Measurement of Lewis Basicity

According to the glossary of terms used in physical organic chemistry published by the International Union of Pure and Applied Chemistry [71], Lewis basicity is defined as 

The thermodynamic tendency of a substance to act as a Lewis base. Comparative measures of this property are provided by the equilibrium constants for Lewis adduct formation for a series of Lewis bases with a common reference Lewis acid. 

the Lewis basicity of a series of bases B will be measured by the thermodynamic equilibrium constants K of the equilibria 

A Lewis basicity scale can be constructed from this set of equilibrium constants. The basicity scale may be expressed as K, or as pAi or as AG. In order that the definition and 

In this way, a Lewis base will be strong if it forms a complex with a large association constant, that is, a low dissociation constant, and a large positive value of pA bao will describe a strong Lewis base towards A°. 

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Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

The donor number (DN) as the measure of Lewis basicity [20] was defined as the negative of the molar enthalpy of formation for the adduct formed between the base in question (donor D) and reference Lewis acid SbClj (acceptor) in a 10 M solution of dichloroethane ... 

Jensen [9] indicated that many reported values are only approximate, as determined indirectly from linear correlation with other measures of Lewis basicity, rather than being directly measured in the laboratory. 

The Measurement of Lewis Basicity and Affinity in the Laboratory 419 7.5.3 Experiment... 

Among the uses of the correlations that have been established between spectroscopic and thermodynamic scales of basicity, the calculation of new thermodynamic values appears of primary importance in the context of the quantitative measurement of Lewis basicity. In fact, it is generally easier and faster to measure a spectroscopic shift than an equilibrium constant. This experiment illustrates the calculation of p Tbi, values for three pyridines. 

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