotopologs.
Note also that the asymmetrically methyl monodeuterated isotopolog is doubly degenerate with respect to the OD species.
The dipole moment was assumed to agree with that of the main species. This assumption should be very good for the large b dipole moment component, but may be very different for the almost vanishing a dipole moment component. The values were taken from
(4) M. Takano, Y. Sasada, and T. Satoh, 1968, J. Mol. Spectrosc. 26, 157.