The Role of Noncovalent Interactions in the Infrared Spectra of Lignin Model Compounds: A DFT Study

Noncovalent interactions are key to the stability of lignin dimers, the primary components of bio-oil. However, their specific influence on the infrared spectra remains poorly understood. Using dispersion-corrected density-functional theory (APFD/6-311++G(d,p)), we conducted a comparative analysis of the structures and infrared spectra of four lignin derivatives—benzene, phenol, anisole, and guaiacol—as monomers and dimers. Our study reveals that distinct vibrational shifts and newly emerging peaks observed in the calculated infrared spectra of the dimers can be attributed to the formation of σ···π and π···π stacking, hydrogen bonding, attractive van der Waals, and steric repulsion interactions. We extended the study to 3,3′-dimethoxy-1,1′-biphenyl-2,2′-diol, a guaiacyl moiety with a C–C linkage. The results indicate that the structure in which both guaiacol units adopt an anti–syn conformation is more stable—minimizing steric repulsion between hydroxyl groups—than configurations in which one guaiacol unit adopts either anti–anti or gauche–anti conformation. These differences are clearly reflected in their distinctive infrared spectral signatures.