Structures of the van der Waals Isomers of Halosulfuric Acids: Microwave Spectra of HX-SO₃ (X= F, Cl, Br)

The complexes of SO₃ with HF, HCl, and HBr have been studied by microwave spectroscopy. In all three systems, the halogen atom approaches the SO₃ on or near its C₃ axis, and the vibrationally averaged structure is that of a symmetric top. The S-X bond lengths are 2.655(10), 3.1328(57), and 3.2339(85) Å for the HF, HCl, and HBr complexes, respectively, and in all three systems the out-of-plane distortion of the SO₃ is negligible. In HF-SO₃, the hydrogen points away from the SO₃ and hyperfine structure in the DF complex gives an average angle of 47.7° with respect to the vibrationally averaged C₃ axis of the complex. In the HCl and HBr complexes, however, the HX unit is nearly parallel to the SO₃ plane. In HCl-SO₃, the HCl forms a 72.8° angle with the average C₃ axis of the complex, with the proton tilting slightly toward the SO₃. In HBr-SO₃, the average orientation of the HBr is 73.0° off the symmetry axis of the complex, but the direction of the tilt (toward or away from the SO₃) is not determined. Although the hydrogen halides react with SO₃ in bulk to produce halosulfuric acids, these gas-phase complexes are much like weakly bound dimers.