Three-site exchange

Fig. 8. Schematic representations of NMR spectra with three-site exchange.

The NMR characteristics of the bromonium ion triflate prove to be interesting in that the proton NMR spectrum of a 1 1 mixture of parent olefin and bromonium ion at room temperature is an average of those of the two isolated species. This suggests that there is a rapid site exchange of the Br+ between the ion and olefin. The phenomenon can be most simply studied by NMR spectroscopy. An approximately 100 mM solution of the ion in CD2C12 held at - 80°C was treated with small aliquots of Ad=Ad, the NMR spectrum was monitored after each addition. At zero added Ad=Ad, the spectrum of the ion consists of a seven line pattern indicative of a species (eqn. 3, 5a) having two perpendicular planes of symmetry that pass, respectively, through the three heavy atoms of the bromonium... 

At temperatures around 50-60°C the three-site jump model is not a good approximation to the multi-site jump model, because the motion is not sufficiently rapid to be in the fast motion limit. However, the calculated spectra are fairly fitted with the observed ones. This is because the calculated spectrum is a superposition of constituent spectra whose rates are spread over several orders, so that the resultant spectrum is governed by the constituent spectra in the fast and slow motion limits having greater intensity than that in the intermediate exchange regime. 

If one or more ligands L in laige excess interact with a metal ion in a metal complex CM in the presence of free metal ions M in solution, then the exchange of ligand L among three sites should be considered. A typical case is when L is a coordinating solvent molecule, e.g. water. The molar fraction of water nuclei is given by... 

One should remember that, for multi-site exchange among n centres, there are only (n — 1) independent populations. Owing to the condition of detailed balancing, any of the four parameters p, pj, Ky, and Kj, can be expressed as a function of the remaining three. Thus the number of independent parameters which describe the equilibrium and the kinetics of a given system does not exceed n(n + l)/2 — 1. 

Unlike kaolinite and montmorillonite, there are several sites for ion exchange reactions to occur on mica and vermiculite (Bolt et al., 1963 Sparks and Jardine, 1984). Bolt et al. (1963) studied potassium exchange on mica and proposed three sites for reactivity. Slow reactions were ascribed to interlattice exchange sites, rapid reactions to external planar sites, and intermediate reactions to readily exposed edge sites. Sawhney (1966) found two distinct reaction rates for cesium exchange on a Ca-vermiculite. The first reaction was ascribed to a rapid exchange of cesium with cations on external planar surface sites and interlattice edges, followed by a second, slow reaction in which cesium diffuses into the interlayers. 

By D-NMR spectroscopy, up to three hindered ligand movements were detected in [Cr(CO)2L(tj4 CH-diene)] complexes, a situation comparable to [Mn(CO)3(t/3 CH-cyclohexenyl)] complexes (46-51). Only small activation barriers are found for exchange processes of the C—H—Cr bridge. Higher barriers are observed for the site exchange of the three monodentate ligands. Similar energies are necessary to activate 1,5-H shifts (Table VI). 

Fig. 27. NMR shift of the H/metal signal on EuroPt-1 (peak fi in Fig. 12) for different coverages (a) or under different pressures (b). Compare the knee in Fig. 27a with that in Fig. 26b. The lines are a fit to a three-site rapid-exchange model. At low coverage, only one site on the metal is occupied (horizontal part in Fig. 27a). At intermediate values there are two metal sites with different shifts (curved part in Fig. 27a). and at high pressure there is a third contribution from hydrogen in the gas phase (right-hand part of F ig. 27b). [Reproduced with permission from Chesters el al (48). Copyright 1996 Royal Society of C hcmistry.j...

In the absence of a dissociative process, the remaining mechanistic options are only intramolecular nondissociative of the pseudorotation family. The nondissociative site-exchange reactions commonly discussed for octahedral complexes are the Ray-Dutt [10] and Bailar twist mechanisms [11], by which three ligands are rotated relative to the other three via a trigonal prism transition state. None of these processes effects a topomerization in one step. At least two consecutive such twists are necessary in order to bring about the observed topomerizations. However, with these mechanisms it would be difficult to rationalize the observation of two distinct barriers in 1-4. 

In view of the very low capacity of the resin used, it may seem remarkable that slopes so close to the theoretical values were obtained. One might expect the ion-exchange sites to be so scattered that it would be impossible for a 3-i- cation to exchange with three sites. However, other work has demonstrated that the exchange sites are in a thin layer on the outer perimeter of the ion-exchange bed. The results obtained here indicate that the concentration of sites in this outer layer is sufficiently dense that essentially theoretical exchange is obtained for polyvalent metal ions. 

Figure 21.15 The calculation ofand p in a three-site PASADENA experiment as a function of the self-exchange rate kgs for an AB system for different production and decay rates (a) completely regio-specific,...

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