A new anisotropic soft-core model for the simulation of liquid crystal mesophases

A new anisotropic soft-core model is presented, which is suitable for the rapid simulation of liquid crystal mesophases. The potential is based on a soft spherocylinder, which can be easily tuned to favor different liquid crystal mesophases. The soft-core nature of the potential makes it suitable for long-time step molecular dynamics or dissipative particle dynamics simulations, particularly as a reference model for mesogens or as an anisotropic solvent for use in combination with atomistic models. Results are presented for two variants of the new potential, which show different mesophase behaviors. Variants of the potential can also be linked together to produce more complicated molecular structures. Here, as an example, results are provided for a model multipedal liquid crystal, which has eight liquid crystalline groups linked to a central core via semiflexible chains. Here, despite the complexity of molecular structure, the model succeeds in showing the spontaneous formation of a liquid crystal phase. The results also demonstrate that there is a very strong coupling between the internal structure of the multipedal mesogen and the molecular order of the phase, with the mesogen spontaneously undergoing major structural rearrangement at the transition to the liquid crystal phase.     

The influence of crystal packing on second-order non-linear optical properties of new chiral ferrocenyl materials

New chiral enantiomerically pure ferrocenyl chromophores for non-linear optics (NLO) have been synthesized and their crystal structures were determined by X-ray analysis. The correlation between crystal packing and bulk NLO efficiency was studied exemplifying again the difficulty to preview crystal packing from the molecular structure. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASferrocene / chiral non-racemic / non-linear optics / X-ray structure/ crystal packing     

A re-examination of the crystal structure and molecular packing of α-nylon 6

During studies of polymorphism of polyamides with an even number of carbon atoms, it was thought worthwhile to re-examine the crystal structure and molecular packing of the α-form of nylon 6 by more advanced instrumentation and computing facilities. X-ray diffraction measurements on mono-oriented fibres confirm that nylon 6 crystallizes in the monoclinic system, with crystallographic parameters: $\text{a = 9.71}\text{A}\text{?}\text{, b = 8.19}\text{A}\text{?}\text{, c = 17.40}\text{A}\text{?}$ (fibre axis), γ = 115°, 8 monomeric units in the unit cell and space group P2₂. The crystal packing can be represented by parallel sheets of hydrogen-bonded antiparallel chains; the sheets are all oriented in the same sense and alternately displaced 3.73 Å along the c axis. The hydrogen bond distance is 2.98 Å. Although the structure was substantially similar to that previously found, some differences were observed; they are discussed on the basis of theoretical calculations.     

A new equilibrium form of zircon crystal

The surface tensions of the {100}, {110}, {101} and {211} faces are calculated by using Machenzie's method. A new equilibrium form of zircon crystal is then derived on the basis of Wulf's Law. This theoretic form consists of {100} prismatic faces and {211} pyramidal faces, which is different from that of the prediction by the periodic bond chain theory. The discovery of the equilibrium form of zircon crystal provides a clue for understanding of the morphology of zircon crystals formed in deep magma chamber, indicating that zircon morphology is an indicator of crystallization conditions.     

Effect of crystal packing on the structures of polymeric metallocenes

The pressure dependencies of the crystal structures of the polymeric metallocenes lithium cyclopentadienide (LiCp) and potassium cyclopentadienide (KCp) have been determined by synchrotron X-ray powder diffraction. The decrease of the volume of LiCp by 34% up to a pressure of p = 12.2 GPa and of KCp by 23% at p = 5.3 GPa as well as the bulk moduli of K = 7.7 GPa for LiCp and 4.9 GPa for KCp indicate a high compressibility for these compounds. The crystal structures of KCp have been determined up to p = 3.9 GPa. An increase of the bend angle is found from 45 degrees at p = 0 GPa up to 51 degrees at p = 3.9 GPa. This variation is completely explained by a model invoking attractive K+ Cp- interaction and repulsive nonbonded carbon-carbon interactions. It is proposed that the bend angle in the polymeric alkali metal metallocenes is the result of the optimization of the crystal packing.     

On crystal chemistry of mercury chromates. Details of fragmentation and packing in crystal structures

In the crystal structures of the minerals edoylerite, deanesmithite, and wattersite and in the structures of the compounds HgCrO₄, Hg₃O₂CrO₄ (analog of the mineral schuetteite), and Pb₂HgO₂CrO₄, there are atomic groups or fragments linked by relatively strong (covalent) bonds. The anion fragments represented by CrO₄ tetrahedra are condensed into pairs and chains, in which adjacent tetrahedra are related by symmetry centers. The cation fragments form various patterns from zigzag ribbons of [Hg₄S₄] rings to ribbons and frameworks with [Hg₆O₂] r-octahedra, [Hg₂CrO] and [Pb₂HgO] triangles, and [Hg₆Cr₂O₄] (stella quadrangula) groups. The symmetry of the fragments and their assemblies is analyzed. Analysis of the reference crystal-forming planes has revealed cation sublattices (close to the F-cubic sublattice in two cases), determining the typical features of the structures.Original Russian Text Copyright © 2004 by S. V. Borisov, S. A. Magarill, and N. V. PervukhinaTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 471–479, May–June, 2004.     

Relationship between crystal packing and high electron mobility in the single crystal of thienyl-substituted methanofullerene

The X-ray crystal structure of 1-(3-methoxy-carbonyl)-propyl-1-thienyl-[6,6]-methanofullerene (ThCBM) was determined and the electron mobility of the single crystal was measured using a TRMC method to reveal high electron mobility (2 cm(2) V(-1) s(-1)) along the long axis (a-axis) and its remarkable anisotropy (7 times).     

A new crystal form of the NSAID dexketoprofen

Dexketoprofen [(2S)‐2‐(3‐benzoylphenyl)propanoic acid], C₁₆H₁₄O₃, is the S‐enantiomer of ketoprofen, a nonsteroidal anti‐inflammatory drug (NSAID) that has analgesic, antipyretic and anti‐inflammatory properties, and finds applications for the short‐term treatment of mild to moderate pain. A new crystalline phase of dexketoprofen is reported. Its solid‐state structure was determined by single‐crystal X‐ray diffraction (SCXRD). The molecular structure of the two independent molecules found in the asymmetric unit of this new phase ( DXKP‐β ) were compared to those of the already known crystal form of dexketoprofen ( DXKP‐α ) and with the S‐enantiomer of the racemic compound. The three different conformers of dexketoprofen found in DXKP‐α and DXKP‐β were then investigated by computational methods. The optimized structures are very close to the corresponding starting geometries and do not differ significantly in energy. The crystal packing of DXKP‐β was studied by means of Hirshfeld surface (HS) analysis; interaction energies were also calculated. A comparison with DXKP‐α shows close similarities between the two crystal forms, i.e. in both cases, molecules assemble through the catemer O—H…O synthon of the carboxylic acid stabilized by additional C—H…O contacts and, accordingly, the interaction energies, as well as the contributions to the HS area, are very similar. Finally, the thermal behaviour of the two polymorphs of dexketoprofen was assessed by means of XRD (both from single crystal and microcrystalline powder) and differential scanning calorimetry (DSC); both crystal forms are stable under the experimental conditions adopted (air, 300–350 K for DXKP‐α and 300–340 K DXKP‐β ) and no solid–solid phase transition occurs between the two crystal forms in the investigated temperature range (from 100 K up to ca 350 K).     

A new strategy of transforming pharmaceutical crystal forms

The robust nature of network materials allows them to (for example) respond to external stimuli such as pressure, temperature, light, or gas/solvent adsorption and desorption. There is difficulty in retaining long-range order in purely molecular organic solids, due to weak intermolecular interactions such as van der Waals forces. Here, we show gas-induced transformations of the well-known pharmaceuticals clarithromycin and lansoprazole. For clarithromycin, the stimulus is capable of converting the kinetic solvate and guest-free crystal forms to the commercial thermodynamically stable polymorph with a huge saving in energy cost relative to industrially employed methods. The synthesis of the marketing form of lansoprazole involves a solvate that readily decomposes and that is stirred in water, filtered, and dried intensively. Our method readily circumvents such synthetic problems and transforms the sensitive solvate to the marketed drug substance with ease. Such expedient transformations hold great implications for the pharmaceutical industry in general when considering the ease of transformation and mild conditions employed.     

Partial fluorination overcomes herringbone crystal packing in small polycyclic aromatics

[structure: see text] We report the synthesis and characterization of partially fluorinated condensed tetracyclic aromatic compounds. Typical edge-to-face/herringbone packing of nonfluorinated analogues is replaced here by columnar stacks with disk planes orthogonal to the columnar axes. Enhanced pi-overlap results with overlaid electron-poor and -rich regions.     

Structural disturbance and lattice relaxation in molecular crystal packing calculations

Lattice calculations on impurity-containing crystals are reported, made with the help of standard intermolecular potentials. A lattice containing a substitutional impurity, or a vacancy, relaxes in its vicinity, the equilibrium structure being little changed in either orientation or translation. The small changes do however enable the strain energy to be greatly reduced. The energies are sensitive to the assumed intermolecular potentials but the optimised structures are rather insensitive. An impurity larger than the host (tetracene in anthracene) adopts closely the host orientation. This applies also to a smaller impurity (e.g. napthalene in anthracene) but in this case the impurity may move away from the lattice site to one of two inversion related displaced positions. The cage surrounding a vacant site relaxes little, leaving the “hole” more or less intact. Lattice relaxations found in this way seem too small to accommodate guest—host photochemical reactions in those lattices which, in the pure crystal, are photochemically stable.     

Single-molecule-magnet behavior and spin changes affected by crystal packing effects

Five Mn 3Zn 2 heterometallic complexes have been synthesized and structurally and magnetically characterized. Spin ground states up to S = 6 have been observed for these complexes and are shown to depend on the cocrystallizing cation and on the terminal ligand. Large axial zero-field interactions ( D = -1.16 K) are the result of near-parallel alignment of the Mn (III) Jahn-Teller axes. High-frequency electron paramagnetic resonance, single-crystal magnetization hysteresis, and alternating current susceptibility measurements are presented to characterize [NEt 4] 3[Mn 3Zn 2(salox) 3O(N 3) 6X 2] [X (-) = Cl (-) ( 1), Br (-) ( 2)] and [AsPh 4] 3[Mn 3Zn 2(salox) 3O(N 3) 6Cl 2] ( 3) and reveal that 1 and 2 are single-molecule magnets ( U eff = 44 K), while 3 is not.     

Vibrational spectra,packing calculations and crystal structure of 1,2-diiodobenzene

The vibrational spectra of crystalline 1,2-diiodobenzene are presented. Spectroscopic evidence in combination with packing and lattice frequence calculations point to a tetramolecular unit cell, space group C⁵_2h- Approximate molecular positions are given.     

Perspectives on the crystal densities and packing coefficients of explosive compounds

We have investigated possible relationships between four crystal properties: experimental densities and computed intrinsic molecular volumes, packing coefficients and amounts of free space per molecule in the crystal lattices. Our focus was upon C-, H-, N-, O-containing explosive compounds. The objectives were to gain some insight into how densities might be increased, to improve detonation performance, and the amounts of free space per molecule decreased, to counter one of the factors promoting undesired sensitivity to accidental stimuli. The issue of molecular planarity was also examined. The best correlation found between the four properties is perhaps a surprising one: The free space per molecule increases as the molecules are bigger. On the other hand, some relationships that seem to be intuitively reasonable turn out to be quite weak. The principal positive conclusions are that it is desirable for explosive compounds to be composed of molecules that are small and preferably planar.

     

Influence of protonation on crystal packing and thermal behaviour of tert-butylammonium decavanadates

Two tert-butylammonium decavanadate(V) salts with the formulae [(CH₃)₃CNH₃]₆[V₁₀O₂₈] · 8H₂O (1) and [(CH₃)₃CNH₃]₄[H₂V₁₀O₂₈] (2), have been synthesized and their crystal structures have been determined by means of single-crystal X-ray diffraction. The crystal structure of compound 1 is stabilized by electrostatic forces and an extensive network of hydrogen contacts involving anions, cations and water molecules. The anions and cations of this compound are arranged in layers perpendicular to the [010] direction following the sequence, cation-anion-cation. In the crystal structure of compound (2), each dihydrogen decavanadate(V) anion is joined to three adjacent polyanions by means of O(6)---H ··· O(4) hydrogen contacts forming layers parallel to the plane ( 01) and the hydrophobic groups of the cations are disposed in layers parallel to the anionic sheets. The thermal behaviour of both compounds has been studied. Compound 1 is an octahydrate and its thermal decomposition begins at 70°C with the loss of water of crystallization, while compound 2 is anhydrous and is consequently more stable, with decomposition starting at 200°C.     

锥角堆积模型应用——-f电子组元素新类型金属有机化合物的推测与合成

过渡金属有机化合物一般遵守18或16电子规律.但对于d电子组的前几个元素(Ti,Zr,Hf)和f电子组元素却并不如此.Marks提出过:铀(IV)、钍(IV)的某些有机化合物分别遵循20或22电子规律.但这种观点无法解释许多实际存在的化合物.我们在最近提出:当配位场作用很弱时,空间堆积作用是不容忽视的.在中心离子周围配位体的堆积达到一定程度时,尽管从表观上看,中心离子的外层电子与配位电子数之和还没有达到“下一级惰性气体结构”,但配位体之间的相互推斥作用阻止了更多的配体进入配位层.从几何角度来进行研究时,这种由空间堆积效应与成键效应达成的平衡应当表现为:化合物结构中配位体的立体角系数(Solid Angle Factor,Q/4π)之总和(SAS)分布在一个“稳定区间”之内.而后者的位置与金属的氧化态有关.我们先后对近300个已知的f电子组元素的配合物与金属有机化合物的结构进行了计算.结果表明中心离子周围的空间占据呈现“饱和”现象.饱和堆积程