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The solvatochromic parameters are intended for use in linear solvation energy relationships (or, in the case of solute/solute interactions, linear complexation energy relationships) of the generalized form XYZ = XYZ0 + s(x* + dS) + aa + 6/3 + WH + e{. This equation may be reduced to a more manageable form by a judicious choice of solvents and reactants or indicators. One-, two-, and three-parameter correlations involving different combinations of the above parameters and various types of physicochemical properties are demonstrated. A comprehensive and up-to-date collection of x*, a, and ß values is presented.The present paper demonstrates how the solvatochromic comparison method may be used to unravel, quantify, correlate and rationalize multiple interacting solvent effects on many types of physicochemical properties and reactivity parameters. A further purpose is to assemble in one convenient reference an up-to-date (as of Feb 1983) and comprehensive collection of the solvatochromic parameters x*, a, and ß. The x* scale is an index of solvent dipolarity/polarizability, which measures the ability of the solvent to stabilize a charge or a dipole by virtue of its dielectric effect.1'4 56789101112Values of x* for "select solvents", nonchlorinated nonprotonic aliphatic solvents with a single dominant bond dipole, have been shown to be generally proportional to molecular dipole moments.4 The a scale of solvent HBD (hydrogen-bond donor) acidities describes the ability of the solvent to donate a proton in a solvent-to-solute hydrogen bond.1,5-7 The ß scale of HBA (hydrogen-bond acceptor) basicities provides a measure of the solvent's ability to accept a proton (donate an electron pair) in a solute-to-solvent hydrogen bond.1,8-11 The ß scale has also been used to evaluate hydrogenbond-acceptor strengths of solid HBA bases dissolved in non-HBA solvents.Rather than being based on solvent effects on single indicators, as has been the case for most earlier solvent property scales,1,12 the solvatochromic parameters were (1)

The solvatochromic comparison method is outlined. Magnitudes of enhanced solvatochromic shifts in HBA (hydrogen-bond acceptor) solvents are determined for 4-nitroaniline (1) relative to [-(1-2)b_h2n] and for 4-nitrophenol (3) relative to 4-nitroanisole (4) [-(3-4) -ho]• Thevalues for the HBD (hydrogen-bond donor) substrates 1 and 3 in corresponding HBA solvents are shown to be proportional to one another, proportional to limiting 19F NMR shifts of hydrogen-bonded complexes of 4-fluorophenol with the same HBA's, and linear with log association constants of hydrogen-bonded complexes between 4-fluorophenol (pKhb) and phenol and the same HBA molecules. The LFE relationships are used to establish a /3-scale of solvent HBA basicities.

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28869.5 ml(20-50-fold excess) of alcohol solution and thermostating for the period of time required for the disappearance of the yellow color of Cr(V1). Sodium hydroxide (100 ml, 2.5 M) was then added with cooling to the mixture to neutralize the acetic acid. Completeness of neutralization was monitored with pH paper. The resulting aqueous solution was twice extracted with 100 ml of ether, and the combined ether extracts were washed with water and dried (MgS04) and evaporated to a volume of ca. 0.5 ml. The mixture was then analyzed on a Hewlett-Packard Model 700 gas chromatograph equipped with dual flame ionization detectors. In each case, only the alcohol starting material and normal oxidation product were detected.Determination of Activation Parameters. Once the rate constants at various temperatures had been determined, Arrhenius plots of the natural logarithm of the rate constant against the reciprocal of the absolute temperature were made. To aid in the accurate plotting of these data and their linear regression analysis, the University of Delaware Burroughs B-6700 computer was utilized. The appropriate program was written by Dr. John J. Stanulonis of these laboratories and made available to us. The program permitted the determination of the least-squares slope of the plot and its intercept, as well as statistical information which indicated the reliability of the data. The statistical equations were those suggested by B a~e r .~~ From the slope of the Arrhenius plot, the energy of activation may be determined, using the relationshipPlotting In k vs. Am. Chem. Soc., 95,5242 (1973).A most recent review is given in H. Taube, A&. "rg. Chem. Radiochem., 1, l(1959).See R. P. Bell. Chem. Soc. Rev., 3,513 (1974). for a recent general review of this subject: see also, J. R. Hulett, 0. Rev., Chem. Abstract: The solvatochromic comparison method is used to evaluate hydrogen-bonding contributions in HBD (hydrogen-bond donor) solvents to several commonly used dye indicator solvent polarity scales (Dimroth's E~o , Brooker's XR. Kosower's Z ) . Hydrogen-bonding effects on other spectral properties, equilibria, and reaction rates are determined, and the results are used to construct an a-scale of solvent HBD acidities.X Y Z = X Y Z o + solvent polarity-polarizability effect Journal of the American Chemical Society / 98:lO / M a y 12, 1976

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