Shielding of Halide Ions

Abstract

Experimental studies of halide ion shielding in solution are concerned with the determination of the chemical shift relative to some reference solution, which may be for example a dilute aqueous solution of the halide. The variation of the chemical shift with the composition of the solution is then interpreted in terms of interactions in the system. In this connection it is important to realize that there is a considerable chemical shift difference between the free ^*) ion and the hydrated ion and that a modification of this chemical shift contribution is often important. For a deeper understanding of the physical nature of the interactions that determine chemical shifts of halide ions in aqueous solutions it would be of greatest importance to have access to the “absolute” values of the shielding constants, σ_X- ,_aq or, equally good, values of the difference σ_X- ,_free — σ_X- ,_aq. As mentioned earlier in this text (cf. Section 3.1) absolute shielding constants of magnetic nuclei in covalent compounds may be calculated from molecular beam measurements of spin-rotation interactions. The diagonal elements of the spin rotation interaction tensor can be used to calculate the diagonal elements in the paramagnetic shielding tensor. Such studies have so far been carried out for a limited number of nuclei and among the halides absolute shielding values are only available for ¹⁹F.