Medel, R.; Springborn, J.R.; Crittenden, D.L.; Suhm, M.A. Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol. Molecules2022, 27, 101.
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.
Medel, R.; Springborn, J.R.; Crittenden, D.L.; Suhm, M.A. Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol. Molecules2022, 27, 101.
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.
Abstract
Rotational microwave jet spectroscopy studies of the monoterpenol α-fenchol have so far failed to identify its second expected torsional conformer, despite computational predictions that it is only very slightly higher in energy than the most stable conformer. Vibrational FTIR and Raman jet spectroscopy investigations reveal unusually complex OH and OD stretching spectra compared to other alcohols. Via modelling 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) cm$^{−1}hc$ for the protiated and deuterated alcohol, respectively, which further shrink to 9(1) and 3(1) cm$^{−1}hc$ 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. Assigned are also for α-fenchol the third – in contrast localized – torsional conformer and the most stable dimer, as well as for propargyl alcohol the two most stable dimers.
Chemistry and Materials Science, Analytical Chemistry
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