Hydrogen-bonding studies of nitroxides with phenols. Effect of donor acidity

Abstract

Electron spin resonance spectroscopy has been used to study the 1:1 hydrogen bonded complexes formed between 2,2,6,6-tetramethylpiperidin-1-oxyl and five phenols in solution in carbon tetrachloride. The free energies and enthalpies of formation of the complexes both show a linear dependence on the free energies of ionization of the phenols. In contrast, the entropies of formation are constant throughout the series and have the value –50 ± 6 J mol^–1 K^–1. Shifts in the g-factor have been measured for three of the complexes and indicate a shift in spin density from the oxygen to the nitrogen atom. This is in accord with the increase in a^N on complexation.