Journal
JOURNAL OF PHYSICAL CHEMISTRY A
Volume 119, Issue 24, Pages 6475-6482Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpca.5b02200
Keywords
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Funding
- Swiss National Science Foundation (SNF), NRP66 Resource Wood [406640136892]
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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.
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