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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 119; no. 24; pp. 6475 - 6482
• Main Authors: Bährle, Christian, Nick, Thomas U, Bennati, Marina, Jeschke, Gunnar, Vogel, Frédéric
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18.06.2015
• Subjects: Benzoquinones - chemistry; Electron Spin Resonance Spectroscopy - methods; Lignin - analysis; Lignin - chemistry; Models, Molecular; Protons; Wood - chemistry
• DOI: 10.1021/acs.jpca.5b02200
• ISSN: 1089-5639

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 g xx , g yy , and g zz 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.