Among the four possible isomers of monothioformic acid, trans-HC(O)SH and cis-HC(O)SH are stable, whereas trans-HC(S)OH and cis-HC(S)OH are not. As for formic acid, the trans isomer is lower than the cis isomer. The experimental lines were reported by
(1) W. H. Hocking and G. Winnewisser, 1976, Z. Naturforsch. 31a, 422.
Most transition frequencies were deemed to be accurate to 10 kHz; few frequencies with comparatively large residuals were omitted from the fit. Predictions with calculated uncertainties larger than 200 kHz should be viewed with caution. The data set is probably extensive enough for astronomical observations.
The dipole moment and the energy difference were determined in
(2) W. H. Hocking and G. Winnewisser, 1976, Z. Naturforsch. 31a, 995.
The energy difference of 231.3 cm^–1 and the separating barrier are high enough that the two isomers may exist as seperate molecules at low to moderately high temperatures (below 100 K). The partition functions were therefore determined independently.