Hydrogen-bonding effects on the properties of phenoxyl radicals. An EPR, kinetic, and computational study

The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH(2) groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH(2) groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the persistency of the alpha-tocopheroxyl radical has been observed in the presence of HFP.