Ethanethiol, aka ethyl mercaptan, occurs in two distinguishable conformers because of the orientation of the SH group. The internal rotation of the CH₃ group can be neglected in most instances. The doubly degenerate gauche conformer is lowest in energy, the anti conformer is higher in energy by about 160 K. This is an average value of several reported ones. Tunneling between the two equivalent gauche conformers lifts the degeneracy, causing a gauche⁺ and a gauche^– state which interact heavily.
The spectroscopic parameters were published by
(1) H. S. P. Müller, A. Belloche, L.-H. Xu, R. M. Lees, R. T. Garrod, A. Walters, J. van Wijngaarden, F. Lewen, S. Schlemmer, and K. M. Menten, 2016, Astron. Astrophys. 587, Art. No. A92.
Initial data are from
(2) R. E. Schmidt and C. R. Quade, 1975, J. Chem. Phys. 62, 3864.
Additional, rather extensive data are from
(3) L. Kolesniková, B. Tercero, J. Cernicharo, J. L. Alonso, A. M. Daly, B. P. Gordon, and S. T. Shipman, 2014, Astrophys. J. 784, Art. No. L7.
The initial parameter set appeared too large with respect to the covered quantum numbers and the amount of the experimental data. Therefore, the data were refit, starting with a much smmaller parameter set. Several higher order parameters were omitted. As a consequence, several transitions with higher values of K_a (about 10) showed frequently rather large residuals. These may be caused by perturbations or by misassignments, possibly in combination with overlapped lines. These high-K_a lines were omitted throughout. In addition, a small number of transitions with lower values of K_a showed larger deviations and were omitted also, see (1).
Predictions may be accurate enough for all observational purposes. Predictions with K_a of 7 or larger should be viewed with some caution. Predictions with K_a of 10 or larger or with uncertainties larger than 0.5 MHz should be viewed with great caution.
The anti conformer was treated as an isolated species because its energy with respect to the gauche conformer is somewhat uncertain. In other words, the partition function does not takes into account the gauche conformer !
The dipole moment was determined in (2).