Effect of Orbital Interactions between Vicinal Bonds and between Hydroxy Groups on the Conformational Stabilities of 1,2-Ethanediol and 2,3-Butanediols

Iodine Gauche Effect Induced by an Intramolecular Hydrogen Bond

The gauche conformer in 1-X,2-Y-disubstituted ethanes, that is, the staggered orientation in which X and Y are in closer contact, is only favored for relatively small substituents that do not give rise to large X···Y steric repulsion. For more diffuse substituents, weakly attractive orbital interactions between antiperiplanar bonds (i.e., hyperconjugation) cannot overrule the repulsive forces between X and Y. Our quantum chemical analyses of the rotational isomerism of XCH₂CH₂Y (X = F, OH; Y = I) at ZORA-BP86-D3(BJ)/QZ4P reveal that indeed the anti conformer is generally favored due to a less destabilizing I···F and I···O-H steric repulsion. The only case when the gauche conformer is preferred is when the hydroxyl hydrogen is oriented toward the iodine atom in the 2-iodoethanol. This is because of the significantly stabilizing covalent component of the I···H-O intramolecular hydrogen bond. Therefore, we show that strong intramolecular interactions can overcome the steric repulsion between bulky substituents in 1,2-disubstituted ethanes and cause the gauche effect. Our quantum chemical computations have guided nuclear magnetic resonance experiments that confirm the increase in the gauche population as X goes from F to OH.

Molecular Conformation and Hydrogen Bond Formation in Liquid Ethylene Glycol

The ethylene glycol (EG) molecule, HOCH₂CH₂OH, adopts a conformation where the central OCCO dihedral is exclusively gauche in the gaseous and crystalline states, but in the liquid state, for close to 20% of the molecules, the central OCCO adopts the energetically unfavorable trans conformation. Here we report calculations, based on ab initio molecular dynamics simulations, on the thermodynamics associated with hydrogen bond formation in the liquid state of EG between donor-acceptor pairs with different molecular conformations. We establish an operational, geometric definition of hydrogen bonds in liquid EG from an analysis of the proton NMR data and show that the key feature, irrespective of the conformation, is marked directionality with almost linear ∠HO···O angles. The free energy for hydrogen bond formation estimated as the potential of mean force for the reversible work associated with the passage from a hypothetical state where hydrogen bonding is absent and donor-acceptor pairs are randomly oriented to the hydrogen-bonded state where the pairs are oriented showed comparable magnitudes irrespective of the molecular conformation of either the donor or acceptor. The results suggest that the presence of the trans conformer in liquid EG would require an understanding of its role in the extended hydrogen-bonded network of the liquid.

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, 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.