Theoretical and Raman spectroscopic studies of phenolic lignin model monomers.

Structural analysis of plant materials supports the growing interest in their use for chemicals, for example, biofuels. Lignin is a main polymer component formed from three phenolic precursors containing none, one or two OMe groups, i.e. H, G and S units, respectively. Raman spectroscopy offers structural information on lignin. This relies on correct assignment of observations to fundamental vibrations, and today this subject is not without controversy. The present work shows the strength of first principles assignment of lignin model bands. Raman spectra of three H, G, and S phenolic end group models are compared with density functional theory predictions of their vibrational properties. H, G, and S marker bands are found and related to specific vibrations. For the S unit, multiple OMe conformations exist that may all contribute to its Raman spectrum. Two ring deformation modes at approximately 1600 cm(-1) offer a potential route of gaining information on the microenvironment.