Problems Associated with the Hydrogen-Bond Geometry

1 Problems Associated with the Hydrogen-Bond Geometry 

It is virtually impossible to comprehend the nature of the hydrogen bonding in a complex biological structure without some knowledge of the location of the hydrogen atoms with respect to their nearest neighbors. This structural information requires resolution of the atomic structure of better than 1.5 A, which is difficult to obtain with macromolecules which have dimensions larger than 30 A. 

The number of crystal structure data correlates with the number of statistical analyses of hydrogen-bond geometries. As the amount of crystal structure data increased, so did the efforts to use it as a basis for statistical analyses of the geometrical properties of hydrogen bonds. Because there was only a limited amount of neutron data available and the X-ray analyses did not have the accuracy to provide reliable hydrogen atom positions, most of the earlier analyses focused on the nonhydrogen donor-acceptor distances, i.e., O- O for O-H - O bonds and N 0 or N- -N for N—H 0 = C or N-H- -N bonds [33, 53, 381-384]. 

The large variation in the X-H A angles and the inability to distinguish between two- and three-center hydrogen bonds tended to obscure many of the systematic features1 that became apparent when more accurate analyses were made accessible. 

Many of the data that are summarized in this Part IB are taken from surveys published since 1980 which have been restricted to particular classes of compounds and have used only data where reasonably reliable hydrogen atom positions are available. 

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