Substituent Effects on13C Chemical Shifts of Aromatic Carbons in β-O-4 and β-5 type Lignin Model Compounds

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Abstract
Substituent effects on the chemical shifts of aromatic carbons in lignin model compounds have been elucidated from 13C 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 13C 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.