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. Internal rotation of the methyl group has been resolved in part in the laboratory. The splittings are small enough to be negligible for observations of hot cores. 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. The analysis was restricted to such transitions because these are the ones most likely to be observed by radioastronomical means. The data have been summarized by
(2) A. Bouchez, A. Walters, H. S. P. Müller, M. Ordu, F. Lewen, M. Koerber, S. Bottinelli, C. P. Endres, and S. Schlemmer, 2012, J. Quant. Spectrosc. Radiat. Transfer 113, 1148.
Additional micowave frequencies have been reported by
(3) J. P. Culot, 1969, Ann. Soc. Sci. Bruxelles 83, 65.
These transitions have not been merged.
Predictions with J + 2K_a > 32 should be viewed with caution. The predictions have been truncated at J = 40.
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 should differ only slightly for ¹³C containing 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.