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A '''hydrogen bond''' is an attraction between [[chemical polarity|polar molecules]]. They are formed when oppositely charged ends of polar molecules are attracted to one another. This weak attractive force is the hydrogen bond.
A '''hydrogen bond''' is an attractive interaction between two species that arises from a link of the form A-H<sup>...</sup>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<sup>-</sup>), 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, <math>\Psi</math><sub>A</sub>, and a hydrogen 1''s'' orbital, <math>\Psi</math><sub>H</sub>, and the lone pair on B occupies an orbital on B, <math>\Psi</math><sub>B</sub>, then, when the two molecules are close together, we can build three molecular orbitals from the three basis orbitals:
<math>\Psi=c_1\Psi _A+c_2\Psi _H+c_3\Psi _B</math>
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 [[protein]]s. Hydrogen bonds are relatively weak, especially compared with [[metallic bond|metallic]], [[ionic bond|ionic]] or [[covalent]] bonds. They are stronger than [[van der Waals]] forces.
Hydrogen bonds are responsible for the double-helical structure of [[DNA]], and for the tertiary and quaternary structure of [[protein]]s. Hydrogen bonds are relatively weak, especially compared with [[metallic bond|metallic]], [[ionic bond|ionic]] or [[covalent]] bonds. They are stronger than [[van der Waals]] forces.
[[Category:Chemistry]]
[[Category:Chemistry]]