Abstract
Broadband chirped-pulse 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 13C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the rC(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 rC(1)-C(2) , rC(3)-C(4) and rC(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 V3= 67 cm-1 . Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.
