19F Nuclear Magnetic Resonance Spectroscopy for the Elucidation of Carbonyl Groups in Lignins. 1. Model Compounds

Abstract

A new method for the detection of different classes of carbonyl groups in a series of carbonyl-containing lignin-like model compounds has been developed. The method is based on the selective fluoride-induced trifluoromethylation of carbonyl groups with (trifluoromethyl)trimethylsilane (TMS-CF3) in the presence of tetramethylammonium fluoride (TMAF), followed by hydrolysis with aqueous HF or TMAF in the case of quinones. In this study a series of ketones, aldehydes, quinones, and dimeric-lignin model compounds were quantitatively trifluoromethylated followed by 19F NMR spectral analyses of the resulting fluorine-containing derivatives, allowing for a thorough understanding of their structure/19F chemical shift relationships. These studies have shown that the 19F-NMR chemical shifts of the trifluoromethyl groups vary significantly and consistently for various classes of carbonyl groups which may be present in complex lignocellulosic materials. These studies are to form the basis for the development of a novel and sensitive method that can be used to obtain quantitative information on the various carbonyl groups present in such materials. Keywords: Carbonyl groups; model compounds; Ruppert's reagent; trifluoromethylation; nuclear magnetic resonance (NMR); chemical shifts