Symmetrical Hydrogen Bonds v2

In one of our prior posts, we left off without mentioning how symmetrical "hydrogen bonds" are created. We mentioned that "hydrogen bonds" are incredibly strong, but these symmetriacl "hydrogen bonds" are even superior in strength. This post will serve to detail what makes a "hydrogen bond" symmetrical, and how they are formed.

Symmetric "Hydrogen Bond"

Symmetrical "hydrogen bonds" have the potential to be one of the strongest bonds, but what makes a regular "hydrogen bond" symmetrical? A study conducted at Yale University in October of 2012 by Schley involved analyzing iridium (III) alkoxides to determine what gives rise to a symmetrical "hydrogen bond". Utilizing x-ray diffraction, he found, along with his coleagues, that very short hydrogen bonds with O···O distances of 2.4 angstroms exist. After a series of calculations, Shley uncovered that a shorter O···O bond distance will give rise to a symmetrical hydrogen bond.



Symmetrical "Hydrogen Bonds" in Iridium (III) Aloxides

One method which attempts to prove that hydrogen bonds are asymmetric is called the NMR, nuclear magnetic resonance, method. It is illustrated with 3-hydroxy-2-phenylpropenal and then applied to
dicarboxylate monoanions. Resulting from this analysis is that the intramolecular "hydrogen bonds" are asymmetric, although can become symmetric in a crystal structure.


3-hydroxy-2-phenylpropenal

Another article by Charles Perrin states that symmetrical "hydrogen bonds" can occur only in a crystal. He supports this claim by stating that a disorded environment, in solution, will produce an asymmetric bond. However, crystals, such as when water is in its solid phase existing as ice, can guarantee symmetry. So does this mean that symmetric hydrogen bonds can occur only in the solid phase, and never in the gas or liquid phase?


Crystal Structure of Water

A paper entitled "A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons" demonstrates how "hydrogen bonds" are stronger when in their symmetric, crystalline form. Potassium and cesium hydrogen bistrifluoroacetates emphasize the covalent element of the O···O bond as well as the ionic nature of the hyrogen proton bond. The resulting of this combination of pairing is an incredibly strong bond.


A Hydrogen Bistrifluoroacetates

As a result of these three different studies, we are left with a symmetric "hydrogen bond's" whose strength exceeds that of a regular "hydrogen bond". There are three elements that must be met if we wish for this "hydrogen bond" to be symmetric:

1. "Hydrogen bonding" must occur in the molecule (this is sort of assumed if we are talking about symmetric "hydrogen bonding"
2. The O···O bond distance must be as small as possible
3. The molecule containing the hydrogen bonding should be in a crystal form

Strong Symmetric "Hydrogen Bond"

With all of these conditions met, a regular, rather strong, "hydrogen bond" becomes an incredibly powerful symmetrical "hydrogen bond"