Calculation of IR-spectra of structural fragments of lignins

Abstract

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 E-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.Lignins, being part of the plant raw, are valuable source of chemical compounds [1]. It is why heightened interest to decoding native lignin structure is saved. Lignin is not individual substance. Lignin is mixture of irregular aromatic polymers of close structure. Therefore the structural formula of lignin does not exist.At the same time it is known that lignins consist of phenylpropane units of guaiacyl, syringyl and p-coumaryl types [2]. Currently it is revealed more than 20 types of linkages between phenylpropane units. It is established that E-alkyl-aryl linkage is the most common one in native lignins.One of useful methods of lignin analysis is FTIR-spectroscopy. 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]. In order to increase the reliability of the interpretation of IR spectra there is a necessity of the comparative analysis of the experimental and theoretical data. To achieve this aim in the work the quantum-chemical calculations of IR spectra of dimmer models of softwood lignin were carried out by method of density functional theory with the B3LYP functional in the basis of 6-31 + G (d, p) using a software package Gaussian08. IR-spectra were calculated for two main structural elements of guaiacyl lignin.Figure shows the lignin experimental spectrum and the simulated IR spectra of the investigated dimmers in the form of the absorption curves in the frequency range 700 -1900 cm -1 . By comparison with experiment the frequencies of normal vibrations of the

Lignin-Based Gel Polymer Electrolyte for Cationic Conductivity

“…The experimental frequencies of the IR absorption bands of main vibrations in the range of 1800–700 cm − 1 are presented in the table, where their possible interpretation is given with allowance for the literature data [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ].…”