Bjerrum postulate

A particularly important question involves the understanding of the role of crystal defects in the peculiar electrical behaviour of ice 4. Upon the application of an electric field, the solid becomes polarized by the thermally activated reorientation of the molecular dipoles. Niels Bjerrum postulated the existence of orientational defects, which represent local disruptions of the hydrogen-bond network of ice 4, to explain the microscopic origin of this phenomenon. 

Bjerrum s derivation of eq. (9) was based on the idea of an intermediate S and seemed simple and straightforward at first, but on second thoughts it appeared to have a fatal flaw. Bjerrum postulated (44) that an equilibrium is first established between the reactants A and B and a purely physical collision complex S (Stosskomplex) ... 

The existence of such complexes was first postulated by Bjerrum in 1908.705 There have been a number of studies in recent years. Several early members of the series of hydrolytic polymers formed on the addition of base to aqueous chromium(III) solutions have been isolated 706 purification was achieved by ion-exchange chromatography on Sephadex-SP C25 resin. The structures suggested for these complexes are illustrated (156-162). The structural unit (162) was held to be singularly important, both as a constituent of higher polymers and in the mechanism of dimerization. Equilibrium data for these complexes are summarized in Table 78. 

The interaction between the protein matrix and its nested coordination complexes is exemplary of outer sphere coordination (OSC). Recently, OSC has guided the design of both small molecules and metalloproteins to tune existing or imbue novel properties. While often cited to explain metalloprotein behavior, OSC is not typically treated in a descriptive manner. We will begin with Bjerrum s favored definition [1] of coordination in the second sphere (chosen from Werner s original postulation [2]) a complex with a fully occupied first sphere has residual affinity to attach groups. This mode of OSC is involved in supramo-lecular chemistry [3, 4], but does not quite suit our present discussion. 

For dissociation reactions in which the reverse, recombination step is slow, dissociation is virtually complete once the pair has separated to the contact distance. Under these conditions the magnitude of the interaction in the dissociated pair, in particular the screening of this interaction by ions, would not affect the dissociation rate. In terms of the Hammond postulate [22] and its extensions [23] an exothermic dissociation process would have its transition state close to the bound state, so that the equilibrium and recombination rate coefficients would change in parallel. In such a case one expects no salt effect on the dissociation reaction, in agreement with the classical picture of Bronsted and Bjerrum for kinetic salt effects [7]. 

The answer to the difficulty with Bjerrum s derivation came with the advent of transition-state theory and was first pointed out by R.P. Bell (48). Transition-state theory (36-38) lays emphasis on the activated complexes, which are species that correspond to the col or saddle-point in a potential-energy surface for a reaction. Unlike an ordinary reaction intermediate, such as the Stosskomplex S postulated by Bjerrum, an activated complex has reached a point of no return it is bound to pass into products and cannot revert to reactants. The process is thus represented as... 

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