Temperature Dependence. Various Kinds of Ion-Pairs

In contrast, the reactivities of ion-pairs vary enormously, especially on changing the solvent. For example, the propagation constant of sodium polystyryl ion-pairs at ambient temperature increases from about 5 M 1s 1 in dioxane to about 5,000 M 1s 1 in dimethoxyethane, raising the question of whether the label ion-pair is sufficient to describe these entities. 

The problem of different kinds of ion-pairs was clearly recognized in 1954 in the publications by Fuoss317) and by Winstein318 . The approaches of these two independent- 

In many systems the alternative forms discussed here correspond to two relatively deep minima in the appropriate potential energy hyper-surface. Thence, it is justified to treat these two kinds of pairs as two thermodynamically distinct species that are in equilibrium with each other. Each of these species possesses specific properties, like reactivities, which could be vastly different288 323.  

Alternatively, in other systems the minima in the potential energy hyper-surface may be shallow and temperature dependent. In such a case, the concept of two distinct kinds of ion-pairs loses its usefulness319,324.  

More complex systems are also known and then it is necessary to differentiate between more than two kinds of pairs. For example, in a solvent containing a small amount of a powerful solvating agent, X, three kinds of ion-pairs were observed319,320) the ordinary tight pairs surrounded by solvent molecules, pairs externally associated with X, and pairs separated by X. Such systems are governed by the equilibria  

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