Simulation And Experimental Verification Of Lignin Raman Spectra

Lignin,the second most abundant natural polymer in nature,is widely found in the xylem of various land plants,and the use of lignin Raman spectroscopy for rapid and nondestructive identification of plant species is of great application.However,due to the very complex conformation of lignin macromolecules with an excessive number of atoms and many polymerization sites,it has been difficult to simulate the study of its Raman characteristic spectra.In this paper,based on the density general function theory,we carried out a study about the Raman spectra of lignin,and proposed a method to analyze the Raman spectral features of large-molecule lignin by using the structure of small-molecule lignin,and realized to explain the complex large-molecule lignin spectral features by simulated spectra of lignin monomer.The main content of this paper is divided into two parts:theoretical simulation calculation and experimental validation:Theoretical calculation part:In this paper,the basic configurations of three lignin monomers were firstly calculated,while structural optimization and analysis of their molecular structures and front-line orbital information were carried out,frequency calculations were performed to obtain the Raman spectra of lignin monomers,and the vibrational attribution of the characteristic peaks of the Raman spectra of the three lignin monomers was analyzed.Subsequently,the spectral peak characteristics of theβ-O-4 dimer with different collocation combinations were used to explain the causes of the drift of the signal positions of the characteristic peaks in the Raman spectra of lignin dimers,and the main basis for judging the signal characteristics of lignin macromolecules was proposed.The results show that the spectral peak signals at 2850 cm^-1,1649 cm^-1,1581 cm^-1 and 1302cm^-1 can be used as signal features to identify lignin.The characteristic peak near 1302cm^-1is affected by both monomer type and polymerization mode,and will form a signal envelope in the Raman spectra of lignin macromolecules,which can be used as an auxiliary criterion to identify lignin.Experimental validation part:In this paper,lignin samples and sodium lignosulfonate samples were measured by using ordinary Raman spectrometer and laser micro Raman spectrometer.The results show that there is no effective Raman signal due to the fluorescence of lignin and sodium lignosulfonate samples,while the measurement results of laser micro Raman spectroscopy are in good agreement with the theoretical simulation results,showing that the lignin samples have obvious Raman signals at 2850 cm^-1,1649 cm^-1,1418 cm^-1,etc.,confirming the simulation conclusions of 2850 cm^-1 and 1649 cm^-1.The characteristic peaks at 2850 cm^-1 and 1649 cm^-1 can be used as the Raman signal to identify lignin.The experimental results provide some data support for the feasibility of the simulation method in this paper,and prove the validity of the simulation method to study the Raman spectra of large-molecule lignin with small-molecule lignin monomer in this paper.This study has some guiding significance for the simulation study of Raman spectra of large-molecule lignin,and has some reference value for the simulation of detecting the spectral characteristics of complex molecules and their physical mechanisms,and also can provide theoretical guidance for the Raman spectral analysis methods of plant examination materials.