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 C-3 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) Angstrom for the HF, HCl, and HBr complexes, respectively, and in all three systems the out-of-plane distortion of the SO3 is negligible. Tn HF-SO3, the hydrogen points away from the SO3 and hyperfine structure in the DF complex gives an average angle of 47.7 degrees with respect to the vibrationally averaged C-3 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 degrees angle with the average C-3 axis of the complex, with the proton tilting slightly toward the SO3. In HBr-SO3, the average orientation of the HBr is 73.0 degrees 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. (C) 1998 Academic Press.