High-Field Electron ParamagneticResonance and DensityFunctional Theory Study of Stable Organic Radicals in Lignin: Influenceof the Extraction Process, Botanical Origin, and Protonation Reactionson the Radical gTensor.

The radical concentrations and gfactors of stableorganic radicals in different lignin preparations were determinedby X-band EPR at 9 GHz. We observed that the gfactorsof these radicals are largely determined by the extraction processand not by the botanical origin of the lignin. The parameter mostlyinfluencing the gfactor is the pH value during ligninextraction. This effect was studied in depth using high-field EPRspectroscopy at 263 GHz. We were able to determine the gxx, gyy, and gzzcomponentsof the gtensor of the stable organic radicals in lignin.With the enhanced resolution of high-field EPR, distinct radical speciescould be found in this complex polymer. The radical species are assignedto substituted o-semiquinone radicals and can existin different protonation states SH3+, SH2, SH1-, and S2-. The proposed model structuresare supported by DFT calculations. The gprincipalvalues of the proposed structure were all in reasonable agreementwith the experiments. [ABSTRACT FROM AUTHOR]