Computational approach in lignin structural models: Influence of non-covalent intramolecular interactions on βO4 bond properties

Lignin is a natural macromolecule that, in general, is formed by the aromatic units S, G and H depending on its origin. From its structure it is possible to obtain molecules with high added value, however the processes face difficulties in the need to break the bond that unites these aromatic units, the βO4 bond. This work, through computational models via DFT, QTAIM and NCI, investigated the main intramolecular interactions and their properties related to this bond. It was possible to observe that lignins with methoxyl linked to the second aromatic unit (SAU) perform interactions that are totally different from models that do not have them. For some specific conformations, hydrogen bonds between the gamma hydroxyl and the SAU methoxyl were characterized and it was found that these are responsible for making the βO4 bond stronger. However, the methoxylated models are dominated by repulsive and destabilizing interactions that make their βO4 bonds, in general, 4.41% weaker than the models formed by H units, which perform attractive and stabilizing dihydrogen interactions.