There are more than 20 million chemicals in the literature, with new materials being synthesized each week. Most of these molecules are stable, and the 3-dimensional arrangement of the atoms in the molecules, in the various solids may be determined by routine x-ray crystallography. When this is done, it is found that this vast range of molecules, with varying sizes and shapes can be accommodated by only a handful of solid structures. This limited number of architectures for the packing of molecules of all shapes and sizes, to maximize attractive intermolecular forces and minimizing repulsive intermolecular forces, allows us to develop simple models of what holds the molecules together in the solid. In this volume we look at the origin of the molecular architecture of crystals; a topic that is becoming increasingly important and is often termed, crystal engineering. Such studies are a means of predicting crystal structures, and of designing crystals with particular properties by manipulating the structure and interaction of large molecules. That is, creating new crystal architectures with desired physical characteristics in which the molecules pack together in particular architectures; a subject of particular interest to the pharmaceutical industry.
There are more than 20 million chemicals in the literature, with new materials being synthesized each week. Most of these molecules are stable, and the 3-dimensional arrangement of the atoms in the molecules, in the various solids may be determined by routine x-ray crystallography. When this is done, it is found that this vast range of molecules, with varying sizes and shapes can be accommodated by only a handful of solid structures. This limited number of architectures for the packing of molecules of all shapes and sizes, to maximize attractive intermolecular forces and minimizing repulsive intermolecular forces, allows us to develop simple models of what holds the molecules together in the solid. In this volume we look at the origin of the molecular architecture of crystals; a topic that is becoming increasingly important and is often termed, crystal engineering. Such studies are a means of predicting crystal structures, and of designing crystals with particular properties by manipulating the structure and interaction of large molecules. That is, creating new crystal architectures with desired physical characteristics in which the molecules pack together in particular architectures; a subject of particular interest to the pharmaceutical industry.
There are more than 20 million chemicals in the literature, with new materials being synthesized each week. Most of these molecules are stable, and the 3-dimensional arrangement of the atoms in the molecules, in the various solids may be determined by routine x-ray crystallography. When this is done, it is found that this vast range of molecules, with varying sizes and shapes can be accommodated by only a handful of solid structures. This limited number of architectures for the packing of molecules of all shapes and sizes, to maximize attractive intermolecular forces and minimizing repulsive intermolecular forces, allows us to develop simple models of what holds the molecules together in the solid. In this volume we look at the origin of the molecular architecture of crystals; a topic that is becoming increasingly important and is often termed, crystal engineering. Such studies are a means of predicting crystal structures, and of designing crystals with particular properties by manipulating the structure and interaction of large molecules. That is, creating new crystal architectures with desired physical characteristics in which the molecules pack together in particular architectures; a subject of particular interest to the pharmaceutical industry.
There are more than 20 million chemicals in the literature, with new materials being synthesized each week. Most of these molecules are stable, and the 3-dimensional arrangement of the atoms in the molecules, in the various solids may be determined by routine x-ray crystallography. When this is done, it is found that this vast range of molecules, with varying sizes and shapes can be accommodated by only a handful of solid structures. This limited number of architectures for the packing of molecules of all shapes and sizes, to maximize attractive intermolecular forces and minimizing repulsive intermolecular forces, allows us to develop simple models of what holds the molecules together in the solid. In this volume we look at the origin of the molecular architecture of crystals; a topic that is becoming increasingly important and is often termed, crystal engineering. Such studies are a means of predicting crystal structures, and of designing crystals with particular properties by manipulating the structure and interaction of large molecules. That is, creating new crystal architectures with desired physical characteristics in which the molecules pack together in particular architectures; a subject of particular interest to the pharmaceutical industry.
There are more than 20 million chemicals in the literature, with new materials being synthesized each week. Most of these molecules are stable, and the 3-dimensional arrangement of the atoms in the molecules, in the various solids may be determined by routine x-ray crystallography. When this is done, it is found that this vast range of molecules, with varying sizes and shapes can be accommodated by only a handful of solid structures. This limited number of architectures for the packing of molecules of all shapes and sizes, to maximize attractive intermolecular forces and minimizing repulsive intermolecular forces, allows us to develop simple models of what holds the molecules together in the solid. In this volume we look at the origin of the molecular architecture of crystals; a topic that is becoming increasingly important and is often termed, crystal engineering. Such studies are a means of predicting crystal structures, and of designing crystals with particular properties by manipulating the structure and interaction of large molecules. That is, creating new crystal architectures with desired physical characteristics in which the molecules pack together in particular architectures; a subject of particular interest to the pharmaceutical industry.
The driving forces for the phase transition and relative stability of the two forms of CuQ(2)-TCNQ molecular crystals have been studied using inelastic neutron scattering (INS), density functional theory (DFT), and Hirshfeld surface analysis. DFT molecular dynamics (MD) simulations show that form-II has a lower enthalpy, but with increasing temperature form-I becomes thermodynamically stable due to the greater entropy. INS and MD simulations both show that the entropy of the hydrogen-bond network that holds molecules together within layers is higher in form-I. The interlayer pi-pi interactions are also weaker in form-I, leading to an overall "loosening" of the structure. The phase transition is kinetically hindered by the requirement to re-optimize the orientation of the layers. The strong H-bond interactions keep the in-plane atomic arrangement stable, while the weak interlayer p-p interactions provide the coupling between layers during the phase-transition. This subtle interplay of the two interactions maintains the integrity of the crystal upon phase transition even with dramatic physical dimension changes.
Thin vacuum sublimed films composed of planar metallophthalocyanines in various proportions have been studied by means of X-ray diffraction. Formation of stable solid solution is proposed from experimental data.
The hybrid density functional/Hartree-Fock methods (DF/HF) UB3LYP and UB1LYP with 6–31 G(d) basis set have been applied to the atomic spin densities and isotropic hyperfine coupling constants (hfcc’s) calculations of the NS, ONS, NN and NO bridged radicals. The calculated hfcc’s are in a good agreement with the experimental values. The results are explained in terms of the molecular geometries and the influence of the substituents.
The previously unknown crystal structure of cyclohexane-1, 3cis, 5cis-tricarboxylic acid (CTA) has been solved from laboratory X-ray powder diffraction data using a simulated annealing algorithm followed by restrained Rietveld refinement (reduced-chi(2) = 8.531 for the refined crystal structure). The structure is triclinic (P (1) over bar, Z = 4). Five out of six of the CO2H groups in the asymmetric unit form R-2(2)(8) hydrogen bond motifs with neighbouring CO2H groups. The motifs connect molecules to form pseudo-hexagonal, ten-molecule rings that fuse into supramolecular buckled honeycomb sheets, which stack and are linked by O-(HO)-O-... hydrogen bonds. (C) 2003 Elsevier Science Ltd. All rights reserved.
Pressure-induced structural changes in molecular crystals that are not accompanied by a change in space group symmetry are considered. Anisotropic structural strain of the same polymorph and isosymmetric polymorphic transition are two possibilities which are illustrated in the examples of Co(III)nitropentaammine chloride, the monoclinic and ortho rhombic polymorphs of paracetamol, the α- and γ-polymorphs of glycine, benzoquinone (anisotropic structural strain) and sodium oxalate (isosymmetric polymorphic transition). The experiments described were carried out in diamond anvil cells, and the structural changes were monitored by X-ray powder and single crystal X-ray diffraction, and IR and Raman spectroscopy.
A Protein Data Bank (PDB) study was undertaken to investigate the role of C–X…π interactions (X=F, Cl, Br, I) in protein structures. From this analysis, it is evident that the propensity for the formation of C–X…π interactions is higher in case of fluorine than in other halogens. These results compare well with the recently reported C–X…π interactions in small molecules and depict the orientational dependence of these interactions.
The single-crystal structures of 2,6-di-tbutyl-4-nitrophenol (dbpH), guanidinium 2,6-di-tbutyl-4-nitrophenolate, piperidinium 2,6-di-tbutyl-4-nitrophenolate, 1,8-diazabicyclo-(5,4,0)-undec-7-enium 2,6-di-tbutyl-4-nitrophenolate and 4-(dimethylamino)pyridinium 2,6-di-tbutyl-4-nitrophenolate are reported. The structure of dbpH is non-centrosymmetric and an ungraded powder sample exhibits five times the second-order nonlinear optical intensity of an ungraded urea sample when irradiated with infrared light. The four salt co-crystals all associate via hydrogen-bonding interactions to the phenolate oxygen atom while two of these also involve hydrogen-bonding associations to the nitro oxygen atoms. The presence of the 2,6-disubstituted t-butyl groups hinder the rotation of the plane of the associated base molecules to >60° with respect to the plane of the phenolate ring, thus introducing limited controllability over the approach and association of these molecules.
The properties of the homodimers of the covalent hydrides of the elements from lithium to fluorine and from sodium to chlorine have been computed by means of ab initio molecular orbital theory. These properties show some interesting discontinuities, illustrating the significant differences between the natures of the dimers of the hydrides of the metals and metalloids of groups 1–3, and those of the electronegative non-metals of groups 5–7. The dimers of the group 1–3 hydrides are very strongly bound, featuring two bridging hydrogen atoms per dimer molecule, in structures similar to that of the well-known diborane species. Those of the hydrides of groups 6 and 7 are stabilized by conventional, almost linear, hydrogen bonds. The same is true of the ammonia dimer, but that of phosphine contains two equivalent pairs of bridging hydrogen atoms in a very weakly bound, doubly bifurcated hydrogen-bonded aggregate. The methane and silane dimers are found to be barely bound at all.
