19F nuclear magnetic resonance spectroscopy for the quantitative detection and classification of carbonyl groups in lignins

A novel method that permits the quantitative detection and classification of various carbonyl groups in lignins has been developed. The proposed method was optimized with the quantitative trifluoromethylation of a series of carbonyl-containing lignin-like model compounds. This effort was followed by 19F NMR spectral analyses of the resulting fluorine derivatives allowing for a thorough understanding of their structure/19F chemical shift relationships. The various carbonyl groups present in lignins were also investigated by trifluoromethylating them in the presence of catalytic amounts of tetramethylammonium fluoride (TMAF), followed by hydrolysis with TMAF in tetrahydrofuran. By using a variety of selective reactions, it became possible to assign a number of prominent 19F NMR signals to a variety of carbonyl groups present in lignins. These studies demonstrated that the proposed method can be applied to the quantitative determination of carbonyl groups that are present in soluble native and technical lignins.