Solvent effects on the visible spectra of nitroxides and relation to nitrogen hyperfine splitting constants. Nonempirical polarity scales for aprotic and hydroxylic solvents

Abstract

The absorption spectra of nitroxides in the visible region are highly solvatochromic. In this paper we report some data on solvent effects on the band energy, ET, of 2,2,6,6-tetramethylpiperidinyl-1 -oxy (TEMPO) and 4-oxo-2,2,6,6-tetramethylpiperidinyl-l-oxy (OTEMPO). Special attention has been given to hydroxylic solvents and particularly to a series of reference solvents of graded hydrophilicity, dodecane, primary alcohols, ethanol-water and methanol-water mixtures, and water. The polarity index H, defined as the ratio of molar concentration of OH dipoles in these solvents to that in water (55.4 M), has been shown to exhibit a good linear correlation with their bulk dielectric constant, D. This relation has been examined in terms of theories of dielectric constants of associated liquids. ET values of both TEMPO and OTEMPO show excellent linear correlations with D and H in hydroxylic solvents including methanol-water and ethanol-water mixtures. The values also show good linear correlations with the nitrogen hyperfine splitting constants (AN) of di-tert-butyl nitroxide (DTBNO) and other similar molecules for both aprotic and hydroxylic solvents. A general theoretical approach for describing aprotic solvent effects has been explored. It is found that a reaction field treatment for estimating the effects of the solvent dielectric constant on the energy of the nitroxide dipole gives a reasonable description of An and ET data in many aprotic solvents. The role of the effective polarizability of the dipole has been investigated. It has been shown that for such systems the refractive index of the solvent plays a relatively minor role, and reasons have been adduced for this in terms of a general theory which indicates that in some other cases refractive index effects can be significant. For hydroxylic solvents equations applicable to aprotic solvents are unsatisfactory because of very pronounced hydrogen-bonding effects on ET and AN. It has been shown that corrections for general medium effects do not materially alter the linear relation of hydrogen-bonding effects to D and H. The hydrogen-bonding effects can be expressed in terms of proportionality to H. The general expressions combining equations for aprotic solvents with a term , where is a proportionality constant, are found to give good descriptions of AN and ET data in both aprotic and hydroxylic solvents. This approach to solvatochromic effects using nonempirical solvent parameters gives as good correlations as empirical polarity scales in representing such data. Preliminary studies indicate the usefulness of such treatments for other systems as well. The relationship of hydrogen-bonding effects to H and D have been analyzed by focusing attention on the electrostatic component of hydrogen-bonding interactions which is expected to be of primary importance. Such analyses have been shown to be consistent with some formal theoretical treatments. Their physical significance has been examined. A possibly important role of a solute-solvent correlation parameter similar ...

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