Identification of Side-Chain Structures in a Poplar Lignin Using Three-Dimensional HMQC−HOHAHA NMR Spectroscopy

The structure of an acetylated 13 C-enriched poplar wood lignin preparation was studied using three-dimensional HMQC−HOHAHA NMR spectroscopy. This method takes advantage of the large dispersion of 13 C chemical shifts to resolve individual 1 H chemical shifts. The whole spin system of a 1 H− 13 C correlation observed in an HMQC spectrum, even for minor components and unknown structural units, can be traced out. It is shown here that both trans- and cis-isomers of 6,7-dihydrodibenzo( e, g )(1,4)-dioxocin, a recently discovered prominent linkage in softwood lignins, occur also in hardwood lignin. Moreover, the poplar lignin preparation contains small amounts of noncyclic α-aryl ether linkages. Signals from a side chain of unknown structure are tentatively assigned to a spiro-cyclohexadienone.