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.
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