Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin
[2];
[4];
[4];
[5]
- Purdue Univ., West Lafayette, IN (United States); Fritz Haber Institute of the Max Planck Society, Berlin (Germany)
- Univ. of Basque Country, Leioa (Spain); Fundación Biofísica Bizkaia/Biofisika Bizkaia Fundazioa, Leioa (Spain)_
- Purdue Univ., West Lafayette, IN (United States)
- Univ. of Basque Country, Leioa (Spain)
- Purdue Univ., West Lafayette, IN (United States); Sandia National Lab. (SNL-CA), Livermore, CA (United States)
In this report 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.
- Research Organization:
- Sandia National Lab. (SNL-CA), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division
- Grant/Contract Number:
- NA0003525; FG02-96ER14656
- OSTI ID:
- 1855798
- Report Number(s):
- SAND2022-2287J; 703733
- Journal Information:
- ChemPhysChem, Vol. 23, Issue 6; ISSN 1439-4235
- Publisher:
- ChemPubSoc EuropeCopyright Statement
- Country of Publication:
- United States
- Language:
- English
Similar Records
Aqueous-phase photoreactions of substituted benzaldehydes and acetophenones as sinks for organic compounds as a source of HOOH in aqueous aerosols, fogs, and clouds: Effects of pH and of ring substituents
Substituent effects on energetics and crystal morphology modulate singlet fission in 9,10-bis(phenylethynyl)anthracenes