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

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 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.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…The p-isomers also possess a very small barrier, because the molecules are electronically and structurally symmetric. This trend also occurs in the cresol derivatives carvacrol (6), thymol (7) [147], and creosol (8) [148], as well as in 3-methylphenylacetylene (13) [153]. Note that the torsional barrier of the methyl group in the methoxy moiety of methylanisole and those of the two methyl groups in the isopropyl moiety of carvacrol and thymol are high.…”

“…Although some chemical properties such as vapor pressure, color, smell, and acidity are similar, the effects of methyl internal rotation, which often depend on the steric and electronic surroundings, are completely different for these isomers, as shown in Figure 5. 1) The prototype toluene [137][138][139][140][141][142], (2) o-cresol [143], (3) m-cresol [144], (4) p-cresol [145], (5) p-thiocresol [146], (6) anti-carvacrol (two conformers, A and B) [147], (7) syn-thymol [147], (8) creosol [148], (9) o-methyl anisole [127], (10) m-methyl anisole [149,150], (11) p-methylanisole [151], ( 12) mephenesin (two conformers, A and B) [152], (13) 3-methylphenylacetylene [153].…”

“…Barriers to methyl internal rotation in toluene and cresol derivatives (in cm −1 ). (1) The prototype toluene[137][138][139][140][141][142], (2) o-cresol[143], (3) m-cresol[144], (4) p-cresol[145], (5) p-thiocresol[146], (6) anti-carvacrol (two conformers, A and B)[147], (7) syn-thymol[147], (8) creosol[148], (9) o-methyl anisole[127], (10) m-methyl anisole[149,150],(11) p-methylanisole[151], (12) mephenesin (two conformers, A and B)[152], (13) 3-methylphenylacetylene[153].…”

Stereo-Recognition of Hydrogen Bond and Its Implications for Lignin Biomimetic Synthesis

“…It occurred within the plane of the ring and locked the guaiacyl moiety into a planar heavy-atom geometry. The literature provided many cases of this pattern with similar geometrical parameters with exceptions appearing only when other protic hydrogen acceptor solvents such as DMSO competed for hydrogen bonding. − To prevent such strong intermolecular H-bonds through interaction with DMSO-d 6 from influencing the NMR results, the poor H-bond-accepting solvent CDCl 3 in this work was therefore used.…”