1:1 Hydrogen bonded complexes of phenol and 4-fluorophenol with 2,2,6,6-tetramethylpiperidin-1-oxyl: an electron spin resonance study
The hydrogen donor molecules phenol and 4-fluorophenol form 1:1 complexes with 2,2,6,6-tetramethylpiperidin-1-oxyl in solution in carbon tetrachloride. The formation constants of both complexes have been obtained over a range of temperatures by measuring aN, the 14N coupling constant of the nitroxide, as a function of donor concentration. The enthalpies of formation were found to be –19.2 ± 0.7 and –23.6 ± 0.4 kJ mol–1 for the phenol and 4-fluorophenol complexes, respectively. The value for phenol is in acceptable agreement with the calorimetric measurements of Lim and Drago. The temperature coefficients of aC, the nitrogen coupling constants for the complexed radicals, are explained in terms of asymmetric double-minima potentials having energy separations between the minima of 8.8 ± 1.3 and 6.5 ± 0.9 kJ mol–1 for the phenol and 4-fluorophenol complexes.