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In order to investigate the delignification mechanism during high-boiling solvent (HBS) pulping, softwood HBS lignin samples from todo fir ( Abies sachalinensis ) were analyzed by  1 H– 13 C correlation NMR spectroscopy (HMQC). Nine lignin model compounds that may represent the substructures in HBS lignin were prepared. The  1 H and  13 C NMR spectral data were compared with HBS lignin samples. The HMQC spectra of HBS lignin samples showed the disappearance of  β - O -4 structures. Introduction of a pulping solvent into lignin was plainly observed with new signals in aliphatic regions. Only the  erythro β -1 structure was detected in softwood milled wood lignin from todo fir by HMQC spectra. The  β -1 structure was very reactive and probably produced the stilbene structure. The  β - β  and  β -5 structures remained in HBS lignin. The 4- O -5 structure was successfully detected by HMQC spectra, and it was found to be stable under the conditions used.

Delignification mechanism during high-boiling solvent pulping. Part 6: Changes in lignin structure analyzed by 1H-13C correlation 2-D NMR spectroscopy