STATISTICAL HYDROGEN BOND STUDIES BASED ON NEUTRON DIFFRACTION DATA. Thomas Steiner, Institut für Kristallographie, Freie Universität Berlin, Takwtr. 6, D-14159 Berlin, Germany

Geometrical characterisation of X-H ^...Y hydrogen bonds requires knowledge of the H-atom position. Since this can be only qualitatively provided by X-ray diffraction, accurate hydrogen bond studies must be based on neutron diffraction data. Hydrogen bonds are soft interactions; therefore, the structural properties of hydrogen bonds cannot be deduced from individual crystal structures, but only from analysis and comparison of possibly large numbers of neutron diffraction studies. In this context, 'crystal correlation studies' using information extracted from crystallographic databases, in particular the CSD, have proven to be a powerful tool to gain insight into the nature of hydrogen bonding.

Only a number of examples can be presented that demonstrate the power of crystal correlation studies in this field:

-Directionality of X-H ^... Y hydrogen bonds, in particular the rapid decrease of directional behaviour with increasing H ^... Y separation. The importance of long-range H ^... Y interactions as components of multi-center hydrogen bonding.

-Lengthening of the X-H bond in X-H ^... Y interactions, which allusively even shows up for the weak C-H donors.

Some recent references: Th. Steiner & W. Saenger (1992). Acta Cryst. B48, 819.

Th. Steiner & W. Saenger (1994). Acta Cryst. B50, 348.

Th. Steiner, (1995) J Chem. Soc., Perkin Trans. 2, 1315.