LSER Case Histories in the Field of Tautomerism

LSER has been applied to tautomerism in two main ways. The first Hes in the elucidation of the solute s molecular properties, as revealed by equations of type (Eq. (11.12)). As LSER coefficients are difference terms, which relate to change in structure, they are rarely simple molecular parameters but often provide important relative information, chiefly through the sign and magnitude of their coefficients, which can help to explain the factors behind tautomeric change. These indications will be explored in detail. Its second main use consists in accurate extrapolation to tautomer ratio in solvents not part of the experimental data set This has special applicability to water, since many important compounds cannot be examined in aqueous solution through lack of analytical techniques that are sufficiently sensitive at the very low concentrations that would have to be employed. Hence all equations 

Enol Formation from -Diketones and Related Compounds 

Most simple ketones, in water, are favored over their enol tautomers by factors in excess of 10, so that there is never likely to be enough of the latter to allow LSER analysis. One solution lies in the photofission technique [26] that has enabled these compounds to be routinely studied, but otherwise some special boost to enol stability is required. Among these is acylation to give a P-diketone, which, on enolization, leads to a conjugated system that, if sterically favorable, results in a chelate whose intramolecular hydrogen bond adds greatly to its stability [27]. 

There results the apparent paradox the generation of a conjugated system leads to a diminishing proportion of this as solvent polarity rises, the opposite behavior to that of their nonchelated analogs. And while the very large p term for 11 testifies to the strong interaction between OH and a proton acceptor solvent, OH in the chelates is trapped and no such interaction can occur. 

While possessing a somewhat different aim, an important paper by Abraham and Leo requires mention [31]. These authors set out to discover whether, when a solute exists as two interconverting tautomers, the global partition coefficient obs 

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LSER Case Histories in the Field of Tautomerism 293 Table 11.6 log(( for the tautomerization of some Schiff bases and azo compounds . 

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