ASSESSMENT OF LIGNIN MODEL QUALITY IN LIGNIN CHEMICAL-SHIFT ASSIGNMENTS - SUBSTITUENT AND SOLVENT EFFECTS

Typical solution-state C-13 NMR spectra of lignins obtained with relat ively modern instrumention contain well over 100 signals. The use of c lassical lignin model compounds is no longer sufficient for accurate i nterpretation of these spectra. Ultimately, the only valid models for etherified C-9 units in lignin (which account for about 80% of the lig nin polymer) must be at least trimeric or tetrameric, depending on the linkages involved. One approach that can be used involves NMR substit uent effects. This approach avoids tedious syntheses while retaining t he much-needed accuracy of chemical shift correlations between models and natural lignins. Appropriate substituent effects have been calcula ted by the use of an extensive NMR database of dimeric, trimeric and t etrameric lignin model compounds. Effects related to acetylation and s olvent have also been determined. For example, accurate chemical shift data for an underivatized milled-wood lignin (MWL) in DMSO-d(6) can b e determined on the basis of the chemical shifts of the corresponding acetylated MWL in acetone-d(6).