Simple models for the quick estimation of ground state hydrogen tunneling splittings in alcohols and other compounds

Abstract

Models for the quick estimation of energy splittings caused by coherent tunneling of hydrogen atoms are evaluated with available experimental data for alcohols and improvements are proposed. The discussed models...

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

“…Observed spectral splittings could be translated into torsional splittings, determined for the main isotopolog of α-fenchol as 16(1) cm −1 and 9(1) cm −1 in the OH stretch ground and excited state, respectively. As the tunneling interaction largely contributes to these splittings, they are characteristically [85] reduced to 7(3) and 3(1) cm −1 upon increasing the tunneling mass by hydroxy deuteration. Despite being calculated to be less delocalized, the deuterated alcohol shows a more even distribution of Raman activity across the split bands.…”

“…Likewise, it has been shown that experimental tunneling splittings of protiated and deuterated alcohols (and also other series of compounds with hydrogen tunneling) can be directly correlated with each other as well, taking advantage of similar changes of the moment of inertia and the torsional wavenumber upon deuteration [85]. If the contributions to the assigned torsional splittings of α-fenchol are dominated by tunneling, and less by asymmetry, the isotope effect should be roughly captured by this model as well.…”

“…To assess the torsional splitting, and the expected contribution from tunneling to it, we employ different computational models. As a first approach, it was demonstrated to be possible to reasonably estimate the tunneling splitting of symmetric alcohols only on the basis of calculated B3LYP properties of the enantiomeric g geometries and the transition state in between [85]. To model the limiting case of the potential of α-fenchol being perfectly symmetric, g−/g+ averages are used for the barrier heights and harmonic torsional wavenumbers in these models.…”

“…Table 2: Predicted tunneling contributions ∆ to the ground-state torsional splitting of α-fenchol-OH and -OD according to different simple models [85] and the torsion 1D code based on B3LYP-D3(BJ)/may-cc-pVTZ calculations, in part with DLNPO-CCSD(T)/augcc-pVQZ corrections. For the simple models barrier heights and torsional wavenumbers were averaged between g− and g+ (used values given in Table S6), for the torsion 1D code the torsional potential was either symmetrized by averaging both half-potentials or an additional barrier at the g−/g+ transition state was added to compare with the localized scenario.…”

“…To validate the performance of the torsion 1D code in combination with the electronic B3LYP potential, we compare results with the experimental benchmarking data set for tunneling splittings for 27 symmetric alcohol species compiled in Ref. 85. The correlation is shown in Figure 9.…”