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Notes: This paper reports three independent studies. In the first study, the infrared band shapes and relative intensities of gaseous thiirane-d4 (ethylene sulfide-d4, C2D4S), the Raman spectrum of liquid thiirane-d4, and infrared spectra of gaseous cis- and trans-1, 2-dideuteriothiirane, (CHD)2S, are reported for the first time. The vibrational spectra of C2H4S, C2D4S, and some bands of cis-(CHD)2S are assigned from the symmetry analysis, group frequencies, infrared band shapes, and Raman polarization data. The frequencies so assigned are used to derive a modified valence force field, (MVFF), which reproduces them well, allows the remaining fundamental frequencies of cis-(CHD)2S to be found, and allows the spectrum of trans-(CHD)2S to be assigned. The MVFF is then further refined to optimize the fit to the 46 assigned frequencies of the four molecules. Twenty four nonzero force constants fit the 46 frequencies with an average error of 0.4%. The assignment is thus well based and self-consistent. Inthe second study, ab initio SCF calculations of optimum geometry, vibrational frequencies, and IR intensities of thiirane, thiirene, and a number of isotopically substituted derivatives are reported for the 6-31G*, 3-21G, and STO 3G bases. The force constants of thiirane from the 6-31G* basis are in good agreement with those of the MVFF when allowance is made for the fact that some were constrained to zero in the MVFF. The potential energy distributions from the ab initio and normal coordinate calculations agree well, with the former confirming some defects in the latter. The 6-31G* force constants multiplied by 0.80 reproduced the 46 observed frequencies with an average error of 1.4%. For thiirene and isotopic derivatives, the 6-31G* IR spectra are in much better agreement with experiment than previous results with smaller bases. In particular, significantly higher frequency C–S stretches are predicted with the 6-31G* basis. Nevertheless, a few discrepancies remain between experiment and the 6-31G* SCF results. In the third study, vibrational frequencies and IR intensities of thiirene and isotopic derivatives were evaluated at the CISD level of theory using a standard DZP basis set. In the DZP-CISD thiirene spectrum, the B1 C–H out-of-plane bend and its position relative to the A1 C–S stretch differ significantly from the 6-31G* SCF results, giving confirmation of the experimental assignments. However, the same DZP-CISD force constants predict that two low-frequency bands of thiirene-d1 are assigned incorrectly. No other significant discrepancies between theory and experiment remain for the thiirene species.