A hydrogen bond is the electromagnetic attraction between a hydrogen atom and an electronegative atom (such as nitrogen, oxygen, or fluorine) that comes from another molecule or chemical group. It is not a true chemical bond. These attractions can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly).
The hydrogen bond (5 to 30 kJ/mole) is stronger than a van der Waals interaction but weaker than a covalent or an ionic bond. This type of bond occurs in both inorganic molecules, such as water, and organic molecules, such as DNA and proteins. Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 °C) as compared to the other group-16 hydrides that have no hydrogen bonds.
Intramolecular hydrogen bonding is partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids. It also plays an important role in the structure of polymers, both synthetic and natural.
A hydrogen atom attached to a relatively electronegative atom is a hydrogen bond donor. This electronegative atom is usually fluorine, oxygen, or nitrogen. An electronegative atom such as fluorine, oxygen, or nitrogen is a hydrogen bond acceptor, regardless of whether it is bonded to a hydrogen atom or not. An example of a hydrogen bond donor is ethanol, which has a hydrogen bonded to oxygen; an example of a hydrogen bond acceptor that does not have a hydrogen atom bonded to it is the oxygen atom on diethyl ether.
A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl3. The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen bond acceptor.The hydrogen bond is often described as an electrostatic dipole-dipole interaction. However, it also has some features of covalent bonding: it is directional and strong, produces interatomic distances shorter than sum of van der Waals radii, and usually involves a limited number of interaction partners, which can be interpreted as a type of valence. These covalent features are more substantial when acceptors bind hydrogens from more electronegative donors.
The partially covalent nature of a hydrogen bond raises the following questions: "To which molecule or atom does the hydrogen nucleus belong?" and "Which should be labeled 'donor' and which 'acceptor'?" Usually, this is simple to determine on the basis of interatomic distances in the X−H…Y system: X−H distance is typically ≈110 pm, whereas H…Y distance is ≈160 to 200 pm. Liquids that display hydrogen bonding are called associated liquids.