Hydrogen atoms in crystals surrounded by oxygen generally have irregular coordination environments containing numerous contacts to other atoms with widely varying lengths. Any attempt to define hydrogen bonds must find a common description for many different geometrys.

In most cases, the coordination number around hydrogen is simply defined as 2. This is especially the case in protein structures, where hydrogen positions are not refined and a hydrogen bond is assumed, id a donor and an acceptor are close enough to each other.

The following material is based on small molecule structures, both inorganic and organic. It can be shown [Lösel, Trömel 93] that the same rules for hydrogen bonds apply for inorganic and organic small crystals.

The reason for investigating small molecules is mainly the availability and quality of data with lots of neutron structures where the hydrogen positions are well known up to standard deviations below 1 pm.

The material shows, that in a lot of cases the consideration of only the two shortest bonds is not sufficient. In all inorganic structures with second shortest bonds that are symmetrically equivalent [e.g. NaOH, LiOH, Ca(OH)2] and organic structures with furcated H-bonds any description of the environment around hydrogen with coordination number two for hydrogen will fail.

But any coordination number higher two will give question to the cut-off criterion, the point where an oxygen atom is no longer treated as coordinated. Most cut-off criterion are based on distance and angle. They may be easy to use and of good help in a given data-set but may fail in some circumstances.

Some selected coordinations around some hydrogen atoms in the next chapter should illustrate this problem.

The use of the term bond in this article

In a lot of discussions about this topic the main misunderstanding arouse with the use of the word bond. Chemists use the term everyday, but the exact meaning is often not clear. In this text the term bond describes the following : in every case where a coordination between hydrogen and an anion exist, I assume an attracting interaction and call it a bond. This doesn't make any distinction between coulomb, ionic, hydrogen- or van der Waals-bonds. There is always a bond if the contact contributes to the valence (described in Chapter 6 ) of the central atom, no matter how small this may be.

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Last Updated: 26 October 1996