Abstract
To study structure of softwood lignins the experimental and theoretical IR-spectra in middle IR-diapason were analyzed. To interpret these data the quantum chemical calculations of IR-spectra of general dimmer fragments of softwood lignins by method of density functional theory (DFT/B3LYP) with 6-31G(d,p) as basis set were carried out. These calculations showed that frequencies of normal vibrations of fragment with β-alkyl-aryl linkage are close to the experimental values of the IR absorption bands of lignin, and infrared spectrum of this structure is similar to the experimental spectrum of lignin. The calculations with accounting for the solvent showed a strong increase in the intensity of the majority of the bands and the solvent effect on the frequencies of vibrations.
Highlights
To study structure of softwood lignins the experimental and theoretical IR-spectra in middle IR-diapason were analyzed
At the same time it is known that lignins consist of phenylpropane units of guaiacyl, syringyl and p-coumaryl types [2]
The calculation results showed that the frequencies of normal vibrations of the dimmer with E-O-4 linkage between aromatic rings are close to the experimental values of the infrared absorption bands of lignin, and the calculated infrared spectrum is similar to the experimental spectrum of lignin
Summary
To study structure of softwood lignins the experimental and theoretical IR-spectra in middle IR-diapason were analyzed. Lignin is mixture of irregular aromatic polymers of close structure. By quantitative analysis of IR-spectra and construction of calibration dependencies similar “structure-properties” the tasks of finding the most informative absorbance bands to characterize chemical structure of native lignins are quite successfully solved [3,4,5,6,7].
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