Polymorph specific RMSD local order parameters for molecular crystals and nuclei: α-, β-, and γ-glycine

Crystal nucleation is important for many processes including pharmaceutical crystallization, biomineralization, and material synthesis. The progression of structural changes which occur during crystal nucleation are often described using order parameters. Polymorph specific order parameters have been developed for crystallization of spherically symmetric particles; however, polymorph specific order parameters for molecular crystals remain a challenge. We introduce template based polymorph specific order parameters for molecular crystals. For each molecule in a simulation, we compute the root mean squared deviation (RMSD) between the local environment around the molecule and a template of the perfect crystal structure for each polymorph. The RMSD order parameters can clearly distinguish the α-, β-, and γ-glycine polymorph crystal structures in the bulk crystal and also in solvated crystallites. Surface melting of glycine crystallites in supersaturated aqueous solution is explored using the newly developed order parameters. The solvated α-glycine crystallite has a thinner surface melted layer than the γ-glycine crystallite. α-glycine forms first out of aqueous solution, so surface melted layer thickness may provide insight into interfacial energy and polymorph selection.     

Two polymorphic forms of N‐(4‐chloro­phenyl)‐5‐[(4‐chlorophenyl)aminomethyl]‐6‐methyl‐2‐phenyl­pyrimidin‐4‐amine

Two polymorphic forms of the title compound, C₂₄H₂₀Cl₂N₄, were obtained and characterized using X‐ray crystal structure analysis. Colourless crystals of polymorph (Ia) were obtained from the oily mother residue. Recrystallization of polymorph (Ia) from an acetone–methanol mixture resulted in pale‐yellow crystals of polymorph (Ib). The major feature distinguishing the two polymorphic forms is their inter­action modes, and hence their packing arrangements. In the crystal structure of polymorph (Ia), there are N—H⋯N hydrogen bonds and also aromatic π–π stacking inter­actions between mol­ecules. The mol­ecules of polymorph (Ib) are linked by N—H⋯Cl hydrogen bonds only.     

Effect of High Pressure on the Polymorphs of Paracetamol

Effect of hydrostatic pressure on the two (I – monoclinic and II – orthorhombic) polymorphs of paracetamol was studied by X-ray diffraction in the diamond anvil cell at pressures up to 4.5 GPa (for the monoclinic form) and up to 5.5 GPa (for the orthorhombic form). The two groups of phenomena were studied: (i) the anisotropic structural distortion of the same polymorph, (ii) transitions between the polymorphs induced by pressure. The anisotropy of structural distortion of polymorphs I and II was well reproducible from sample to sample, also from powder samples to single crystals. The bulk compressibility of the two forms was shown to be practically the same. However, a noticeable qualitative difference in the anisotropy of structural distortion was observed: with increasing pressure the structure of polymorph II contracted in all the directions showing isotropic compression in the planes of hydrogen-bonded molecular layers, whereas the layers in the structure of the polymorph I expanded in some directions. Maximum compression in both polymorphs I and II was observed in the directions normal to the molecular layers. The transitions between the polymorphs induced by pressure were poorly reproducible and depended strongly on the sample and on the procedure of increasing/decreasing pressure. No phase transitions were induced in the single crystals of the monoclinic polymorph at pressures at least up to 4GPa, although a partial transformation of polymorph I into polymorph II was observed at increased pressure in powder samples. Polymorph II transformed partly into the polymorph I during grinding. The transformation could be hindered if grinding was carried out in CCl₄. This revised version was published online in August 2006 with corrections to the Cover Date.     

双亲水嵌段共聚物存在下特殊形貌的BaC2O4晶体合成

近年来 ,无机新型材料已经广泛应用于各个技术领域 ,而对于所合成材料的大小和形貌的调控已经成为该材料能够应用于催化、药物、电子、陶瓷、染料和化妆品等领域的一个关键因素[1 ] .利用水溶性有机添加剂作为晶体生长的诱导剂来控制无机晶体的形貌和晶型已有很多报道 .例如 :多种生物分子[2～ 5] 和普通合成的大分子添加剂[6] 对 Ca CO3 晶体在成核、晶化和生长过程中的晶型和形貌有着明显的控制作用 .近年来 ,双亲水嵌段共聚物的功能型高分子 (DHBC) [7]已经发展成为能够有效控制无机晶化过程的新型晶体生长的调控剂 .这类高分子通常是…     

Experimental investigation and population balance equation modeling of solid lipid nanoparticle aggregation dynamics

Solid lipid nanoparticles (SLNs) have applications in drug delivery and the encapsulation of bioactive, lipophilic compounds. However, SLNs tend to aggregate when stored due to the lipid crystals undergoing a polymorphic transformation from the unstable α form to the stable β form. We developed a population balance equation (PBE) model for prediction of average polymorph content and aggregate size distribution to better understand this undesirable behavior. Experiments with SLNs stored at room temperature showed that polymorphic transformation was the rate determining step for our system, SLNs with smaller initial size distributions aggregated more rapidly, and aggregates contained particles with both α and β crystals. Using parameter values estimated from our data, the PBE model was able to capture the bimodal nature of aggregate size distributions, the α-to-β polymorph ratio, and the faster aggregation dynamics of SLNs with smaller initial size distributions. However, the model was unable to adequately capture the fast disappearance rate of primary particles, the broad size distributions of formed aggregates, and the significant α content of aggregating particles. These discrepancies suggest that a PBE model which accounts for polymorph content as an internal variable along with aggregate size may be required to better reproduce experimental observations.     

Self-assembled monolayers: solidification of carbamazepine in Form II at an interface

A self-assembled monolayer incorporating well-spaced biphenyl moieties initiates solidification of carbamazepine at its interface. A detailed analysis of the resulting crystals using X-ray powder diffraction, FTIR-ATR as well as thermomicroscopy, indicates a preference for its crystallization in trigonal (Form II) polymorph.     

DSC and adiabatic calorimetry study of the polymorphs of paracetamol

Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs. This revised version was published online in July 2006 with corrections to the Cover Date.     

A twinned triclinic polymorph of dibromidotetra­kis(tetra­hydro­furan‐κO)magnesium(II)

The title compound, [MgBr₂(C₄H₈O)₄], forms crystals which appear to be monoclinic but are actually twinned triclinic. The current form is a new triclinic polymorph, with Z′= 2, in addition to the already known tetra­gonal polymorph. Although the geometric parameters of the two polymorphs agree well, their packing patterns are completely different.     

蔗糖/精氨酸体系中碳酸钙结晶的仿生合成

该文以更加接近生物矿化的方法研究了蔗糖/精氨酸体系对碳酸钙晶体取向、形貌和晶型的控制作用.XRD 分析表明,在蔗糖/L-精氨酸混合体系中合成的晶体主要为碳酸钙的球霰石晶型及少量的方解石型,在单独的蔗糖或L-精氨酸溶液中基本是球霰石晶型.SEM分析表明,蔗糖和L-精氨酸均可诱导形成特殊形貌的碳酸钙.实验结果表明,蔗糖/精...     

Nucleation of calcium carbonate as polymorphic crystals in the presence of lipid A-diphosphate

The well-defined structure of lipid A-diphosphate in aqueous solutions provides a way of observing the formation of calcium carbonate crystals. The crystals are either tetrahedral or rhombohedral calcite at a volume fraction of phi = 5.4 x 10 (-4) at pH 5.8 or the vaterite polymorph of CaCO(3) at a volume fraction of phi = 7.8 x 10 (-4) at pH 5.8. In both cases, nucleation, adsorption pH, and the shape-dependent template of lipid A-diphosphate control the formation of the calcite and vaterite.     

A general set of order parameters for molecular crystals

Crystallization is fundamental to many aspects of physics and chemistry in addition to being of technological relevance, for example, in the chemical, food, and pharmaceutical industries. However, the design of crystalline materials and crystallization processes is often challenging due to the many variables that can influence the process. As a part of an effort to gain a molecular-level understanding of the way molecules aggregate and organize themselves into crystal structures, in this work we present a new method to construct order parameters suitable for the study of crystallization and polymorph transformations in molecular systems. Our order parameters can be systematically defined for complex systems using information that can be obtained from simple molecular dynamics simulations of the crystals. We show how to construct the order parameters for the study of three different systems: the formation of α-glycine crystals in solution, the crystallization of benzene from the melt, and the polymorph transformation of terephthalic acid. Finally, we suggest how these order parameters could be used to study order-disorder transitions in molecular systems.     

Structure and morphology of nylon 46 lamellar crystals: Electron microscopy and energy calculations

A detailed electron microscopy study of the structure and morphology of lamellar crystals of nylon 46 obtained by crystallization from solution has been carried out. Electron diffraction of crystals supported by X‐ray diffraction of their sediments revealed that they consist of a twinned crystal lattice made of hydrogen‐bonded sheets separated 0.376 nm and shifted along the a‐axis (H‐bond direction) with a shearing angle of 65°. The interchain distance within the sheets is 0.482 nm. These parameters are similar to those previously described for nylon 46 lamellar crystals grown at lower temperatures. A combined energy calculation and modeling simulation analysis of all possible arrangements for the crystal‐packing of nylon 46 chains, in fully extended conformation, was performed. Molecular mechanics calculations showed very small energy differences between α (alternating intersheet shearing) and β (progressive intersheet shearing) structures with energy minima for successive sheets sheared at approximately 1/6 c and 1/3 c. A mixed lattice composed of a statistical array of α and β structures with such sheet displacements was found to be fully compatible with experimental data and most appropriate to describe nylon 46 lamellar crystals. Annealing of the crystals at temperatures closely below the Brill transition induced enrichment in β structure and increased chain‐folding order. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 41–52, 2000     

Controlled synthesis of calcium carbonate in a mixed aqueous solution of PSMA and CTAB

A mixed system of poly (styrene-alt-maleic acid) (PSMA) and cetyltrimethylammonium bromide (CTAB) was used as a very effective crystal growth modifier to direct the controlled synthesis of CaCO₃ crystals with various morphologies and polymorphs. The as-prepared products were characterized with scanning electron microscopy and X-ray diffraction. It was found that the concentrations and relative ratios of PSMA and CTAB in the mixed aqueous solution were turned out to be important parameters for the morphology and polymorph of CaCO₃ crystals. Various morphologies of CaCO₃ crystals, such as hollow microsphere, peanut and so on, were produced depending on the concentrations and relative ratios of PSMA and CTAB. Moreover, the formation mechanisms of CaCO₃ crystals with different morphologies were discussed.     

Biosynthesis of CaCO3 crystals of complex morphology using a fungus and an actinomycete

The biosynthesis of CaCO3 by reaction of aqueous Ca2+ ions with a fungus, Fusarium sp., and an actinomycete, Rhodococcus sp. (both plant organisms), is described. In the case of the fungus, cruciform-shaped calcite crystals are obtained (SEM picture A) while the actinomycete yielded the unstable polymorph of CaCO3, vaterite (SEM picture B). Specific proteins secreted by the microorganisms are responsible for the morphology and crystallography control observed. A highlight of this approach is that the microorganisms also provide CO2 for reaction with the Ca2+ ions, making the crystals completely biogenic.     

水溶液中P123对CaCO3微粒形貌的调控

在三嵌段共聚物P123水溶液体系中,合成了特殊形貌的层面光滑的碳酸钙层状聚集体、具有多级结构的碳酸钙层状聚集体和仙人球状的碳酸钙粒子.探讨了反应时间、聚合物浓度和反应温度对碳酸钙粒子形貌和晶型的影响,采用扫描电子显微镜(SEM)、X射线粉末衍射(XRD)及红外吸收光谱对合成样品的形貌、结构进行了表征.结果表明,聚合物浓度和反应温度对碳酸钙粒子形貌和晶型具有重要的影响.利用周期键链(PBC)理论模型解释了层状结构碳酸钙聚集体的形成过程.     

The Polymorphs of L‐Phenylalanine

The solid‐state structure of the amino acid phenylalanine (Phe) offers a potential key to understanding the behavior of a large class of important aromatic compounds. Obtaining good single crystals is, however, notoriously difficult. The structure of the common polymorph of Phe, form I, was first reported by Weissbuch et al. (as D ‐Phe) in 1990, but the correctness of the published C2 unit cell with two disordered molecules in the asymmetric unit was later questioned and other space groups suggested. The identity of form I of L ‐Phe is here established to be P2₁ with Z′=4, based on data from a well‐diffracting single crystal grown from an acetic acid solution of the amino acid. A second new polymorph, form IV, together with the two recently described forms II and III provide unprecedented information on the structural complexity of this essential amino acid. It is furthermore documented that the racemate, dl ‐Phe, does not grow proper single crystals.     

Polymorphism and phase transition behavior of 6,6′‐bis(chloromethyl)‐1,1′,4,4′‐tetramethyl‐3,3′‐(p‐phenylenedimethylene)bis(piperazine‐2,5‐dione)

A crystallographic investigation of the title compound, C₂₂H₂₈Cl₂N₄O₄, using crystals obtained under different crystallization conditions, revealed the presence of two distinct polymorphic forms. The molecular conformation in the two polymorphs is very different: one adopts a `C' shape, whereas the other adopts an `S' shape. In the latter, the molecule lies across a crystallographic twofold axis. The `S'‐shaped polymorph undergoes a reversible orthorhombic‐to‐monoclinic phase transition on cooling, whereas the structure of the `C'‐shaped polymorph is temperature insensitive.     

Crystals of Benzamide,the First Polymorphous Molecular Compound,Are Helicoidal

The growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II ) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X‐ray diffraction reveal a complex lamellar structure of benzamide form II needle‐like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non‐classical morphologies.     

Crystals of Benzamide,the First Polymorphous Molecular Compound,Are Helicoidal

The growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II ) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X-ray diffraction reveal a complex lamellar structure of benzamide form II needle-like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non-classical morphologies.     

Temperature-dependent Crystallization and Phase Transition of Poly(L-lactic acid)/CO_2 Complex Crystals

Semicrystalline polymers can crystallize in the unique crystalline polymorph and show different phase behaviors under the highpressure CO_2 treatment. Understanding such unique crystallization and phase transition behavior is of fundamental importance for the CO_2-assisited processing of semicrystalline polymers. Herein, we investigated the polymorphic crystalline structure, phase transition, and structureproperty relationships of poly(L-lactic acid)(PLLA) treated by CO_2 at different pressures(1-13 MPa) and crystallization temperatures(T_c's, 10-110 °C). PLLA crystallized in the PLLA/CO_2 complex crystals under 7-13 MPa CO_2 at T_c≤50 °C but the common α crystals under the high-pressure CO_2 at T_c≥70 °C. Solid-state nuclear magnetic resonance analysis indicated that the PLLA/CO_2 complex crystals possessed weaker interactions between the PLLA chains than the common α crystals. The PLLA/CO_2 complex crystals were metastable and transformed into the thermally stableα crystals via the solid-to-solid route during heating or annealing at the temperature above 50 °C. The complex crystals of PLLA produced at low T_c was more ductile than the α crystals due to the lower crystallinity and the plasticizing effect of CO_2.