THE APT PULSE SEQUENCE IN C-13 NMR-STUDIES OF LIGNIN STRUCTURE

The aim of the present work was to demonstrate on two acetylated lignins (dichloromethane extract of industrial kraft lignin and the remaining lignin residue) that the APT pulse sequence (Patt and Shoolery 1982) is a useful technique in C-13-NMR studies of lignin structure. Quantitative C-quat and C-all APT spectra with suppressed NOE were measured using the evolution delays tau-chosen with respect to the average heteronuclear coupling constant (1J(CH) = 143 Hz) found in models representing the basic lignin structures.The error caused by differences between the real and chosen coupling constants was estimated as less than 5%. The adequacy of measuring conditions was confirmed by negligible signal residues and by a comparison of APT spectra with "normal" inverse-gated decoupled spectrum. In quantitative evaluation, the signals of methoxyl and acetoxyl groups served as internal standards for transformation of spectral data to common basis. The following lignin structural characteristics were thus determined: content of acetoxyl groups (phenolic, primary and secondary), quaternary aromatic carbons, sp2-hybridized CH-carbons, aryl ethers, and aromatic carbons not bound to oxygen. The aromatic rings of the lignin extract were found to have higher content of hydroxyl groups, to be less condensed and, surprisingly, to participate in more aryl ether bonds than those of the lignin residue.