Intermolecular Interaction between CO or CO₂ and Ethylene Oxide or Ethylene Sulfide in a Complex, Investigated by Fourier Transform Microwave Spectroscopy and Ab Initio Calculations

The rotational spectra of the CO-ethylene oxide (EO), CO-ethylene sulfide (ES), CO₂-EO, and CO₂-ES complexes were measured by Fourier transform microwave spectroscopy in the frequency region from 4 up to 31 GHz. The isotopologues with a single ¹³C atom in the EO or ES, ¹⁸O in the EO, ³⁴S in the ES, and ¹³C in the CO₂ moiety, respectively, were observed in natural abundance, and enriched samples, ¹³CO or C¹⁸O in the CO-EO or CO-ES complex and C¹⁸OO and C¹⁸O₂ in the CO₂-EO or CO₂-ES complex, were employed to record respective rotational transitions. The rotational spectra observed for the CO-EO, CO-ES, CO₂-EO, and CO₂-ES complexes were analyzed by using an asymmetric-rotor <i>S</i>-reduced Hamiltonian to determine rotational and centrifugal distortion constants. The <i>r</i>_s coordinates of the atoms in the four complexes, which were calculated from the observed rotational constants, led to a structure in which the CO or CO₂ moiety is located in a plane perpendicular to the EO or ES skeletal plane and bisecting the COC or CSC angle. We have also carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G­(<i>d</i>,<i>p</i>) and aug-cc-pVDZ using the Gaussian 09 package. The MP2/6-311++G­(<i>d</i>,<i>p</i>) calculations yield rotational constants in better agreement with the experimental values than with the other basis set; in other words, the molecular structures calculated using this basis set are close to those experimentally found for the ground state. The estimated dissociation energies of the complexes, including the zero-point vibrational energy corrections ΔZPV and the basis set superposition errors (BSSE) calculated with the counterpoise correction (CP), are in good agreement with the experimentally obtained binding energies <i>E</i>_B. We have applied an NBO analysis to the complexes to calculate the stabilization energy CT (=Δ<i>E</i>_σσ*), which we found are closely correlated with the binding energies <i>E</i>_B. We have thus achieved a consistent overview on the intermolecular interaction in the complexes under consideration. It is to be noted that the spectral intensities of the inner OC¹⁸O-EO and OC¹⁸O-ES complexes were larger by a factor of 2 than those of the outer ¹⁸OCO-EO/ES complexes. This observation was explained by the zero-point energy of the inner conformer being a little smaller than that of the outer one