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High field EPR and DFT study of stable organic radicals in lignin: Influence of the extraction process, botanical origin and protonation reactions on the radical g-tensor.

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Nick,  T. U.
Research Group of Electron Paramagnetic Resonance, MPI for biophysical chemistry, Max Planck Society;

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Bennati,  M.
Research Group of Electron Paramagnetic Resonance, MPI for biophysical chemistry, Max Planck Society;

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Bährle, C., Nick, T. U., Bennati, M., Jeschke, G., & Vogel, F. (2015). High field EPR and DFT study of stable organic radicals in lignin: Influence of the extraction process, botanical origin and protonation reactions on the radical g-tensor. Journal of Physical Chemistry A, 119(24), 6475-6482. doi:10.1021/acs.jpca.5b02200.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0027-12DE-7
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 factor 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 component 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.