Hydrogen bond donating ability of meta and para hydroxy phenoxyl radicals

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 (alpha(H)(2)) 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 alpha(H)(2) values for OH-substituted phenoxyl radicals that were not persistent enough to be studied by EPR spectroscopy. The alpha(H)(2) value calculated for the 2,6-di-methyl analogue of 1 was 0.76, in good agreement with EPR experiments. The alpha(H)(2) 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 alpha(H)(2) values were dependent on the presence of electron donating substituents and on the relative positions of the -OH and -O-center dot groups. By correlating the alpha(H)(2) 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.