Definition of a Hydrogen Bond

Since its first suggestion many years ago , the hydrogen bond has continued to fascinate chemists. This interaction is intimately involved in the structure and properties of water in its various phases, and of molecules in aqueous solution. In addition to the traditional role of the H-bond as a structural element in large molecules such as proteins and nucleic acids °, a series of such bonds appear to be vital to the functioning of a number of en- 

There are some indications that H-bonds play an even more important role in 

A Lewis structure of a H-bond violates the octet principle of striving toward two electrons around each hydrogen. Much weaker than a conventional covalent bond, the H-bond is stronger than the van der Waals forces that bind together nonpolar molecules. While Coulombic attraction between polar molecules certainly contributes toward the interactive force, the H-bond is nonetheless considered to be more than a simple electrostatic interaction. 

There are several consequences of this interaction that are commonly taken as criteria for formation of a H-bond .  

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A definition of a hydrogen bond must be based upon operational criteria and two types of evidence are necessary before the existence of a hydrogen bond can be said t o be well established. 

Hydrogen bonding (introduced in Section 3.4) is relevant to most scientific disciplines. The definition of a hydrogen bond in terms of its covalent and ionic contributions continues to be debated. Its traditional definition, a force that arises between molecules that have an H atom bonded to a small, highly electronegative atom with lone pairs, usually N, O, or F, has been considerably broadened. The lUPAC Physical and Biophysical Chanistry Division has recommended a new definition that includes the following themes. 

The sharp first peak and following deep minimum of gon(A ) for normal liquid water (at 1.8 A and 2.4 A, respectively) become much less pronounced under supercritical conditions (Fig. 7b). These two characteristic features of gon( ) observed in computer simulations, as well as experimentally (Soper et. al. 1997), are the basis of a simple geometric definition of a hydrogen bond, whereby the bond is assumed to exist between any pair of H2O molecules whose respective O and H atoms are separated by less than i HB = 2.4 A. Integration under gon(A ) up to the chosen threshold distance provides a convenient way to quantitatively estimate the average number of H-bonds in which an individual molecule participates under various thermodynamic conditions (e.g.,... 

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