1:1 Hydrogen bonded complexes of phenol and 4-fluorophenol with 2,2,6,6-tetramethylpiperidin-1-oxyl: an electron spin resonance study

Abstract

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 a^N, 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 a_C, 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.