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 s pectroscopy. In all three systems, the halogen atom approaches the SO3 on o r 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(8 5) Angstrom for the HF, HCl, and HBr complexes, respectively, and in all th ree 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 c omplex gives an average angle of 47.7 degrees with respect to the vibration ally averaged C-3 axis of the complex. In the HCl and HBr complexes, howeve r, the HX unit is nearly parallel to the SO3 plane. In HCl-SO3, the HCl for ms 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 d irection of the tilt (toward or away from the SO3) is not determined. Altho ugh the hydrogen halides react with SO3 in bulk to produce halosulfuric aci ds, these gas-phase complexes are much like weakly bound dimers. (C) 1998 A cademic Press.