Experimental and theoretical study of IR spectra of lignins

Abstract

Vibration spectra of hardwood lignin samples (G-S type of lignin) were investigated by IR-Fourier spectroscopy and quantum-chemical calculations. The density functional (DFT/B3LYP) method with 6-31G(d,p) as basis set was applied to calculate IR spectrum of dimmer model of hardwood lignin consisting of two aromatic rings of siringyl type. These calculations showed that frequencies of normal vibrations of this dimmer with β-alkyl-aryl linkage are close to the experimental values of the IR absorption bands of dioxan lignin, and infrared spectrum of this model is similar to the experimental spectrum of hardwood lignin in the frequency range of 900–1600 cm−1. Comparison of theoretical spectrum of dimmer with experimental spectrum of dioxan lignin allowed to carry out more detail interpretation of the most intensive bands of absorbance of aromatic rings of siringyl type in experimental lignin spectra which they use to evaluate lignin structure. It was shown that the intensive bands at 1125, 1215, 1328, 1505 and 1600 cm−1 are connected with planar vibrations of siringyl rings, changing O-C bonds in methoxyl groups and deformation of CCH and COH angles of aromatic rings and aliphatic chains. All these vibrations are strongly localized on one ring and its closest environment.