Shielding of Halide Ions

Introductory Aspects. Relaxation in Molecules or Ions with Covalently Bonded Halogens. Shielding Effects in Covalent Halogen Compounds. Scalar Spin Couplings. Relaxation of Chloride, Bromide and Iodide Ions. Shielding of Halide Ions. Quadru-pole Splittings in Liquid Crystals. Halide Ions in Biological Systems. Studies of the Perchlorate Ion. 

Rb NMR shieldings have been shown experimentally to depend linearly on the distance of the Rb" ion to its nearest neighbor shell of halide ions in the family of Rb-doped alkali halides, and theoretical calculations bear out the experimental linear trends. The shielding derivatives with respect to the ionic distance are positive and are very similar, largely independent of the identity of the halide ion for the distances that naturally occur in the alkali halide lattices. At much shorter distances, we would not expect this to be the case. 

It was first suggested by Itoh and Yamagata [247] that the same effects which cause shielding changes of halide ions in solution also may affect quadrupole relaxation. This possibility was investigated in detail by Deverell [245 246] and recently a systematic theoretical treatment... 

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... 

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. 

However, because they cannot form hydrogen bonds and because their positive centers are well shielded by steric effects from any interaction with anions, aprotic solvents do not solvate anions to any appreciable extent. In these solvents anions are unencumbered by a layer of solvent molecules and they are therefore poorly stabilized by solvation. These naked anions are highly reactive both as bases and nucleophiles. In DMSO, for example, the relative order of reactivity of halide ions is opposite to that in protic solvents, and it follows the same trend as their relative basicity ... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Boron, which occurs in the second period of the periodic system as a tervalent ion, is naturally very small (see Table F), and, in the compounds which it forms with the halogens, the boron ion will be completely shielded by the three halogen ions, so that the boron halides are thus compounds with low boiling points. Of the halogen ions, iodine, which contains the most electrons, is the largest. In aluminium iodide the three iodine ions will shield the aluminium ion, as also do the somewhat smaller bromine or chlorine ions in... 

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

If the attack were purely random, we would expect equal amounts of the two isomers that is to say, we would expect only the racemic modification. But the product is not completely racemized, for the inverted product exce ds its enantiomer. How do we account for this The simplest explanation is that attack by the nucleophilic reagent occurs before the departing halide ion has completely left the neighborhood of the carbonium ion to a certain extent the departing ion thus shields the front side of the ion from attack. As a result, back-side attack is somewhat preferred. 

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