Can 2‑X-Ethanols Form Intramolecular Hydrogen
Bonds?
Version 2 2019-08-23, 14:39
Version 1 2019-08-23, 14:36
Posted on 2019-08-23 - 14:39
For
2-X-ethanols, where X = F, OH, or NH2, the gauche
conformer is favored over the trans conformer by at least 2 kcal/mol.
Initially, this preference, ΔE, was attributed
to an intramolecular hydrogen bond, IMHB, between the OH and X groups.
Over the years, this conclusion has been challenged by two major arguments.
One claim is that the entirety of ΔE can be
accounted for by the gauche effect. Against this, calculations using
five different methods show that the maximum contribution of the gauche
effect to ΔE is less than 1 kcal/mol. A second
argument employs the quantum theory of atoms in molecules to contend
that the absence of a bond critical point (BCP) between the OH and
X groups in 2-X-ethanols denotes the lack of an IMHB. By looking at
the 2-X-ethanols at fixed XCCO torsional angles ranging from 0°
to 60°, it is shown that the BCP criterion is inconsistent with
other properties such as energy, bond lengths, and stretching frequencies.
These inconsistencies are removed when the theory of noncovalent interactions
is used. The IMHBs in 2-X-ethanols are found to be similar in form
but smaller in magnitude than their intermolecular counterparts. This
work concludes that 2-X-ethanols form IMHBs.
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Rosenberg, Robert E. (2019). Can 2‑X-Ethanols Form Intramolecular Hydrogen
Bonds?. ACS Publications. Collection. https://doi.org/10.1021/acs.jpca.9b06801
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