The Rotational Spectrum of Monothioformic Acid IV. cis- and trans-H13C(:O)SH and HC(:18O)SH

The microwave and millimeter wave spectra of two isotopically substituted species of monothioformic acid, H13COSH and HC18OSH. have been investigated. Over 60 rotational transitions in the ground vibrational state have been assigned and measured for both the eis and trans thiol rotamers. HC(:O)SH. of each isotopic species. Rotational constants and quartic centrifugal distortion comstants of H13COSH and HC18OSH have been determined from the observed spectra. These data have been combined with previously published results on HCOSH, DCOSH. HCOSD and HCO34SH to obtain complete substitutions structures for the planar eis and trans rotamers of HC(:O)SH. All of the in-plane bond angles as well as the SH distance are significantly different for the two rotamers. For the eis rotamer the HSC and HCS angles increase by more than 2°, the OCS angle decreases by more than 3°, and the SH bond shortens compared to the trans rotamer geometry. The eis structure is: r(C-H) = 1.104 ± 0.003 Å, r(C = O) 1.203 ± 0.003 Å. r(C-S) = 1.771 ± 0.003 Å, r(S-H) = 1.335 ± 0.002 Å, ∢(OCS) = 122.5° ± 0.3°, ∢(HSC) = 94.9° ± 0.2° and ∢(HCS) = 114.4° ± 0.2°; whereas the trans structure is: r(C-S) = 1.104 ± 0.003 Å. r(C = O) = 1.205 ± 0.003 Å, r(C-S) = 1.768 ± 0.003 Å, r(S-H) = 1.354 ± 0.002 Å. ∢(OCS) = 125.9° ± 0.3°, (HSC) = 92.5° ± 0.2° and ∢(HCS) = 111.0° ± 0.2°