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A survey of 113 published neutron diffraction crystal structures is described. The major results are as follows. Hydrogen atoms that are covalently bonded to carbon have a statistically significant tendency to form short intermolecular contacts to oxygen atoms rather than to carbon or hydrogen atoms. This phenomenon is probably due to electrostatic stabilization of the short C-H-O contacts. The proton in the majority of short C-H-O contacts lies within 30°of the plane containing the lone-pair orbitals of the oxygen atom. C-H groups that are adjacent to nitrogen atoms are particularly likely to participate in short C-H-O interactions, but the environment of the oxygen atom is unimportant. The crystal structures also contain several short, intermolecular C--N and C-H-Cl contacts. It is concluded that the C-H-O, C--N, and C-H-Cl 3 Acetamide (ACEMID03)

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The Cambridge crystallographic database comprises files of bibliographic, chemical connectivity and numeric structural data, for organics, organometallics and metal complexes studied by X-ray and neutron diffraction. The files, covering the literature from 1935 and maintained on a current basis, presently contain information on some 25 000 structural studies. Certain categories of information, particularly bibliographic, are disseminated in printed form via the Molecular Structures and Dimensions series. The full potential of the total database depends, however, on its response to specific user queries. The present paper describes the computer-based search and retrieval system, with special reference to question coding. The retrieved data may then be used for extensive and systematic geometric analysis, and for the visual display of crystal and molecular structures. The availability of the database is also discussed.as well as the establishment of abstracting, input and evaluation procedures. The utilization of sections of the database in the production of the reference-book series Molecular Structures and Dimensions (MSD) (Ken-nard &

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