title: High-Field Electron Paramagnetic Resonance and Density Functional Theory Study of Stable Organic Radicals in Lignin: Influence of the Extraction Process, Botanical Origin, and Protonation Reactions on the Radical g Tensor.
abstract: The radical concentrations and g factors of stable organic radicals in different lignin preparations were determined by X-band EPR at 9 GHz. We observed that the g factors of these radicals are largely determined by the extraction process and not by the botanical origin of the lignin. The parameter mostly influencing the g factor is the pH value during lignin extraction. This effect was studied in depth using high-field EPR spectroscopy at 263 GHz. We were able to determine the gxx, gyy, and gzz components of the g tensor of the stable organic radicals in lignin. With the enhanced resolution of high-field EPR, distinct radical species could be found in this complex polymer. The radical species are assigned to substituted o-semiquinone radicals and can exist in different protonation states SH3+, SH2, SH1-, and S2-. The proposed model structures are supported by DFT calculations. The g principal values of the proposed structure were all in reasonable agreement with the experiments.
authors.affiliation: †Paul Scherrer Institut, Research Department General Energy, 5232 Villigen PSI, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Bährle
authors.affiliation: ‡Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
authors.forename: Thomas U
authors.initials: TU
authors.lastname: Nick
authors.affiliation: ‡Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.
authors.forename: Marina
authors.initials: M
authors.lastname: Bennati
authors.affiliation: §EPR Research Group, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland.
authors.forename: Gunnar
authors.initials: G
authors.lastname: Jeschke
authors.affiliation: †Paul Scherrer Institut, Research Department General Energy, 5232 Villigen PSI, Switzerland.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Vogel
mesh_terms: D016227:Benzoquinones / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D008031:Lignin / Q000032:analysis / Q000737:chemistry*; D008958:Models, Molecular; D011522:Protons; D014934:Wood / Q000737:chemistry
chemical_list: D016227:Benzoquinones; D011522:Protons; C025232:semiquinone radicals; D008031:Lignin