Solvation spectra. Part 51.—Di-t-butyl nitroxide as a probe for studying water and aqueous solutions

Abstract

Dilute aqueous solutions of di-t-butyl nitroxide have been studied by e.s.r. spectroscopy, and changes in the ¹⁴N hyperfine coupling and the line-widths have been monitored as a function of the temperature and of the concentrations of a wide range of added solutes.

The ¹⁴N hyperfine coupling is strongly dependent upon hydrogen-bonding, reaching a maximum in cold water, and low values in aprotic media. The alcohols give intermediate values, because of the presence of hydrogen bonded and non-hydrogen bonded radicals. Trends in A(¹⁴N) as basic solvents are added to aqueous solutions show a preference for the non-hydrogen bonded form, especially for strong bases such as hexamethylphosphoramide. These changes are interpreted in terms of a competition for water molecules, rather than preferential solvation of the nitroxide by the aprotic media. At 25°C or above, the widths of all three components increase equally as non-aqueous solvent is added. We conclude that spin-rotational relaxation and changes in A(¹H) are the width-controlling phenomena. Estimates of the latter were obtained using the perdeuterated nitroxide and computer simulations. The width increases correlate linearly with the changes in A(¹⁴N) which suggests that it is mainly the “free” nitroxide that contributes to the width enhancements. Values of τJ, the spin-rotational correlation time have been calculated in selected cases.

Differential line-broadening is observed for dilute aqueous solutions at low temperatures. This effect is greatly enhanced by certain additives such as t-butyl alcohol and triethylenediamine, which are thought to be water structure enhancers. Two possible causes are a reduced tumbling rate, with a consequent broadening resulting from the anisotropy of the g- and A-tensors, and a slowing down of the equilibrium between hydrogen-bonded and “free” nitroxide. The control exerted by viscosity is shown by the very large asymmetric broadening caused by added glycerol at 0°C.

Added methanol causes a slow fall in A(¹⁴N) to the value in pure methanol, with a concomitant line-width enhancement, again linear with A(¹⁴N). This is interpreted in terms of an increase in the concentration of “free” nitroxide, which results primarily because non-hydrogen bonded lone-pair electrons of methanol compete effectively with the nitroxide for hydrogen bonds. At low temperatures, added t-butyl alcohol initially has no effect on A(¹⁴N), but beyond about 0.05 M.F. (mole fraction), A(¹⁴N) falls very rapidly to the value for nitroxide in the pure alcohol. However, at 70°C, this initial plateau in the 0 to 0.05 M.F. region is absent. Again, the widths of the lines follow exactly the complex pattern of changes in A(¹⁴N).

These results are compared with those recently obtained by Jolicoeur and Friedman for similar systems.