Kamlet-Taft solvatochromic equation

Rates of esterification of 2-substituted nicotinic acids by diazodiphenylmethane in nine aprotic solvents at 303 K were correlated using the Kamlet-Taft solvatochromic equation. ... 

The Kamlet-Taft solvatochromic parameters )t a, and p for a micellar medium are usually determined by measuring the spectroscopic parameters of certain probes using known correlation equations. The correlation equations are, however, obtained from studies in pure solvents. It is assumed that the correlation equation obtained for pure solvents is also valid for micellar media. Both absorption [14-17] and fluorescence [18,90] probes have been used for estimating values of a, p, and it for the micellar media. 

Kamlet M. J., Abboud J.-L., Abraham M. H. and Taft R. W. (1983) Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, 7i, a, and ft, and Some Methods for Simplifying the Generalized Solvatochromic Equation, J. Org. Chem. 48, 2877-2887. 

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. 

Kamlet MJ, Abboud JLM, Abraham MH, Taft RW (1983) Linear solvation energy relationships. 23. A comprehensive collection of the solvatochromic parameters, jr, a, and /3, and some methods for simplifying the generalized solvatochromic equation. J Org Chem 48 2877-2887. 

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. 

The scale was proposed for solute HBD acidity of monomer amphihydrogen-bonding compounds acting as non-self-associated solutes [Taft, Abraham et al., 1985 Kamlet, Doherty et al, 1986a]. In particular, a, values were derived from logfC values for complexation with pyridine N-oxide in cyclohexane this set of values was successively extended through various back-calculations using the solvatochromic equation. 

A cmcial component of this treatment is the value of P°w for the distribution of the solute molecules between 1-octanol and water. This could be estimated from group contributions (Hansch and Leo 1979) but could also be related to other properties of the solutes, namely, their solvatochromic parameters according to Kamlet et al. (1984,1988). Their equation was tested for 245 solutes of all kinds with a correlation coefficient of 0.996 and a standard deviation of 0.13. Earlier, Kamlet, Taft and coworkers (Taft et al. 1985, Kamlet et al. 1986) applied the LSER concept directly to the solubility of non-electrolytes in water, independently of the octanol/water distribution. Subsequently, important differences between aliphatic and aromatic solutes were stressed (Kamlet et al. 1987). For liquid aliphatic solutes the expression ... 

The relative importance of the hafide anion - HO - Cell interactions can be inferred from application of the Taft-Kamlet-Abboud equation to the UV-Vis absorbance data of solvatochromic probes, dissolved in cellulose solutions in different solvent systems, including LiCl/DMAc and LiCl/N-methyl-2-pyrrolidinone [96]. According to this equation, the microscopic polarity measured by the indicator, Ej (indicator), in kcalmol is correlated with the properties of the solvents by Eq. 1 ... 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

The SA acidity scale was estabhshed by comparing the solvatochromism of the probes TBSB and DTBSB using flic method of Kamlet and Taft. In this method, the solvatochromism of DTBSB in a solvent is used as die reference for zero acidity. Consequently, non-acidic solvents obey flic equation... 

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