The Effect of Solvent on Spectra. I. A New Empirical Measure of Solvent Polarity: Z-Values

The position of the charge-transfer (c.-t.) absorption band of 1-alkylpyridinium iodide complexes is remarkably sensitive to the nature of the solvent in which it is measured. The transition energies derived from the band positions are linear in the Winstein-Grunwald Y-value (a kinetic measure of solvent ionizing power) over a wide range of methanol-water, ethanol-water and acetone-water mixtures. The c.-t. bands can be measured in solvents for which it is not possible or convenient to seek a Y-value. It is proposed that the transition energies, in kcal./mole, be adopted as empirical measures of solvent polarity, and that they be called Z-values. The complex formed from 1-ethyl-4-carbomethoxypyridinium iodide was selected as a standard, and pyridine-1-oxide was used to extend the Z-value scale to isooctane, in which the salt is insoluble. As an initial check on the validity of the Z-values, the transition energies for the c.-t. bands of the 1-ethyl-4-cyanopyridinium iodide complex in six solvents have been found to be linear in Z. A theoretical analysis of the data based on a reasonable model for the 1-alkylpyridinium iodide complex provides the free energy of solution of the complex dipole. The difference in free energy of solution of the 1-ethyl-4-carbomethoxypyridinium iodide complex dipole between isoöctane and water is 17.3 kcal./mole. Using the apparent absorption coefficient as a measure of the effect of the solvent on the association of the pyridinium and iodide ions into complex, it is found that certain solvents (dimethylformamide, dimethyl sulfoxide) are dissociating, and others are associating (acetic acid), in comparison with “normal” solvents (chloroform, alcohols, etc.).