Studies in the Organic Solid State
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Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
Figure 1. Postulated sheet structure of the complex between cyanuric acid and melamine. Hydrogen bonding continues from the molecules at the periphery to give an infinite two-dimensional network. The lines indicate substructures, or .motifs, that could be obtained if hydrogen-bonding were prevented by substituents: A = linear tape, B = crinkled tape, C = rosette.
character of these molecules is also beneficial in generating simple structures. Hydrogen-bonding moieties connected by flexible chains (for example, an aliphatic dicarboxylic acid) would again weaken our chances to predict intermolecular geometry. A number of workers have described regularities in the packing of hydrogen-bonded molecular solids. Etter and her colleagues have performed pathfinding work on systematic categorization of the patterns that hydrogenbonding moieties are likely to adopt in solids. 12 ,13 This work has been critical in suggesting tools for use in crystal engineering. Leiserowitz and coworkers have comprehensively catalogued packing features found in carboxylic acids and amides. 14 Others have ingeniously employed hydrogen-bonding molecules in a
variety of solid-phase systems. Lehn has used three-fold hydrogen bonding in mesogenic compounds. 15 Kunitake has made microassemblies in the form of discs of controlled sizes. 16 Researchers at the Weizmann Institute have explored recognition properties at surfaces of crystals. 17 ,18 Lauher and Fowler have begun systematic work19toward constructing crystalline arrays with the potential for solidstate reactivity.
RESULTS General Structural Features. The structural motifs that we have repeatedly obtained are those that are suggested by the sheet structure of Fig. 1, or "tapes" that are crystallographically infinite in one dimension and are held together by hydrogen bonds. These motifs provide starting points for an approach to crystal engineering based on hierarchies of crystalline architecture (Fig. 2). We label these
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aes(0
0
Moleciules (10)
Dimers and/or 1
SheetsI
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a
Solid (4°)7
Figure 2. Schematic view of hierarchies of crystalline architecture that we observe in 1:1 co-crystals of melamines and barbituric acids.
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successive levels of aggregation by analogy with the nomenclature that is already in use for describing protein structure. Primary structure is simply the molecular
composition and/or sequence of a molecular crystal, which can be built from a single component or from a complex of varying stoichiometry, in the order ABAB, AABB, etc. The secondary level of architecure consists of the initial motif of aggregation of molecules into linear tapes, crinkled tapes, or rosettes (see Fig. 1). Tertiary structure refers to aggregates of secondary elements (e.g. "sheets" or dimers). Finally, quaternarystructure is built up from aggregates of tertiary elements, giving the final three-dimensional solid. Specific Families of Complexes. We chose initially to restrict the scope of our investi
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