Infrared spectra of lignin model compounds and of lignins from Eucalyptus regnans

Abstract

Infrared spectra in the O-H and C=O stretching regions of lignin model compounds and of lignins isolated in various ways from Eucalyptus regnans F. Muell. have been obtained for samples both in the solid state and in solution. The effects of solvents on the stretching vibrations of free and intramolecularly bonded hydroxyl groups have been examined for a number of the model compounds, including some in which the groups are subject to steric hindrance. The results have been used to interpret the O-H stretching frequencies of the lignins in the same solvents. In the lignin macromolecules all hydroxyl groups appear to be involved in hydrogen bonds, and the phenolic groups are sterically hindered.    Different solvent effects have also been observed in model compounds between the C=O stretching frequencies of carboxyl groups and those of aldehyde, ketone, and ester groups. The solvent dependence of the 1665 cm^-1 band in the lignin spectra supports its assignment to a conjugated aldehyde or ketone group, but the dependence of the 1725 cm^-1 band suggests that both carboxyl and carbony1 groups may contribute to the band envelope.