Dynamic complex exchange

Compensarion temperature, 199,271,272 for reversed-phaM chromatography, 273 for solubility of hydrocabons in water, 273 Complex exchange with displacement of hetaeron, 254 see also Dynamic complex exchange... 

The third possible mechanism, Ilia, proposed here involves a transfer of solute from the complex formed in the mobile phase to the bound hetaeron and complex formation at the surface. The corresponding equilibrium for this metathetical process, which may be called dynamic complex exchange," can be written as... 

The third mechanism, proposed by Melander and Horvath (1980b), is that of dynamic complex exchange where counter-ions are present both in the mobile phase and at the surface of the stationary phase. Sample molecules which have formed an ion-pair with the counter-ion in the mobile phase can then transfer to the bound counter-ions to form a complex at the surface of the stationary phase. A modification of this process envisages that the complex is initially formed at the stationary phase surface. 

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

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