On the Reactivity of Lignin Models with Oxygen- Centered Radicals. I. Computations of Proton and Electron Affinities and O-h Bond Dissociation Energies

The electronic structures of three lignin model compounds were computed by means of semi-empirical (PM3) and density functional methods (B3LYP), both in vacuo and in solution. The computed values of proton affinities, electron affinities and O-H bond dissociation energies for 4-methylphenol, 2-methoxy-4methylphenol and 2,6-dimethoxy-4-methylphenol were compared with those of the oxygen-derived radicals, hydroxyl radical, superoxide radical anion, hydroperoxyl radical, oxyl radical anion and triplet oxygen. The data suggest that the hydroxyl radical is the strongest oxidizing and dehydrogenating reagent, followed by the oxyl radical anion and the hydroperoxyl radical. These three radicals could oxidize or dehydrogenate the lignin models at an appropriate pH. The superoxide radical anion and the triplet oxygen are somewhat weaker reagents. Thermodynamic calculations indicate that formation of aromatic peroxides is to be expected during peroxide bleaching and that the pKa of the phenolic hydroxyls significantly decreases when these peroxides are formed. The results obtained demonstrate the usefulness of molecular orbital calculations for elucidating the radical reactions in pulp bleaching, in the attempt to improve the selectivity of alkaline oxygen and peroxide delignification processes.