What are the conditions for hydrogen bonding

Hydrogen bond

Author: Hans Lohninger

Hydrogen bond is understood to be the electrostatic attraction between a partially positively charged hydrogen atom and an electronegative atom of another functional group or another molecule. The prerequisite for the formation of hydrogen bonds is, on the one hand, the (positive) polarization of the hydrogen atom in question by a strongly electronegative atom (O, N, or F) bound to the hydrogen atom and, on the other hand, the presence of suitable atoms that carry a partial negative charge and / or have a lone pair of electrons.

In general terms, the following simplified model can be used:

The hydrogen atom is bound to the electronegative atom X and is attracted by the electron pair of the atom Y.

If you look at water, then - due to the strong electronegativity of oxygen - the covalent OH bonds are polarized. The hydrogen carries a positive partial charge, the oxygen a negative one. Since the water molecule is angled, it is easy for the positive hydrogen atoms to interact with the negative oxygen atoms of neighboring molecules (see figure on the right). These interactions (the "hydrogen bonds") mean that individual water molecules cannot easily escape from the liquid when it evaporates, which increases the boiling point of the water.

Hydrogen bonds have typical binding energies of 5 to 40 kJ / mol (with the participation of ions up to 140 kJ / mol) and are therefore stronger than Van der Waals interactions, but weaker than classic covalent or ionic bonds. The weaker hydrogen bonds also have significantly longer bond lengths than covalent bonds. The bond length of the hydrogen bonds in ice is around 180 pm while the covalent OH bond in the water molecule is around 100 pm long.

Hydrogen bonds are usually linear, as this is the only way to keep the repulsive forces between the negatively polarized atoms X and Y minimal while at the same time the attraction between H and Y is maximal. In most hydrogen bonds, the hydrogen atom has a coordination number of two, but there are a number of compounds (such as crystalline carbohydrates or amino acids) in which the coordination number is 3, i.e. the hydrogen bond is bifurcated.

Hydrogen bonds can occur both intermolecularly (e.g. in water or in acetic acid) but also intramolecularly (e.g. in proteins or in salicylaldehyde) and contribute significantly to the properties of the respective substances. The comparatively high boiling point of water, ammonia and hydrogen fluoride is due to hydrogen bonds.

Relationship between boiling point and molar mass for small molecules. Measured by the size of the respective molecule, water, hydrogen fluoride and ammonia show clear deviations upwards, which can be explained by the formation of hydrogen bonds.

The hydrogen bond also plays a major role in the folding of proteins and base pairing in DNA. Hydrogen bonds are formed between guanine and cytosine and between adenine and thymine. The hydrogen bond also plays an essential role in the adsorption of water on surfaces. It leads to the fact that in a normal atmospheric environment the adsorbed water quickly organizes itself in hexagonal double-layer structures.