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

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.