The gauche conformer of mercaptoacetonitrile displays large amplitude tunneling motion. Transitions with a- and b-type selection rules connect rotational levels within each state. No c-type transitions have been measured yet. In addition, no strongly perturbed transitions have been assigned. Therefore, the energy difference between the states is very uncertain. Transition frequencies were taken from
(1) H. Møllendal, S. Samdal, and J.-C. Guillemin, 2016, J. Phys. Chem. A, 120, 1992.
Considerably fewer parameters were used in the present fit than in the work above. The energy difference may be considerably smaller, about 1 GHz, than the reported 8 GHz. However, the present fit is not much better than the one reported, and several lines have rather large residuals. Therefore, no experimental lines have been merged. Furthermore, all predicted transition frequencies should be viewed with caution, those with predicted uncertainties larger than 1 MHz should be viewed with great caution.
The ab initio dipole moment components were determined in (1).