Frank H. Allen

The Cambridge Structural Database: a quarter of a million crystal structures and rising.

The Cambridge Structural Database (CSD) now contains data for more than a quarter of a million small-molecule crystal structures. The information content of the CSD, together with methods for data acquisition, processing and validation, are summarized, with particular emphasis on the chemical information added by CSD editors. Nearly 80% of new structural data arrives electronically, mostly in CIF format, and the CCDC acts as the official crystal structure data depository for 51 major journals. The CCDC now maintains both a CIF archive (more than 73,000 CIFs dating from 1996), as well as the distributed binary CSD archive; the availability of data in both archives is discussed. A statistical survey of the CSD is also presented and projections concerning future accession rates indicate that the CSD will contain at least 500,000 crystal structures by the year 2010.

The Cambridge Structural Database in Retrospect and Prospect

The Cambridge Crystallographic Data Centre (CCDC) was established in 1965 to record numerical, chemical and bibliographic data relating to published organic and metal-organic crystal structures. The Cambridge Structural Database (CSD) now stores data for nearly 700,000 structures and is a comprehensive and fully retrospective historical archive of small-molecule crystallography. Nearly 40,000 new structures are added each year. As X-ray crystallography celebrates its centenary as a subject, and the CCDC approaches its own 50th year, this article traces the origins of the CCDC as a publicly funded organization and its onward development into a self-financing charitable institution. Principally, however, we describe the growth of the CSD and its extensive associated software system, and summarize its impact and value as a basis for research in structural chemistry, materials science and the life sciences, including drug discovery and drug development. Finally, the article considers the CCDCs funding model in relation to open access and open data paradigms.

CIF applications. XV. enCIFer: a program for viewing, editing and visualizing CIFs

The enCIFer program permits the location, reporting and correction of syntax and format violations in single- or multi-block crystallographic information files (CIFs). The program also permits the editing of existing individual or looped data items and the addition of new data in these categories, and provides data-entry wizards for the addition of two types of standard information for small-molecule structural studies, namely publication data and chemical and physical property information. Facilities for the graphical visualization and manipulation of structure(s) in a CIF are also provided.

Tables of bond lengths determined by X-ray and neutron diffraction. Part 1. Bond lengths in organic compounds

The average lengths of bonds involving the elements H, B, C, N, O, F, Si, P, S, Cl, As, Se, Br, Te, and l in organic compounds are reported.

Applications of the Cambridge Structural Database in organic chemistry and crystal chemistry

The Cambridge Structural Database (CSD) and its associated software systems have formed the basis for more than 800 research applications in structural chemistry, crystallography and the life sciences. Relevant references, dating from the mid-1970s, and brief synopses of these papers are collected in a database, DBUse, which is freely available via the CCDC website. This database has been used to review research applications of the CSD in organic chemistry, including supramolecular applications, and in organic crystal chemistry. The review concentrates on applications that have been published since 1990 and covers a wide range of topics, including structure correlation, conformational analysis, hydrogen bonding and other intermolecular interactions, studies of crystal packing, extended structural motifs, crystal engineering and polymorphism, and crystal structure prediction. Applications of CSD information in studies of crystal structure precision, the determination of crystal structures from powder diffraction data, together with applications in chemical informatics, are also discussed.

The crystallographic information file (CIF) : a new standard archive file for crystallography

The specification of a new standard Crystallographic Information File (CIF) is described. Its development is based on the Self-Defining Text Archive and Retrieval (STAR) procedure [Hall (1991). J. Chem. Inf. Comput. Sci. 31, 326-333]. The CIF is a general, flexible and easily extensible free-format archive file; it is human and machine readable and can be edited by a simple text editor. The CIF is designed for the electronic transmission of crystallographic data between individual laboratories, journals and databases: it has been adopted by the International Union of Crystallography as the recommended medium for this purpose. The file consists of data names and data items, together with a loop facility for repeated items. The data names, constructed hierarchically so as to form data categories, are self-descriptive within a 32-character limit. The sorted list of data names, together with their precise definitions, constitutes the CIF Dictionary (Core Version 1991). The CIF Core Dictionary is presented in full and covers the fundamental and most commonly used data items relevant to crystal structure analysis. The Dictionary is also available as an electronic file suitable for CIF computer applications. Future extensions to the Dictionary will include data items used in more specialized areas of crystallography.

Carbonyl-carbonyl interactions can be competitive with hydrogen bonds

The geometries and attractive energies of carbonyl–carbonyl interactions have been investigated using crystallographic data and ab initio molecular-orbital calculations. Analysis of crystallographic data for 9049 carbon-substituted >C=O groups shows that 1328 (15%) form contacts with other >C=O groups, in which d(C⋯O) < 3.6 A. Three common interaction motifs are observed in crystal structures: (a) a slightly sheared antiparallel motif (650 instances) involving a pair of short C⋯O interactions, together with (b) a perpendicular motif (116 instances) and (c) a highly sheared parallel motif (130 instances), which both involve a single short C⋯O interaction. Together, these motifs account for 945 (71%) of the observed interactions. Ab-initio-based molecular-orbital calculations (6-31G** basis sets), using intermolecular perturbation theory (IMPT) applied to a bis-propanone dimer model, yield an attractive interaction energy of −22.3 kJ mol−1 for a perfect rectangular antiparallel dimer having both d(C⋯O) = 3.02 A and attractive energies < −20 kJ mol−1 over the d(C⋯O) range 2.92–3.32 A. These energies are comparable to those of medium-strength hydrogen bonds. The IMPT calculations indicate a slight shearing of the antiparallel motif with increasing d(C⋯O). For the perpendicular motif, IMPT yields an attractive interaction energy of −7.6 kJ mol−1, comparable in strength to a C—H⋯O hydrogen bond and with the single d(C⋯O) again at 3.02 A.

Systematic analysis of the probabilities of formation of bimolecular hydrogen-bonded ring motifs in organic crystal structures

A methodology has been developed for characterising hydrogen-bonded ring motifs formed between two organic molecules without any prior knowledge of the topology or chemical constitution of the motifs. The method has been implemented by modifying the current Cambridge Structural Database (CSD) System programs. All intermolecular ring motifs comprising ⩽20 atoms formed with N—H···N, N—H···O, O—H···N and O—H···O hydrogen bonds in organic structures in the CSD have been classified. The 75 bimolecular motifs occurring in >12 structures in the CSD are described in terms of their graph sets and chemical functionalities. Motifs are ranked according to their frequency of occurrence and according to their probabilities of formation, i.e. their frequency relative to the number of possible motifs which could have formed. These probabilities provide insights into the relative robustness of known and potential supramolecular synthons.

Hydrogen-bond directionality at the donor H atom—analysis of interaction energies and database statistics

A thorough analysis has been performed on the effects of varying the angle at the donor hydrogen in hydrogen bonds using database studies and ab initio intermolecular interaction energy calculations. Non-activated C–H⋯acceptor interactions are seen to have a weak energy dependence on the D–H⋯A angle, but for all of the conventional hydrogen-bonds studied the D–H⋯A angle has a range of 40–65° within an energy penalty of RT (2.5 kJ mol−1). Interactions with D–H⋯A angle in the range 120–140° are seen to have substantially reduced stabilisation energies and angles below 120° are generally unlikely to correspond to significant interactions.

Supplement. Tables of bond lengths determined by X-ray and neutron diffraction. Part 2. Organometallic compounds and co-ordination complexes of the d- and f-block metals

Average lengths for metal–ligand bonds are reported, together with some intraligand distances, for complexes of the d- and f-block metals. Mean values are presented for 325 different bond types involving metal atoms bonded to H, B, C, N, O, F, Si, P, S, Cl, As, Se, Br, Te, or I atoms of the ligands.