SMALL-MOLECULE STRUCTURES WITH LARGE-MOLECULE PROBLEMS. Larry R. Falvello. University of Zaragoza, Department of Inorganic Chemistry, Faculty of Science, Plaza San Francisco, E-50009 Zaragoza, Spain.

The causes and effects of data and model deficiencies in small- molecule structures are explored, with emphasis on problems analogous to those commonly encountered in macromolecular crystallography -- poor resolution, weak scattering, and incomplete structural models. Possible methods of treating such problems are discussed, using examples from organic and inorganic chemistry, and with special consideration of the limits of the validity of the results. Although reference is made to specialized methods, the examples presented are taken from problems treated with widely available crystallographic techniques.

Some causes of weak scattering, sometimes inherent and not a result of poor sample quality, are described, with reference to two structure determinations from samples with two quite distinct features leading to deficient data. In one of these examples, a control experiment is used to estimate the validity of the information derived from the defective data set. Another example is used to explore the problems caused by an incomplete structural model. As a means of estimating the limits of common crystallographic techniques for treating small-molecule structures with data or model deficiencies, an unsuccessful structure determination is described.