Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G), syringol (2,6-dimethoxy phenol, S), 4-methyl guaiacol (MG), and 4-vinyl guaiacol (VG), under jet-cooled conditions in the gas phase. Using a combination of C-13 isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the r(C(1)-C(6)) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at r(C(1)-C(2)), r(C(3)-C(4)) and r(C(5)-C(6)). Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm(-1), a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH...OCH3 H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V-3=67 cm(-1). Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.