Microwave spectrum and structure of the SO3−SO2 weakly bound complex

Abstract

The microwave spectrum and structure of the weakly bound complex SO3–SO2 are reported. The parent species, as well as the isotopologues with 34S and 33S single substitution on each sulfur atom have been observed. Together with the measured 33S nuclear quadrupole coupling constants, the structure of the complex has been determined. MP2/6-311++G(3df) calculations identify two nearly isoenergetic conformations, one of which corresponds to the observed structure. In the observed complex, an oxygen atom of the SO2 approaches the sulfur of the SO3 with an intermolecular S⋯O distance of 2.635(28) Å. The experimental S⋯S distance is 3.7517(83) Å, and the SO2 is positioned so as to direct an oxygen lone electron pair roughly along the C3 axis of the SO3. This structure reflects the strong acidity of SO3 insofar as it is suggestive of incipient electron pair donation from the SO2 oxygen to the acidic site of the SO3.