Defining the Hydrogen Bond

2) there is evidence that this bond specifically involves a hydrogen atom already bonded to another atom. 

The evidence called for by criterion (1) can be drawn from a wide range of types of measurements. All of the classical methods for detecting chemical bonds have been cited as providing evidence for H bonds. Methods which can be grossly classified as molecular weight determinations are important cryoscopic, vapor pressure, and vapor density measurements are typical. Of course any measurement that can satisfy criterion (2) may also suffice for criterion (1). 

For the label of specificity, criterion (2), we must appeal to a more limited area of study, to spectroscopic and diffraction data. The most definitive data are, no doubt, those which indicate atom positions in the molecular aggregate. Thus, x-ray diffraction, neutron diffraction, and certain nuclear resonance studies of solids can provide more or less direct evidence that there are H atoms which occupy positions of close approach (hence bonding distance) to two other atoms. Electron diffraction spectra can yield the same information for gaseous species. More easily obtained, however, are IR and Raman spectra, which reveal specific involvement of H atoms by peculiarities in their vibrationeil degrees of freedom in the molecular aggregate. Finally, high resolution proton magnetic resonance studies provide a sensitive index of the electronic environment of the H atoms. 

With this ground work established, we shall exzunine briefly the well known H bonding systems, then deal with the less well recognized types. 

4 Operational Criteria for H Bond Formation. In these well  

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Hydrogen bonding is without doubt the most studied intermolecular interaction, presumably due to its frequent presence in organic solids and biological molecules. Various attempts have been made to define the hydrogen bond [24,... 

Arunan E, Desiraju GR, Klein RA et al (2011) Defining the hydrogen bond an account (lUPAC technical report). Pure Appl Chem 83 1619-1636... 

London dispersion forces, and/or similar interactions is not a simple task. Chemical bonding, whether noncovalent, covalent, ionic, or metallic, covers a broad, continuous spectrum of electronic interactions and energies. Consequently, the classification of a bond or interaction (e.g., double versus triple or covalent versus noncovalent ) is sometimes open to interpretation. As a result, there is no unique criterion or set of criteria that can be used to define weak interactions or noncovalent interactions. In the second volume of this review series, Scheiner already notes this issue and highlighted the difficulties associated with defining the hydrogen bond. Here, matters are even more complicated because other weak interactions are also considered. 

Clearly, in the Mezei-Beveridge definition, gom ) fo " -/ QQ defines the hydrogen bonding structure around a molecule so that oh( oo ) gives the number of molecules hydrogen-bonded. 

Figure 1 Single-flip rearrangement of the water trimer. This figure was produced using Mathematical and a distance cut-off to define the hydrogen bonds...

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