University Federico II
of Naples, Italy
European Chemistry
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Hydrogen bond

Hydrogen bond is a special type of interaction between molecules: it forms whenever a hydrogen atom, bound to a strongly electronegative (able to attract electrons) atom, at the same time interacts with another strongly electronegative atom having a lone pair of electrons, like oxygen, nitrogen or fluorine. The bond is commonly represented as a dotted line between the hydrogen atom and the other electronegative atom, as shown in the figure.

When a hydrogen atom is bound to a strongly electronegative atom, a charge separation occurs. The molecule, though it is electrically neutral, has a partial positive charge (indicated as δ+) on one side and a partial negative charge (δ-) on the opposite side. The hydrogen atom is the positive end of the molecule. It interacts with the negative end of a neighboring molecule, forming a "bridge" between the two molecules. Hydrogen bond is a directional bond, meaning that it is stronger when the hydrogen atom is aligned with the two electronegative atoms.

A single hydrogen bond is relatively weak: usually, however, a high number of such bonds forms simultaneously. All together, they play a central role in determining the chemical and physical properties of polar substances like hydrogen fluoride and water.

Hydrogen bonds in hydrogen fluoride
Hydrogen atoms in white, Fluorine atoms in green

Hydrogen fluoride is composed of HF molecules. Because of the difference in electronegativity between H and F, a hydrogen bond occurs between the hydrogen atom of a molecule and the fluorine atom of a neighboring molecule.

Hydrogen bond between two water molecules

Because of the extensive network of hydrogen bonds between molecules, HF has a higher boiling point than similar molecules (HCl, HBr, HI) containing elements of the same group of the Periodic Table, but heavier and less electronegative. For such reason, HF is liquid, whereas HCl, HBr, and HI are gaseous at temperatures close to room temperature.

Likewise, the peculiar properties of water are due to the presence of hydrogen bonds between molecules. Water molecules, H2O, interact each other through the attraction of a positive tail (the hydrogen atom) with a negative head (the oxygen atom). More precisely, a hydrogen atom, bonded to oxygen in a water molecule, binds by hydrogen bond to the oxygen atom of another molecule, creating a bridge between the two oxygen atoms.

Examples of hydrogen bonds. From left to right: 1) between an alcohol and water; 2) between a ketone and water; 3) between peptide groups of polypeptides; 4) between thymine and adenine in deoxyribonucleic acid (DNA)
DNA double helix
The two strands are held together by hydrogen bonds and other intermolecular interactions

Interaction through hydrogen bonds is important for the properties of many substances. For instance, water molecules interact with polar solutes, determining their solubility. Biological macromolecules, like proteins and nucleic acids, interact with water or between themselves, determining their conformation in solution. Four examples of hydrogen bonds are shown in the figure.

In deoxyribonucleic acid (DNA) hydrogen bonding between purine and pyrimidine bases are responsible for the double helix structure. Although the amount of energy needed to break a single hydrogen bond is rather small, in normal physiological conditions the double helix is stabilized by a very high number of hydrogen bonds.