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Racemic Crystals (racemic + crystal)
Selected AbstractsA solution to the observed Z, = 2 preference in the crystal structures of hydrophobic amino acidsACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2009Carl Henrik Görbitz Chiral amino acids without functional groups in their side chains (hydrophobic amino acids) systematically form crystals with two molecules in the asymmetric unit. In contrast, racemates of the same compounds form crystals with Z, = 1. The present investigation addresses the origin of this important difference between enantiomeric and racemic crystals. Through a series of ab initio calculations on infinite two-dimensional slabs, derived from crystal structures, as well as calculations on full crystal structures it is shown that it is indeed possible to explain the observed behaviour. Additionally, the (not unexpected) observation that amino acids usually form racemates in the solid phase rather than undergoing racemic separation upon crystallization is rationalized on the basis of energy calculations. [source] S,S -1,2-Dicyclohexylethane-1,2-diol and its racemic compound: a striking exception to Wallach's ruleACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2006Brian O. Patrick The structures of enantiopure S,S -1,2-dicyclohexylethane-1,2-diol and its racemic compound (rac - S,S -1,2-dicyclohexylethane-1,2-diol) have been determined at 295 and 173,K. The crystals of the enantiopure material are more than 4% denser than the crystals of the racemic compound, but the melting points indicate that the crystals of the less dense racemic compound are considerably more stable than those of the racemic conglomerate. This apparent exception to the correlation of crystal density and melting point is explained. The enantiopure crystals have four molecules in the asymmetric unit (Z, = 4). Two of the molecules have the conformation observed for the one independent molecule of the racemic compound and two have a higher energy conformation; the overall P21 structure is a perturbed version of a P212121 structure with Z, = 2. The enantiopure and racemic crystals have the same hydrogen-bonding motif, but the motif in the former appears to be significantly strained. A reason why crystals of enantiopure material might be systematically less dense than crystals of its racemic compound and to be more likely to have Z, > 1 is suggested. [source] Basic forms of supramolecular self-assembly organized by parallel and antiparallel hydrogen bonds in the racemic crystal structures of six disubstituted and trisubstituted cyclopentane derivativesACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2001Alajos Kálmán A selection of stereoisomeric 2-hydroxy-1-cyclopentanecarboxamides, a 4- tert -butyl derivative and three tert -butyl derivatives of the respective carboxylic acid were subjected to X-ray crystallography. The optically active molecules (I),(VI) form racemic crystals. Each racemic structure is basically determined by two intermolecular hydrogen bonds of O,H,O=C,XH and O=C,X,H,OH types (X = O, NH). The partially similar patterns of close packing observed reflect five basic forms of supramolecular self-assembly. In the racemic crystals of chiral molecules, there are homo- and heterochiral chains of molecules formed by the principal (O,H,O=C) hydrogen bonds. These chains assemble either in a parallel or antiparallel mode. The parallel homochiral chains (hop) observed in structure (II), (1R*,2R*)-2-hydroxy-1-cyclopentanecarboxamide, demand the polar space group Pca21, while the parallel heterochiral chains (hep) are organized in antiparallel layers with space group P21/n in structure (VI), (1R*,2S*,5R*-5- tert -butyl-2-hydroxy-1-cyclopentanecarboxylic acid). Heterochiral chains in an antiparallel array (hea) are found in (I), (1R*,2S*)-2-hydroxy-1-cyclopentanecarboxamide, and (V) [(1R*,2S*4S*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxylic acid, space group P21/c]. Structures (IV), (1R*,2S*,4R*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxylic acid, and (III), (1R*,2R*,4S*)-4- tert -butyl-2-hydroxy-1-cyclopentanecarboxamide, reveal that homochiral chains in an antiparallel array (hoa; cross-linked by heterochiral dimers held together by the second hydrogen bonds) can be formed by either translation (space group P) or a screw axis (space group P21/c). These alternatives are denoted hoa1 and hoa2. Similarly, within each pattern (hea, hep and hop) two slightly different alternatives can be expected. The partial similarities in the identified five patterns of hydrogen bonding are described by graph-set notations. Structures (I), (IV) and (V) can be characterized by a common supramolecular synthon, while the highest degree of similarity is shown by the isostructurality of (I) and (V). [source] Chloro(1-{3-[2-(diphenylphosphanyl-,P)ethyl]-,6 -benzyl}-3,5-dimethyl-1H -pyrazole-,N2)ruthenium(II) trifluoromethanesulfonateACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2000Bruno Therrien To address the question of the role of chirality at the metal in enantioselective catalysis, a pseudo-tetrahedral three-legged piano-stool complex has been prepared, i.e. [RuCl(C26H27N2P)](CF3SO3). Anchoring a phosphine and a pyrazole tether to an arene (PArN) yields, after ,6:,1:,1 coordination to ruthenium, [{,6:,1:,1 -(PArN)}RuCl]+ as a 1:1 mixture of enantiomers. Unfortunately, all attempts to resolve the enantiomers failed. The structure solution revealed the presence of racemic crystals. [source] Induction and Inhibition of Preferential Enrichment by Controlling the Mode of the Polymorphic Transition with Seed CrystalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2006Rui Tamura Prof. Abstract Both induction and inhibition of "preferential enrichment", an unusual symmetry-breaking enantiomeric-resolution phenomenon observed upon simple recrystallization of a certain kind of racemic crystals from organic solvents, have been successfully achieved by controlling the mode of the polymorphic transition during crystallization with appropriate seed crystals. Such control of the polymorphic transition can be interpreted in terms of a novel phenomenon consisting of 1) the adsorption of prenucleation aggregates, 2) the heterogeneous nucleation and crystal growth of a metastable crystalline form, and 3) the subsequent polymorphic transition into the more stable form; these three processes occur on the same surface of a seed crystal. We refer to this phenomenon as an "epitaxial transition", which has been confirmed by means of in situ attenuated total reflection (ATR) FTIR spectroscopy in solution and the solid state, differential scanning calorimetry (DSC) measurements of the deposited crystals, and X-ray crystallographic analysis of the single crystals or the direct-space approach employing the Monte Carlo method with the Rietveld refinement for the structure solution from the powder X-ray diffraction data. [source] | ||||||||||