Hydrogen bond

A hydrogen bond is an attractive interaction between two species that arises from a link of the form A-H^...B, where A and B are highly electronegative elements and B possesses a lone pair of electrons. Hydrogen bonding is conventionally regarded as being limited to N, O, and F but, if B is an anionic species (such as Cl^-), it may also participate in hydrogen bonding. There is no strict cutoff for an ability to participate in hydrogen bonding, but N, O, and F participate most effectively.

The formation of a hydrogen bond can be regarded either as the approach between a partil positive charge of H and a partial negative charge of B or as a particular example of delocalized molecular orbital formation in which A, H, and B each supply one atomic orbital from which three molecular orbitals are constructed. Thus, if the A-H bond is regarded as formed from the overlap of an orbital on A, $\Psi$_A, and a hydrogen 1s orbital, $\Psi$_H, and the lone pair on B occupies an orbital on B, $\Psi$_B, then, when the two molecules are close together, we can build three molecular orbitals from the three basis orbitals:

$\Psi =c_{1}\Psi _{A}+c_{2}\Psi _{H}+c_{3}\Psi _{B}$

One of the molecular orbitals is bonding, one almost nonbonding, and the third antibonding. These three orbitals need to accompodate four electrons (two from the orginal A-H nond and two from the lone pair of B), so two enter the bonding orbital and two enter the nonbonding orbital. Because the antibonding orbital remains empty, the net effect-depending on the precise location of the almost nonbonding orrbital-may be a lowering of energy.

Hydrogen bonds are responsible for the double-helical structure of DNA, and for the tertiary and quaternary structure of proteins. Hydrogen bonds are relatively weak, especially compared with metallic, ionic or covalent bonds. They are stronger than van der Waals forces.