Journal of Physical Chemistry A, Vol.123, No.35, 7651-7660, 2019
Can 2-X-Ethanols Form Intramolecular Hydrogen Bonds?
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, Delta E, 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 Delta E 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 Delta E 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 degrees to 60 degrees, 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.