Fred Y. Fujiwara

Proton NMR Studies of Bihalide Ion Equilibria in Solution

In aprotic solvents, bihalide ions may be formed by adding the tetra‐n‐butylammonium halide to a solution of the hydrogen halide. The shieldings of the bihalides are obtained by analysis of the salt concentration dependence of the proton shieldings, which are averaged by rapid exchange. The deduced shieldings are insensitive to temperature and the nature of the solvent, and, relative to internal TMS, are: ClHCl−, −13.9 ppm; BrHBr−, −10.2 ppm; IHI−, −0.9 ppm. These represent deshieldings of about 13 ppm in each case, relative to the corresponding solvated hydrogen halide. The complete and rapid equilibration of all protonated species precludes characterization of the heterobihalide ions: addition of Cl− or Br− to another hydrogen halide results in quantitative formation, in excess salt, of the homobihalide ion ClHCl− or BrHBr−. However, ClHI− and BrHI− may be formed in solutions with excess iodide. The equilibrium constants are in the order consistent with a simple electrostatic model of ion‐molecule assoc...

Proton Shielding in the Bihalide Ions

The absolute proton shieldings of the bihalide ions, FHF−, ClHCl−, BrHBr−, and IHI−, are consistently about 14 ppm less than those in the corresponding gas‐phase hydrogen halide molecules. It can be shown that in both species, the dominant contributions to the shielding are all proportional to the hydrogen charge density. Thus, the charge density in each ion must be less than that in the corresponding molecule, consistent with SCF calculations on the bifluoride ion. This is the charge shift predicted by a simple electrostatic model, but not by the independent‐electron MO model.