Substituent effects on carbon-13 chemical shifts of aromatic carbons in β-O-4 and β-5 type lignin model compounds.


Substituent effects on the chemical shifts of aromatic carbons in lignin model compounds have been elucidated from C NMR spectra of guaiacyl and syringyl type monomerio and β-O-4 model compounds and guaiacyl type β-5 model compounds. Evaluation of the observed values of substituent chemical shift (SCS) for the aromatic carbons leads to elucidation of a generalized SCS additivity rule, for estimation of the chemical shifts of aromatic carbons in ring A of β-O-4 and β-5 type substructures in model compounds and in ring B of β-O-4 substructures in lignin preparations, with errors of less than 1 ppm. The rule is applicable to substructures of both guaiacyl and syringyl types, using an appropriate parent compound as reference instead of benzene. Signals in the aromatic region of the C NMR spectra of β-O-4 and β-5 type model compounds are reassigned on the basis of the observed SCS’s as well as APT spectra of the compounds.

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