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# Hydrogen bond

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In chemistry and biochemistry one of a number of types of chemical bond.

As the name implies, one portion of the bond involves a hydrogen atom. In biochemical terms, this hydrogen is most likely attached to a polarizing heteroatom, such as an oxygen or nitrogen, the hydrogen-bond donor. The other half of the bond is another such heteroatom (N or O), called the hydrogen-bond acceptor.

Generally speaking, the donor is that atom to which, in the absence of the hydrogen bond, the attachment of the hydrogen atom would not increase the positive charge on the molecule, whereas attaching the hydrogen to the acceptor atom would leave that portion of the molecule with a positive charge.

The most ubiquitous, and perhaps simplest, example of a hydrogen bond is found in the interaction among water molecules. In a discrete water molecule, water has two hydrogen atoms and one oxygen atom. Two molecules of water can form a hydrogen bond between one of the first atom's hydrogens and the second atoms oxygen. This bond is weaker than the bonds between a water molecules oxygen and each of it's own hydrogens.

     H-O-H...O-H2

The presence of numerous hydrogen bonds in liquid water is credited with water's relatively high boiling point, compared with those of other liquids with similar [molecular mass]?. In solid water (ie, ice?), the crystalline lattice is dominated by a regular array of hydrogen bonds which space the water molecules further apart than is found in liquid water. This manifests itself in water's decrease in density upon freezing. In other words, the presence of hydrogen bonds enables ice to float.

Were the bond strengths more equivalent, one might instead find the atoms of two interacting water molecules partitioned into two complex ions of opposite charge, specifically hydroxide? and hydronium?.

     H-O-  H3O+

Indeed, in pure water under conditions of standard temperature and pressure, this latter formulation is applicable only rarely, on average but once in every 10-14 times (which is the value of the dissociation constant for water under such conditions).

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Last edited July 29, 2001 2:46 pm (diff)
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