Ab initio studies on the molecular conformation of lignin model compounds I.: Conformational preferences of the phenolic hydroxyl and methoxy groups in guaiacol

The conformational preferences of the lignin guaiacyl structural unit were studied at the MP2/6-311G(d, p) level of theory using guaiacol (2- methoxyphenol) as model compound. The potential energy surface of guaiacol was investigated by the ab initio method with full geometry optimization by varying the torsion angles of the guaiacol functional groups ( hydroxyl and methoxy). An overall of nine stationary points were located, four of which were found to be minima and the other five transition structures between them. The energy minima of guaiacol can adopt one cisoid and three transoid conformations for the hydroxyl and methoxy groups. The transoid structures differ by the orientation of the methoxy group inside and outside of the aromatic plane. The most stable cisoid conformer has an intramolecular hydrogen bond between phenolic hydrogen and methoxy oxygen with a binding energy of 18.09 - 18.51 kJ/mol as calculated with the second- order (MP2) and fourth-order (MP4SDQ) Moller- Plesset methods and with larger polarized basis sets including diffuse functions. When comparing the geometrical parameters of the global energy structure with relevant experimental data from crystallographic structures good agreement between the data was found. The saddle points, the effect of calculation level on the energy relative stability, the rotational barrier heights, and the relative concentrations of the conformers are also discussed.