Infrared and Raman spectra of lignin substructures: Dibenzodioxocin

Vibrational spectroscopy is a very suitable tool for investigating the plant cell wall in situ with almost no sample preparation. The structural information of all different constituents is contained in a single spectrum. Interpretation therefore heavily relies on reference spectra and understanding of the vibrational behavior of the components under study. For the first time, we show infrared (IR) and Raman spectra of dibenzodioxocin (DBDO), an important lignin substructure. A detailed vibrational assignment of the molecule, based on quantum chemical computations, is given in the Supporting Information; the main results are found in the paper. Furthermore, we show IR and Raman spectra of synthetic guaiacyl lignin (dehydrogenation polymer—G‐DHP). Raman spectra of DBDO and G‐DHP both differ with respect to the excitation wavelength and therefore reveal different features of the substructure/polymer. This study confirms the idea previously put forward that Raman at 532 nm selectively probes end groups of lignin, whereas Raman at 785 nm and IR seem to represent the majority of lignin substructures. Lignin is a heterogeneous aromatic plant polymer and can be studied in situ in the plant cell wall using infrared (IR) and Raman microscopy. Analysis relies on the availability and understanding of reference spectra. IR and Raman spectra of dibenzodioxocin, a lignin substructure, are shown, leading to a new assignment of lignin. Results strengthen the idea that end groups can be highlighted using Raman at 532 nm, whereas the entire polymer can be studied using Raman at 785 nm and IR spectroscopy.