The structure of halonium ions in superacidic solutions
Isotopic perturbation of equilibrium was applied to 1,2-bridged halonium ions to determine whether they exist as single symmetric structures or as a rapid equilibrium of asymmetric structures. The observed deuterium isotope shifts are qualitatively and quantitatively consistent with the presence of intrinsic and equilibrium isotope shifts. The presence of equilibrium shifts suggests that these ions exist as a rapid equilibrium of asymmetric structures. Though the asymmetric structures were initially ascribed to β-halocarbenium ions, subsequent computational data suggest that 1,2-bridged halonium ions react with sulfur dioxide (SO2), the experimental solvent. Our current hypothesis is that the equilibrium isotope shifts result from rapid labile addition of SO2 to the halonium ions. Other hypotheses have been invoked to explain the results and are considered in the context of the available data.