Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin. Issue 6 (15th February 2022)

• Record Type: Journal Article
• Authors: Hernandez‐Castillo, A. O.; Calabrese, Camilla; Fritz, Sean M.; Uriarte, Iciar; Cocinero, Emilio J.; Zwier, Timothy S.
• Is Part Of: Chemphyschem. Volume 23:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-15
• Subjects: tunneling -- resonance structures -- internal rotation -- bond length -- high resolution; Chemistry, Physical and theoretical -- Periodicals; 541.05
• DOI: 10.1002/cphc.202100808
• Languages: English
• ISSNs: 1439-4235

Abstract

Abstract: 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 13 C 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 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. Abstract : Detailed structural characterization of a series of model aromatic substituents of lignin is carried out in this high‐resolution gas phase rotational spectroscopy study. Clear bond length alternation and tunneling dynamics are reported in this experimental work.