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Article

Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol

1
Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstr. 6, 37077 Goettingen, Germany
2
School of Physical and Chemical Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
*
Author to whom correspondence should be addressed.
Academic Editor: Maria Eugenia Sanz
Molecules 2022, 27(1), 101; https://doi.org/10.3390/molecules27010101
Received: 10 November 2021 / Revised: 11 December 2021 / Accepted: 15 December 2021 / Published: 24 December 2021
Rotational microwave jet spectroscopy studies of the monoterpenol α-fenchol have so far failed to identify its second most stable torsional conformer, despite computational predictions that it is only very slightly higher in energy than the global minimum. Vibrational FTIR and Raman jet spectroscopy investigations reveal unusually complex OH and OD stretching spectra compared to other alcohols. Via modeling of the torsional states, observed spectral splittings are explained by delocalization of the hydroxy hydrogen atom through quantum tunneling between the two non-equivalent but accidentally near-degenerate conformers separated by a low and narrow barrier. The energy differences between the torsional states are determined to be only 16(1) and 7(1) cm1hc for the protiated and deuterated alcohol, respectively, which further shrink to 9(1) and 3(1) cm1hc upon OH or OD stretch excitation. Comparisons are made with the more strongly asymmetric monoterpenols borneol and isopinocampheol as well as with the symmetric, rapidly tunneling propargyl alcohol. In addition, the third—in contrast localized—torsional conformer and the most stable dimer are assigned for α-fenchol, as well as the two most stable dimers for propargyl alcohol. View Full-Text
Keywords: delocalization; tunneling; terpene; alcohol; vibrational spectroscopy; supersonic jet expansion; structural determination delocalization; tunneling; terpene; alcohol; vibrational spectroscopy; supersonic jet expansion; structural determination
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MDPI and ACS Style

Medel, R.; Springborn, J.R.; Crittenden, D.L.; Suhm, M.A. Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol. Molecules 2022, 27, 101. https://doi.org/10.3390/molecules27010101

AMA Style

Medel R, Springborn JR, Crittenden DL, Suhm MA. Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol. Molecules. 2022; 27(1):101. https://doi.org/10.3390/molecules27010101

Chicago/Turabian Style

Medel, Robert, Johann R. Springborn, Deborah L. Crittenden, and Martin A. Suhm. 2022. "Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol" Molecules 27, no. 1: 101. https://doi.org/10.3390/molecules27010101

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