Dimethyl Sulfide–Dimethyl Ether and Ethylene Oxide–Ethylene Sulfide Complexes Investigated by Fourier Transform Microwave Spectroscopy and Ab Initio Calculation

The ground-state rotational spectra of the dimethyl sulfide–dimethyl ether (DMS–DME) and the ethylene oxide–ethylene sulfide (EO–ES) complexes were observed by Fourier transform microwave spectroscopy, and <i>a</i>-type and <i>c</i>-type transitions were assigned for the normal, ³⁴S, and three ¹³C species of the DMS–DME and <i>a</i>-type and <i>b</i>-type transitions for the normal, ³⁴S, and two ¹³C species of the EO–ES complexes. The transition frequencies measured for both the complexes were analyzed by using an <i>S</i>-reduced asymmetric-top rotational Hamiltonian. The rotational parameters thus derived for the DMS–DME were found to be consistent with a structure of <i>C</i>_<i>s</i> symmetry with the DMS bound to the DME by two C–H­(DMS)···O and one S···H–C­(DME) hydrogen bonds. Some high-<i>K</i>_a lines were found to be split, and we have interpreted these splittings in terms of internal rotations of the two methyl groups of the DMS and of the “free”, i.e., outer group, of the DME. Some forbidden transitions were also observed in cases where <i>K</i>_a = 3 levels were involved, for the DMS–DME complex in the internal-rotation <i>E</i> state. The barrier height, <i>V</i>₃, to internal rotation of the CH₃ in the DME thus derived is smaller than that of the DME monomer, while the <i>V</i>₃ of the CH₃ groups in the DMS is nearly the same as that of the DMS monomer. For the EO–ES complex, the observed data were interpreted in terms of an antiparallel structure of <i>C</i>_<i>s</i> symmetry with the EO bound to the ES by two C–H­(ES)···O and two S···H–C­(EO) hydrogen bonds. An attempt was also made to observe <i>a</i>-type transitions of the DMS dimer without success. We have applied a natural bond orbital analysis to the DMS–DME and EO–ES to calculate the stabilization energy CT (= Δ<i>E</i>_σσ*), which was correlated closely with the binding energy as found for other related complexes