The pressure-induced structural transformation of CHD involves, apart from other effects, the interchange of donor and acceptor sites by the enolic H atoms in the hydrogen bonds. The structure, despite its close similarities with antiferroelectric crystals, is ordered in the high-pressure phase, also retaining its low-pressure symmetry. It is possible that the crystals have a domain structure in both low-and highpressure phases. The proposed mechanism of the transformation strongly suggests an important role of electrostatic interactions in the transformation and in the jump of the enolic H atom to its other site in the OH---O bond. This mechanism also affords an explanation of the phase transition observed in CPD and of the exceptional stability of MCPD at high pressures; however, several points still need to be confirmed and further studies are being carried out on the CHD crystals.The author is grateful to Professor Z. Katuski for encouragement, to Dr R. J. Nelmes for his invitation to use the high-pressure and X-ray facilities of the Physics Department, University of Edinburgh, to Professor M. C. Etter of the Department of Chemistry, University of Minnesota, for stimulating discussions and providing the 1,3-cyclohexanedione samples crystallized from 2-pentanone, and to Dr J. Koput (Adam Mickiewicz University) for his expertise in the MNDO calculations. This study was partly supported by the British Council and by the Polish Academy of Sciences, Project CPBP, 01.12.References ALLMANN, R. (1977 AbstractA method is presented based on graph theory for categorizing hydrogen-bond motifs in such a way that complex hydrogen-bond patterns can be disentangled, or decoded, systematically and consistently. * Alfred P. Sloan Foundation Fellow, 1989-1991 This method is based on viewing hydrogen-bond patterns topologically as if they were intertwined nets with molecules as the nodes and hydrogen bonds as the lines. Surprisingly, very few parameters are needed to define the hydrogen-bond motifs comprising these networks. The methods for making these assignments, and examples of their chemical utility are given.

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The supramolecular chemistry and crystal structures of five bis(imidazolium 2,6-pyridinedicarboxylate)M(II) trihydrate complexes, where M ) Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , or Zn 2+ (1-5, respectively), are reported. These complexes serve as supramolecular building blocks that self-assemble when crystallized to generate a single, well-defined, predictable structure in the solid state. 2,6-Pyridinedicarboxylate anions and imidazolium cations form strong ionic hydrogen bonds that dominate crystal packing in compounds 1-5 by forming twodimensional networks, or layers of molecules. This layer motif serves as a platform with which to control and predict molecular packing by design for engineering the structures of crystals. Moreover, compounds 1-5 create a robust organic host lattice that accommodates five different transition metals without significantly altering molecular packing. Growth of crystals from solutions that contain two or more different metal complexes produces mixed crystals in which mixtures of the different metal complexes are incorporated in the same relative molar ratio present in solution. Epitaxial growth of crystals from one metal complex on the surface of a seed crystal that contains a second metal complex generates composite crystals in which the different metal complexes are segregated into different regions of the crystals. Compounds 1-5 form crystalline solids that represent a new class of modular materials in which the organic ligands serve as a structural component that defines a single packing arrangement that persists over a range of structures, and in which the metal serves as an interchangeable component with which to vary the physical properties of the material.

This paper demonstrates that scanning probe microscopies (SPMs) can image patterned self-assembled monolayers (SAMs) formed by microcontact printing. Lateral force microscopy (LFM) and force modulation microscopy (FMM) showed contrast between regions of patterned SAMs terminated by different chemical functionalities. Normal force microscopy (NFM) typically showed less contrast than LFM or FMM but provided information about the topography of the surface. Chemical functionality (at the interface between the SAM and air) dominated imaging in these experiments. Changes in the morphology of the surface or changes in the humidity of the environment for imaging did not influence significantly the contrast in our experiments. The sharpness of the contrast suggests the use of LFM and SAMs in studying tribology on the submicrometer scale.

The supramolecular chemistry and crystal structures of salts of imidazole with one monocarboxylic acid (1), nine different dicarboxylic acids (2−10), and one tetracarboxylic acid (11) are reported. Salts 2−11 serve as building blocks that self-assemble via ionic O−H···O and N−H···O hydrogen bonds when crystallized. These strong hydrogen bonds generate two types of chains that intersect at the anions and form polar hydrogen-bonded layers with four different motifs. These layers serve as scaffolds with which to control molecular packing in two dimensions for engineering the structures of crystals. All imidazolium cations function as multidentate proton donors by forming two or three C−H···O hydrogen bonds in addition to two N−H···O hydrogen bonds. Strong O−H···O and N−H···O hydrogen bonds define structure and connectivity within layers, while weaker C−H···O hydrogen bonds dominate interactions between layers in these salts.

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A series of eight symmetrically substituted diketopiperazines (DKPs) derived from 1-amino-1-carboxycycloalkanes (n = 3−7; 3,3,5,5-tetramethylcyclohexane; 4,4-dimethylcyclohexane; 2-indan) were synthesized and their crystal structures determined. In the solid state, all eight compounds form two pairs of hydrogen bonds with two adjacent molecules to form a one-dimensional structure that we refer to as “tapes”. These molecules represent a range of volumes and shapes that contain a common molecular fragment (DKP ring). We examined this series of compounds with three objectives in mind:  (i) to establish the ability of the hydrogen-bonded “tape” motif to persist through these differences in volume and shape; (ii) to provide a series of structurally related compounds to use to test computational methods of predicting crystal structure from molecular structure; (iii) to search for qualitative correlations between molecular structure and crystal packing. All compounds form tapes and with one exception, all tapes pack with their long axes parallel. When viewed down their long axis, two types of tapes emerge:  planar and nonplanar. The type of tape that forms reflects the conformation adapted by the DKP ringplanar or boat. Planar tapes form when the angle (α) between the two planes defined by the cis-amides in the DKP ring is 180°; nonplanar tapes form when α < 180°. Five of the eight compounds studied form planar tapes, the remaining three compounds form nonplanar tapes. Despite the variability in volume and shape represented by this series of molecules, the persistence of the tape motif in their crystalline solids suggests that the hydrogen-bonding interactions between DKPs dominate the packing arrangement of these molecules. Void space in the crystalline solid is minimized by parallel alignment of tapes that pack in a manner that permits the interdigitation of substituents on adjacent tapes.

Six derivatives of 2-benzimidazolone, disubstituted in the 4 and 5 positions, have been synthesized, and their structures have been determined in the solid state. Four of these compounds crystallize as molecular tapes. Compounds 1-(CH3)2 , 1-Cl2 , and 1-Br2 form tapes that pack with their long axes parallel; compounds 1-H2 and 2-H2 form tapes that pack with their long axes at an angle to one another. Compounds 1-F2 and 1-I2 crystallize with a three-dimensional network of hydrogen bonds. The packing arrangement of molecular tapes is rationalized on the basis of closest packing and electrostatic interactions between aromatic rings. The occurrence of the network motif rather than the tape motif for 1-F2 and 1-I2 is rationalized on the basis of secondary interactions involving the halogen atoms.