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

Abstract The complexes of SO 3 with HF, HCl, and HBr have been studied by microwave spectroscopy. In all three systems, the halogen atom approaches the SO 3 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) A for the HF, HCl, and HBr complexes, respectively, and in all three systems the out-of-plane distortion of the SO 3 is negligible. In HF–SO 3 , the hydrogen points away from the SO 3 and hyperfine structure in the DF complex gives an average angle of 47.7° 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 SO 3 plane. In HCl–SO 3 , the HCl forms a 72.8° angle with the average C 3 axis of the complex, with the proton tilting slightly toward the SO 3 . In HBr–SO 3 , 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 SO 3 ) is not determined. Although the hydrogen halides react with SO 3 in bulk to produce halosulfuric acids, these gas-phase complexes are much like weakly bound dimers.