The Functional Approach to Ionization

The Functional Approach to Ionization Phenomena in Solutions U. Mayer and V. Gutmann... 

The functional approach to chemical interactions provides a general pattern for the electron dynamics of all types of chemical interaction, as has been discussed in detail elsewhere (97). In this connection it may be desirable to mention briefly how a covalent bond is ionized by the nucleophilic attack of an EPD molecule (50) ... 

Now, it can be postulated that solvolysis rate should be a function of two properties of the solvent one is its ionizing power, and the other is its nucleo-philicity. An SnI process should be promoted by high ionizing power, and an Sn2 process by high solvent nucleophilicity. At this point, we are ready to bring the extrathermodynamic approach to bear on this problem. This was initiated by Grun-wald and Winstein, who defined a solvent ionizing power parameter Y by... 

Ionization reactions in homogeneous liquid phase usually proceed as reactions between closed-shell molecules. According to the functional approach, these reactions are regarded as EPD-EPA reactions in the strict sense of the word. Owing to the inherently high stability of closed-shell molecules or ions, these reactions usually lead to heterolytic bond cleavage. 

In cases where Koopmans approximation fails badly, the direct way of calculating ionization energies, the method of Green s functions, has been applied successfully, although we are not aware of its use in case of organometallic compounds. Applications of a simplified Green s function approach to transition metal complexes, however, have been reported The scattered-wave method , on the contrary, has wide applications in describing metal-metal bonds and the nature of multicentre interactions within metal-lacycles. ... 

It is possible to measure and compare the acidities of compounds that are not acidic enough to be ionized in water by using a more basic medium. Stewart and O Donnell established the H scale for solutions of tetraalkyl-ammonium hydroxide in sulfolane, DMSO-water, pyridine-water, and water. In essence, the H scale extends upward the range for which pfCg values can be measured (which is effectively from 0 to 14 in aqueous solution) by another 12 pK units of basicity. A number of workers have modified the Hammett approach to acidity functions. In particular, Cox and Yates proposed the excess acidity function, X, which represents the difference between the acidity observed for a system and the acidity that would be observed if the system were ideal. In this approach, the activity coefficient ratio in equation 7.27 is taken to be the product of a coefficient m times the activity coefficient ratio, X, for a hypothetical standard base, B. Here m is a constant characteristic of each base, while X is a constant characteristic of the med-... 

A number of approaches to predict ionization based on structure have been published (for a review, see [53]) and some of these are commercially available. Predictions tend to be good for structures with already known and measured functional groups. However, predictions can be poor for new innovative structures. Nevertheless, pfCa predictions can still be used to drive a project in the desired direction and the rank order of the compounds is often correct. More recently training algorithms have also become available which use in-house data to improve the predictions. This is obviously the way forward. 

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