The conformational structures and dipole moments of ethyl sulfide in the gas phase

The pure rotational spectrum of ethyl sulfide has been measured from 12 to 21 GHz in a 1 K jet-cooled expansion using a Fourier-transform microwave (F TMW) spectrometer. Prominent features in the spectrum are assigned to trans itions from three conformational isomers. Additional assignments of the C-1 3 and S-34 isotopomer spectra of these conformers effectively account for a ll of the remaining transitions in the spectrum. Accurate "heavy-atom" subs titution structures are obtained via a Kraitchman analysis of 14 rotational parameter sets, permitting definitive identification of the molecular stru ctures of the three conformers. Two of the structures designated as the gau che-gauche (GG) and trans-trans (TT) conformers have symmetric forms with C -2 and C-2v symmetries, respectively, and the third trans-gauche (TG) confi guration is asymmetric. The components of the electric dipole moment along the principal inertial axes have been determined from Stark measurements an d are consistent with these structural assignments. Detailed comparisons ar e made with the calculated geometries, dipole moments, and energy-level ord ering at both the HF (Hartree-Fock)/6-31* and MP2 (second-order Moller-Ples set)/6-311** levels of theory. Significant discrepancies are found, which a re mainly attributed to errors in the calculated dihedral angles that defin e the different conformations. A graphical-user-interface computer program has aided in the identification and assignment of entangled hybrid-band spe ctra from the different conformers and isotopomers in this study. The progr am includes features that enable real-time refinement of rotational constan ts and hybrid band intensities through visual comparisons of the experiment al data with simulated spectra. Capacities also exist to rapidly assign qua ntum number labels for least-squares fitting purposes. (C) 2001 American In stitute of Physics.