Shielding in Aqueous Alkali Halide Solutions

The difficulties, described above, in accounting for alkali and halide ion shielding in the simple situation of the ion at infinite dilution may look discouraging for the use of ion shielding data to probe into the interactions of the ions with other species in more complex systems. Indeed, quantitative interpretations of the changes in halide ion chemical shift with changes in solution composition are difficult and considerations in the literature have mainly been confined to 

Even though, as is apparent from Table 6.1, there is considerable controversy regarding the values of the shielding of aqueous halide ions relative to the free ions, it is clear that the effect of ion-water interactions is sizeable. Consequently, we must when we consider chemical shift changes in aqueous solution also take into account modifications of the ion-water contribution in addition to effects due to other solutes. For many systems it appears that these two terms are of the same order of magnitude. The most thorough discussions of halide ion shielding in solution have been presented by Deverell [246], by Deverell and Richards [250] and by Halliday et al, [575] and we will here mainly follow their treatment. 

Here r is the expectation value of r for an outer p-electron of the halide ion and the A s are appropriate sums of squares of overlap integrals between the outer p orbitals of the halide ion (X ) and the orbitals of other halide ions, cations (M ) and water molecules (W). denotes the infinite dilution situation and is constant 

The first to report halide ion chemical shifts in aqueous alkali 

From Figs. 6.1-6.3 it can be seen that plots of chemical shift versus concentration are appreciably non-linear in many cases. Halliday et al, [373] have made a detailed analysis of the shape of these curves. They assumed then that the concentration dependence of the shift lies mainly in the term of Eq. (6.4). This term is expected 

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C. Deverell, Nuclear Magnetic Shielding of Hydrated Alkali and Halide Ions in Aqueous Solution, Mol. Phys., 16 (1969), 491-500. 

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