Solution state NMR of lignins

Abstract

Despite the rather random and heterogeneous nature of isolated lignins, many of their intimate structural details are revealed by diagnostic NMR experiments. 13C-NMR was recognized early-on as a high-resolution method for detailed structural characterization, aided by the almost exact agreement between chemical shifts of carbons in good low-molecular-mass model compounds and in the polymer (1, 2). Although this correspondence cannot generally be expected for proton NMR, lignin dimers and their counterpart units in the polymer also closely match proton NMR chemical shifts and coupling constants attesting to similar conformations of models and their polymeric counterparts (3-5). Lignin trimers provide slightly better data, and deliver revealing insight into the incredible stereochemical complexity of lignin oligomers and polymers and its impact on ligninsa NMR spectra (5). Despite the broad featureless ID proton spectra, which have nevertheless allowed substantive interpretation (6), 2D spectra both homo- and heteronuclear, are informationally rich and strikingly useful. With modem NMR methods, there is the potential to elucidate structures without first preparing model compounds, although such models are still crucial for most studies, and for obtaining additional evidence of the identity of new structural features in the polymer.

Citation

Ralph, John.; Marita, Jane M.; Ralph, Sally A.; Hatfield, Ronald D.; Lu, Fachuang.; Ede, Richard M.; Peng, Junpeng.; Landucci, Larry L. 1999. Solution state NMR of lignins. Advances in lignocellulosics characterization. Atlanta, Ga. : TAPPI Press,c1999.:p. 55-108 : ill.