Hydrogen bond donating ability of meta and parahydroxy phenoxyl radicals

Abstract

H-bond complexes between 3- or 4-OH phenoxyl radicals and various H-bond accepting molecules were investigated by experimental and computational methods. The H-bond donating ability (α₂^H) of 2,6-di-tert-butyl-4-hydroxyphenoxyl radical (1) was determined as 0.79 ± 0.05 by measuring, using EPR spectroscopy, the variations of the hyperfine splitting constants of 1 as a function of the acceptor concentrations. A computational approach, based on DFT calculations, was employed to estimate the α₂^H values for OH-substituted phenoxyl radicals that were not persistent enough to be studied by EPR spectroscopy. The α₂^H value calculated for the 2,6-di-methyl analogue of 1 was 0.76, in good agreement with EPR experiments. The α₂^H values for 2-methoxy-4-hydroxy (3), 4-hydroxy (4), 4,6-di-methyl-3-hydroxy (5) and 3-hydroxy (6) phenoxyl radicals were computed as 0.77, 0.84, 0.66 and 0.71, respectively, indicating that α₂^H values were dependent on the presence of electron donating substituents and on the relative positions of the –OH and –O˙ groups. By correlating the α₂^H values for 4 and 6 with their water and gas-phase acidities, an unexpected role of water in promoting proton dissociation from these radicals was evidenced.