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Switching On/Off the Intramolecular Hydrogen Bonding of 2-Methoxyphenol Conformers: An NMR Study

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posted on 2024-07-11, 14:01 authored by Frederick Backler, Feng WangFeng Wang
Intramolecular hydrogen bonding of 2-methoxyphenol (2-MP, guaiacol) is studied using NMR spectroscopy combined with quantum mechanical density functional theory (DFT) calculations. The hydrogen bonding of OH⋯O and HO⋯H is switched on in the conformers of anti-syn (AS, 99.64 % dominance) and anti-gauche (AG), respectively, with respect to the anti-anti (AA) conformer (without either such hydrogen bonding interactions). It confirms that the 13C and 1H NMR chemical shift of AS dominates the measured NMR spectra, as the AS conformer reproduces the measurements in CDCl3 solvent (RMSD of 1.86 ppm for 13C NMR and of 0.27 ppm for 1H NMR). The chemical shift of hydroxyl H(1) at 5.66 pm is identified as the fingerprint of the OH(1)⋯OCH3 hydrogen bonding in AS, as it exhibits a significant deshielding from H(1) of AA (4.24 ppm) and H(1) of AG (4.38 ppm) without such OH(1)⋯OCH3 hydrogen bonding. The AG conformer (C1 point group symmetry) possesses a less strong hydrogen bonding of HO⋯HCH2O, with the methoxyl group out of the aromatic phenol plane. The substituent effect of AG due to the resonance interaction of methoxyl being out of plane in a concentrated solution shifts the ortho- and para-aromatic carbons, C(3)/C(5), of the AG to ∼125.05/125.44 ppm from the corresponding carbons in AS at 108.81/121.60 ppm. The hydrogen bonding exhibits inwards reduction of IR frequency regions of AS and AG from AA. Finally, energy decomposition analysis (EDA) indicates that there is a steric energy of 45.01 kcal mol-1 between the AS and AG when different intramolecular hydrogen bonding is switched on.

History

ISSN

1445-0038

Journal title

Australian Journal of Chemistry

Volume

73

Issue

3

Pagination

222-

Publisher

CSIRO Publishing

Copyright statement

Copyright © CSIRO 2020. The accepted manuscript is made available for open access with the permission of the publisher.

Language

eng

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