Phase transition and domain structure in iodoform crystals

Phase transition in an iodoform crystal has been established from the temperature dependence of the Raman spectra of “external” crystal vibrations at 260 K. The transformation mechanism is interpreted under the assumption on the existence in the crystal of domains where dipole moments of the molecules have parallel or antiparallel orientations. Below the phase-transition temperature, the structure is more ordered, and the crystal consists mainly of domains with the parallel orientation of dipole moments of the molecules, whereas above this point, the domains with the antiparallel orientation of dipole moments prevail. The crystal as a whole is not a pyroelectric.     

X-Ray structure investigation of 1,3-indandione and 1,3-dicyanomethyleneindan derivatives: A review

This paper generalizes the results of the X-ray structure investigations of crystals of the 1,3-indandione and 1,3-dicyanomethyleneindan derivatives (primarily, individual compounds, as well as crystal hydrates, crystal solvates, ionic salts, and betaine-like intraionic derivatives of 1,3-indandione). The structural features of molecules and crystals, the geometric parameters, and the intramolecular and intermolecular hydrogen bonds are discussed. Structural formulas of 92 compounds and selected geometric parameters of molecules and hydrogen bonds are presented in six tables.     

Molecular and crystal structures of 1-methyl-2-Oxo-3-acetoxy-4-hydroxy-4-(Phenyl)piperidine, 1-ethyl-2-oxo-3,4-dihydroxy-4-(Pyridyl)piperidine, and 1-benzyl-2-oxo-3,4-dihydroxy-4-(Pyridyl)piperidine and the role of hydrogen bonds in molecular packing in crystals

The molecular and crystal structures of 1-methyl-2-oxo-3-acetoxy-4-hydroxy-4-(phenyl)hydropyridine, 1-ethyl-2-oxo-3,4-dihydroxy-4-(pyridyl)piperidine, and 1-benzyl-2-oxo-3,4-dihydroxy-4-(pyridyl)piperidine are determined by X-ray diffraction analysis. These three hydroxy derivatives of hydropyridine are the convenient model compounds in studies of the influence of intramolecular and intermolecular hydrogen bonds on the conformation and packing of molecules in crystals. The molecules of hydroxy derivatives possess a considerable conformational flexibility and can form an extended network of intramolecular and intermolecular hydrogen bonds. It is shown that these compounds more often than others form noncentrosymmetric and polysystem crystals. The role of the molecular association and the type of hydrogen-bonded associates (syntons) in crystal packing are discussed.     

Design of molecules and noncentrosymmetric crystals of 3-(4-nitrophenyl)-2-phenylacrylic acid derivatives

The derivatives of 3-(4-nitrophenyl)-2-phenylacrylic acid are synthesized. It is shown that the chemical modification of this acid results in a change in the symmetry of the supramolecular fragment formed by molecules in the crystal. In turn, this change leads to a change in the crystal symmetry and favors the formation of a noncentrosymmetric crystal structure. The crystal structures of 3-(4-nitrophenyl)-2-phenylacrylic acid (I), 3-(4-nitrophenyl)-2-phenyl-triethyl aminoacrylate (II), and 3-(4-nitrophenyl)-2-phenylacrylic acid (3-methoxy)phenyl ester (III) are investigated by X-ray diffraction.     

Crystal and molecular structure of an amber polymorph of 4-methyl-2-nitroacetanilide (MNA)

The crystal and molecular structure of a third polymorph of 4-methyl-2-nitroacetanilide has been determined by X-ray diffraction. It crystallizes in the monoclinic system, space groupP2₁/c,Z=4,a=10.158(2),b=11.635(2),c=8.041(2) Å and=94.55(2)°. Using 1027 unique reflections, the structure was solved by the method of vector verification and refined by full-matrix least squares, which gave convergence toR=0.080. The structure consists of nearly planar molecules, all approximately parallel to one another with their longitudinal axes parallel tob. The amide group forms an intramolecular hydrogen bond with the nitro group. Molecules related byc-glide are stacked alongc in a very distinct columnar form. The columns are held together by dipole-dipole interactions between close antiparallel carbonyl groups or between close antiparallel nitro groups.     

Crystal and molecular structures of selected oxidative nitration products of aminopyrazine and 2-amino-3-hydroxypyridine

2,3,5,6-Tetraoxopiperazine (I), ammonium 2,3,5,6-tetraoxo-4-nitropyridinate crystal hydrate (II), co-crystallized dimethylammonium 2,3,5,6-tetraoxo-4-nitropyridinate and dimethylammonium chloride (III), and ammonium 2,6-dioxo-3,5-dinitropyridinate (IV) were isolated in the crystalline state and studied by X-ray diffraction. All heterocycles are planar and nonaromatic. They consist of two conjugated moieties linked together by C-C single bonds. The short C...O contacts (2.841–3.048 Å), which link the molecules to form zigzag two-dimensional chains, make a considerable contribution to the formation of the crystal structure of I, whereas hydrogen bonds that form three-dimensional networks play the most important role in the formation of the crystals of II–IV.     

Stereochemistry of benzophenone derivatives: crystal and molecular structure of 3,3′-dibromobenzophenone

The structure of 3,3′-dibromobenzophenone has been reinvestigated using new three-dimensional data. Crystals of 3,3′-dibromobenzophenone are orthorhombic:a = 4·05,b = 11·74,c = 24·80 Å,Z = 4, space groupPbcn. The structure has been refined by three-dimensional methods, including anisotropic full-matrix least squares, to anR index 0·104. The conformation of isolated molecules of benzophenone,pp′-dimethoxy benzophenone and 3,3′-dibromobenzophenone have been calculated by semi-empirical methods, and the results are compared with those obtained from X-ray crystallographic investigations on these compounds. It has been found that the calculated geometries of isolated molecules of benzophenone and its derivatives are different from those observed in the crystal structures of these compounds.     

Crystal and anion structure, TGA, DTA, and infrared and Raman spectra of managanese(II) nitroprusside dihydrate, Mn [Fe(CN)5NO]·2H2O

The single crystal and anion structure of Mn[Fe(CN)₅NO·2H₂O, obtained by slow interdiffusion of reactant solutions through a TMS gel, was solved by X-ray diffraction methods and refined toR1=0.036. Spatial group: orthorhombic,Pnma,a=14.069(2),b=7.538(1),c=10.543(1)Å,Z=4. The Mn(II) ion and the water molecules are sited on mirror planes, which bisect the nitroprusside ions. One of the water molecules is coordinated to Mn(II) and the other, strongly hydrogen (as acceptor) bonded to the first molecule. The IR spectrum confirms the bonding of the water molecules and TGA results are in accordance with the dihydrate character of the substance and its dehydration in two successive steps. DTA results and the Raman spectrum agree with other results and the comparison between IR and RamanvNO wavenumbers confirms the expected strong vibrational interaction between the closely packed antiparallel (eclipsed) NO groups. There is a topotactic relationship between the dihydrate and the trihydrate, which crystallizes in the space subgroupP2 _1/n.     

Polar structure of N,N′-dimethylurea crystals

In the crystal structure of N,N′-dimethylurea, there is one molecule in the asymmetric unit, but with two halves related by an axis of pseudosymmetry. All nonhydrogen atoms of each molecule lie on a plane. In the crystal the molecules are held together by hydrogen bonds in the direction of the “a” axis. They form parallel rows with the same sense orientation, so that the crystal has a polar structure.     

Molecular structures and crystal packings of 2-styrylquinoxaline derivatives

The crystal and molecular structures of 2-styrylquinoxaline derivatives with different substituents in the styryl fragment are determined. The degree of planarity of the molecules studied varies in a very wide range, from 1.7° to 33.5°. In the ethylene fragment, the double bond is essentially localized. The bicycle-pedal disordering of the ethylene fragment is found in the crystals of the methoxy and oxyacetyl derivatives of 2-styrylquinoxaline. None of the packings contains packing motifs favorable for the photocycloaddition (PCA) reaction with single crystal retention. The crystal packings of these compounds and that of 2-(4-methylstyryl)quinoxaline are characterized by a stacking motif of the head-to-head type, which eliminates the possibility of PCA taking place with single crystal retention but is suitable for this reaction in polycrystalline films. The crystal packing of 2-(3,4-dimethoxystyryl)quinoxaline does not contain elements with stacking interactions.     

Synthesis and crystal structure investigation of pyridine‐2‐(3′‐mercaptopropanoic acid)‐N ‐oxide

Pyridine‐2‐(3′‐mercaptopropanoic acid)‐N ‐oxide (I), is a higher homologue of 1‐oxopyridinium‐2‐thioacetic acid (II) [1]. It crystallizes in monoclinic space group P2₁ with a = 9.2168(2) Å, b = 4.1423(2) Å, c = 11.3904(4) Å, β = 98.65(2)°, V = 429.93(3) ų and Z = 2. The least‐squares refinement gave residual index R = 0.024 for 1070 observed reflections. The introduction of an additional methylene group in (II) causes a flip in the carboxylic acid group of (I) that facilitates the molecules to align infinite antiparallel chains through strong C–H···O interactions. The molecules are interlinked by O–H···O hydrogen bonding across the chains and forming an infinite screw chain along y‐direction. The molecular packing is stabilized by O–H···O and C–H···O hydrogen bonding and π‐π electron interactions. This is an important facet of the crystal packing. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and crystal structure of 3,10-dimethyl-10H-dibenzo[b,e]iodinium tetrafluoroborate

The crystal and molecular structures of a chiral cyclic iodonium salt, namely, 3,10-dimethyl-10H-dibenzo[b,e]iodinium tetrafluoroborate, is solved by the direct method (a = 20.678(5) Å, b = 20.738(5) Å, c = 8.408(2) Å, γ =120.404(5)° Z = 4, sp. gr. P2₁/b). The crystals of the title compound consist of the iodonium cations and the BF ₄ ^? anions. The benzene rings A and B of the cations are planar. The angles between these rings are 56.1(4)° and 58.3(4)° in two independent molecules, and the rings are virtually coplanar with the corresponding halves of the central heterocycle C adopting the I,C(3)-boat conformation. The structural units are packed in the crystal so that each I atom forms short contacts with the fluorine atoms of two independent anions.     

Crystal structure of 4'‐O‐methylalpinumisoflavone at 90K

4′‐O‐Methylalpinumisoflavone crystals which hitherto had been difficult to crystallize for X‐ray diffraction analyses have been obtained from a demethylated product of O, O‐dimethylalpinumisoflavone. The structural and conformational features of the crystal measured at 90 K are reported and compared with previously reported work of O,O‐dimethylalpinumisoflavone and 5‐O‐methyl‐4′‐O‐(3‐methyl‐but‐2‐en‐1‐yl) alpinumisoflavone. Each of these compounds is characterized by a six membered ring that is further fused to a benzopyrone isoflavone moiety resulting in a tricyclic ring system. This six membered ring shows a half‐chair conformation in all the molecules studied, with the ring in the title compound showing the least Cremer‐Pople puckering amplitude Q and shorter inter and intramolecular contacts. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics and intimate mechanism of protein crystal nucleation

Experimental and theoretical investigations on protein crystal nucleation are reviewed. Various experimental applications of the classical principle, which requires separation of the nucleation and growth stages of the crystallization process, are described. Temperature control is used most frequently, hypergravity and concentration changes being auxiliary techniques. Nucleation time-lags are measured by imposing temperature evoked supersaturation gradients. Application perspectives are revealed. Nucleation rates are measured according to the classical principle mentioned above, and energy barriers for crystal nucleation and numbers of molecules constituting the critical nuclei are calculated. Surprisingly, although requiring unusually high supersaturation, protein crystal nucleation occurs much more slowly than that with small molecule substances. On this basis novel notions are suggested for the elementary mechanism of protein crystal bond formation. Due to the selection of the crystalline bonding patches a successful collision between protein molecules, resulting in the formation of a crystalline connection, requires not only sufficiently close approach of the species, but also their proper spatial orientation. Imposing a rigid steric constraint, the latter requirement postpones the crystal nucleus formation. Besides, it was shown that cluster coalescence is not a factor, hampering the protein crystal nucleation. The comparison of the model predictions with experimental results proved that nucleation kinetics is governed by kinetic (not by energetic) factors.     

Synthesis and Mesomorphic Properties of New Columnar Liquid Crystal Containing 1,3,5-triiminebenzene with Pendant 1,3,4-thiadiazole Group

A novel discotic liquid crystal series based on 1,3,5-benzenetrisazomethine derivatives with three pendant 2-amino-5-(4′-n-alkoxy)phenyl-1,3,4-thiadiazole has been synthesized, which is the first columnar molecules containing 1,3,4-thiadiazole moiety exhibiting a discotic liquid crystal. The molecular structure of compounds was confirmed by FT-IR, ¹H-NMR, and mass spectroscopy and elemental analysis. The electron excitation properties of these compounds were investigated by UV-vis absorption spectroscopy. Their liquid crystalline properties were studied by polarizing optical microscopy and differential scanning calorimetry. The formation of a columnar mesophase was found to be dependent on the number of methylene unit in alkoxy side chains.     

Crystal and molecular structure of Zoapatanolide B,a sesquiterpene lactone

The crystal and molecular structure of the natural product Zoapatanolide B has been determined by X-ray crystallographic analysis. The compound, obtained from the leaves ofMontanoa tomentosa, is a heliangolide sesquiterpene lactone. The two molecules of the asymmetric unit are linked together by hydrogen bonding and this results in conformational differences when the ester side chains (at C(9)) are compared.     

Polymorphism and design of noncentrosymmetric crystals of 4-hydroxybenzaldehyde-4-nitrophenylhydrazone and N′-(2-phenyl-1H-indole-3-aldehyde)-4-nitrophenylhydrazone

The products of crystallization of 4-hydroxybenzaldehyde-4-nitrophenylhydrazone (I) and N′-(2-phenyl-1H-indole-3-aldehyde)-4-nitrophenylhydrazone (II) from different solvents are studied by X-ray diffraction with the aim of examining the polymorphism of hydrazone derivatives. The structural features of the crystals prepared are analyzed. It is shown that, in the case when molecules of organic compounds with a high molecular hyperpolarizability are capable of forming stable acentric supramolecular associates in the crystal, knowledge of their polymorphism offers strong possibilities for designing the noncentrosymmetric crystal structure necessary for a manifestation of the nonlinear optical activity.