Kamlet-Taft solvatochromic parameter approach, solvents

One such methodology is the Kamlet-Taft Solvatochromic parameter approach. In this methodology, a solvent can be characterized by three parameters, tt, a measure of the polarity and polarizability of the fluid, a, the acidity or hydrogen bond donor capability and P, the hydrogen bond acceptor capability or basicity. Each of these parameters is determined from the shift in UV-visible absorbance of a series of select indicator species dissolved in the solvent. Rather than depending on the bulk properties of the fluid, as is the case with the cohesive energy approaches, the solvatochromic parameters are derived from the interactions between the indicator solute and the immediate solvent shell, in effect they are a measure of how a solute sees the solvent. In each case, the scale of values has been normalized to between 0.0 for cyclohexane... 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

There have been other phenomenological approaches to rationalize (or even predict) the experimentally observed solvent effect on the chemical shift. Many chemists use the Kamlet-Abbout-Taft (KAT) set of solvatochromic parameters it, a and 8 [31], KAT parameters can be used together with the multiple linear analysis to describe the variation in the chemical shift of the solute as the solvent is varied. An extensive study of this type was conducted by Witanowski et al. to interpret the solvent effects on the shielding of l4N in a large set of compounds (see ref. [32] and references cited therein). For a nitroso aliphatic and aromatic series, solvent-induced shielding was indeed found to depend on the polarity of the solvent. However, other experience with this model suggests the need for caution. 

This approach to separating the different types of interactions contributing to a net solvent effect has elicited much interest. Tests of the ir, a, and p scales on other solvatochromic or related processes have been made, an alternative ir scale based on chemically different solvatochromic dyes has been proposed, and the contribution of solvent polarizability to it has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system constitutes the best or ultimate extrathermodynamic approach to the study of solvent effects. There are two objections One of these is to the averaging process by which many model phenomena are combined to yield a single best-fit value. We encountered this problem in Section 7.2 when we considered alternative definitions of the Hammett substituent constant, and similar comments apply here Reichardt has discussed this in the context of the Kamlet-Taft parameters. The second objection is to the claim of generality for the parameters and the correlation equation we will return to this controversy later. 

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