Spectroscopic Study of Lignin Monomer Analogs: 2,3-dimethoxyphenol and 1,3,5-trimethoxybenzene

Lignin is the second most abundant biopolymer on our planet. It supports the structure of the plant to withstand the environmental conditions and protect the sugars inside the plant that can be potentially used as biofuels. The composition of lignin varies from plant to plant, with main monomer units as substituted phenols, guaiacols and syringols providing rigidity of the plant walls. Previous research focused on five monomer analogs, and current research is studying two more compounds related to lignin – 2,3-dimethoxyphenol and 1,3,5-trimethoxybenzene. Molecules are collisionally cooled in a supersonic expansion down to temperatures within a few degrees of absolute zero. Laser Induced Fluorescence (LIF) and Dispersed Fluorescence (DFL) spectra have been recorded for both compounds. LIF spectra are indicative of a large geometry change and Franck-Condon activity in the excited state. Microwave spectroscopy experiments were performed on the compounds to determine rotational constants and the direction of the dipole moment. These can be used to test the planarity of the lowest energy conformers of the molecules. Finally, we performed a fluorescence-dip Infrared Spectroscopy experiment on the compounds to observe the interaction of hydroxyl- and methoxy- substituents on the pi-cloud of the phenyl rings. Theoretical calculations were completed on both compounds using various exchange methods and basis sets. Hartree-Fock (HF) method and Density Functional Theory (DFT) were used as exchange methods, and 6-31G*, 6-31+G* and 6-311++G** were the studied basis sets. DFT/6-311++G** was proven to be the most accurate way to represent the energies of the ground and excited states. Calculations on bright excited states were performed to obtain adiabatic excitation energies and geometries of the states