Radical sites in humic acids: A theoretical study on protocatechuic and gallic acids

Protocatechuic and gallic acids (PCA/GA) were used as models for studying the radical sites in humic acids. Neutral, anionic, and anion radical conformers were investigated by means of density functional theory in gas phase and solvent, respectively. For the most stable radical structures electron spin density and EPR parameters such as g-tensors and isotropic hyperfine coupling constants (HFCCs) were calculated using the B3LYP functional. Additionally, the performance of several other popular functionals of the DFT theory including BP86, B97D, M06, TPSS, TPSSh and B2PLYP was evaluated. Obtained g-tensors are strongly anisotropic having two principal components (g_xx, g_yy) lying in the plane of the aromatic ring. The difference between the g-tensors of PCA and GA anion radicals are minimal. Inclusion of environmental effects (explicit consideration of hydrogen bonds plus continuum solvation) has a significant impact on g-tensor components and HFC constants shifting their values closer to available experimental data. In summary, good agreement of the B3LYP results with experiment is obtained and from the set of the additional DFT functionals used only the hybrid variant of the TPSSh functional provides good results. The calculated values of g-tensors and HFCCs support the hypothesis that polyphenolic fragments are main sources of transient and native radical sites in structurally complex humic acids.