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

Abstract

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 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 transitionsmore » due to two conformers differing in the relative orientations of the vinyl and OH groups.« less

Authors:

Hernandez‐Castillo, A. O. ^[1]; Calabrese, Camilla  ^[2];

• Search DOE PAGES for author "Calabrese, Camilla"
• Search DOE PAGES for ORCID "0000-0003-4299-2098"

Fritz, Sean M. ^[3]; Uriarte, Iciar  ^[4];

• Search DOE PAGES for author "Uriarte, Iciar"
• Search DOE PAGES for ORCID "0000-0003-3726-1389"

Cocinero, Emilio J.  ^[4];

• Search DOE PAGES for author "Cocinero, Emilio J."
• Search DOE PAGES for ORCID "0000-0001-7632-3728"

Zwier, Timothy S.  ^[5]

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• Search DOE PAGES for ORCID "0000-0002-4468-5748"

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+ Show Author Affiliations

1. Purdue Univ., West Lafayette, IN (United States); Fritz Haber Institute of the Max Planck Society, Berlin (Germany)
2. Univ. of Basque Country, Leioa (Spain); Fundación Biofísica Bizkaia/Biofisika Bizkaia Fundazioa, Leioa (Spain)_
3. Purdue Univ., West Lafayette, IN (United States)
4. Univ. of Basque Country, Leioa (Spain)
5. Purdue Univ., West Lafayette, IN (United States); Sandia National Lab. (SNL-CA), Livermore, CA (United States)

Publication Date:

Mon Jan 31 00:00:00 EST 2022

Research Org.:

Sandia National Lab. (SNL-CA), Livermore, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division

OSTI Identifier:

1855798

Report Number(s):

SAND2022-2287J
Journal ID: ISSN 1439-4235; 703733

Grant/Contract Number:  

NA0003525; FG02-96ER14656

Resource Type:

Accepted Manuscript

Journal Name:

ChemPhysChem

Additional Journal Information:

Journal Volume: 23; Journal Issue: 6; Journal ID: ISSN 1439-4235

Publisher:

ChemPubSoc Europe

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY