Knowledge Bank

The structures of $HF-SO3$ and $HCl-SO3$ have been determined by Fourier transform microwave spectroscopy. Both HF and HCL react with $SO3$ to form halosulfonic acids, $HSO3X(X=F.Cl)$, but the isomers observed correspond to more weakly bound minima on the potential energy surface. The halogen atom lies above the $SO3$ plane in both systems, with S-F and S-Cl distances of $2.68\backslash AA$ and $3.11\backslash AA$, respectively. The equilibrium position of the proton is most likely off-axis, but large amplitude vibrational averaging produces an effective symmetric top structure in both cases. The observed geometries are indicative of weak donor-acceptor type interactions, with out-of-plane distortions of the $SO3$ subunit of less than $1\circ$. The complexes are compared with those of $BF3$, whose Lewis acidity is generally regarded as less than that of $SO3$.