Characterization of solvent mixtures: preferential solvation of chemical probes in binary solvent mixtures of polar hydrogen-bond acceptor solvents with polychlorinated co-solvents

The use of chemical probes for the characterization of chemical properties is explored for aprotic binary solvent mixtures. The solvatochromic indicat ors N,N-diethyl-4-nitroaniline, 4-nitroanisole, 4-nitroaniline and 4-nitrop henol were used to characterize binary solvent mixtures of a polar aprotic hydrogen-bond acceptor solvent (ethyl acetate, acetonitrile and dimethyl su lfoxide) with a polychlorinated hydrogen-bond donor solvent (chloroform or dichloromethane). The solvent parameters pi*, alpha and beta of the binary mixtures were calculated from the solvatochromic shifts of the indicators. In each case the degree of convergence for a solvent property values obtain ed from different probes was analyzed. Data obtained by using the non-polar solvatochromic indicator beta-carotene are additionally presented. The beh avior of the solvent systems was analyzed according to their deviation from ideality due to preferential solvation of the solutes and the complicated intermolecular interactions of the two components of the solvent mixture. T he validity of the concept of an intrinsic absolute property of a solvent m ixture and whether such a property can be defined by means of chemical prob es is discussed. Theoretical equations (preferential solvation models) were used to compute the solvatochromic data. The results were analyzed and rel ated to the solvent effects on some aromatic nucleophilic substitution reac tions, comparing the application of single- and multiparametric treatments of solvent effects. Copyright (C) 1999 John Wiley & Sons, Ltd.