X射线晶体学结合电子晶体学在复杂无机晶体结构解析中的应用

自然界中的材料,比如无机材料,有机材料,生物材料等等,均有其独特的物理和化学性质。而材料的性能又与材料的结构息息相关,只有充分了解了材料的结构,才能更加深入的研究材料性质。因此,材料结构的确定在化学、物理、生物等学科中的显得尤为重要。X射线晶体学作为传统的结构解析技术仍然是目前最重要的结构解析手段,但是对于复杂结构,X射线衍射晶体学解析结构也存在一些不足,往往需要其他技术手段相补充才能完成复杂结构的结构解析。电子晶体学虽然起步比X射线晶体学晚,但是,经过近几十年的发展,已经是结构解析领域一个非常重要的手段。本文将主要介绍X射线晶体学结合电子晶体学在复杂无机晶体结构解析中的应用。     

Electron crystallography and organic materials with non-linear optical properties

Results of electron microscopic studies of crystal structures of a number of bis-benzylidene cyclohexanones are presented. It is shown that some of these compounds are efficient crystalline non-linear optically active (NLO) chromophores with second harmonic generation (SHG) properties. Appropriately functionalized chromophores of this type can be used as a polycondensation comonomer to produce partly crystalline main-chain NLO-active polymers. Electron diffraction crystal structural data, obtained for very small crystals, allowed us to get reasonable estimations of macroscopic crystal NLO-coefficients, relating quantum-chemically calculated molecular first hyperpolarizability components to crystal axes.     

Morphology and Crystal Structure of Wholly Aromatic All-Para Polyamide-Hydrazide Polymers

The morphology of some amide-hydrazide polymers of the type useful for high-modulus X-500 class fibers has been characterized by transmission electron microscopy of thin films crystallized from dilute solution. Selected area electron diffraction was used to characterize the crystallinity and crystal structure of the thin films and precipitated polymer. The films were cast from concentrated solutions and crystallized by heating the films. The results of these studies revealed several unique features relative to the crystal structure of the all-para polymers. Thin films of the crystallized polymer showed a distinctive crystalline texture—the molecular chains were found to be preferentially oriented parallel to the film plane and randomly oriented about an axis normal to the film plane. Electron diffraction measurements showed equatorial reflection maxima at tilt angles of = 30, ±48, and =59 when the films were tilted on an axis parallel to the film plane. From these results a tentative crystal unit cell and theoretical crystal density were determined: a = 8.5 [Agrave], b = 4.9 Å, c (chain axis) = 29.6 Å, p (density) =1.51 g/cc. The value a/b = 1.735, which is very near 3¹/², implies essentially hexagonal packing of the chains. Crystallization from dilute solution revealed lamellar structures resembling “single crystals” in the electron microscope similar to those observed in other crystalline polymers. However, in contrast to these other polymers, these “crystals” are not likely to contain folded chains because of the very rigid nature of the all-para poiyamide-hydrazide.     

New Approaches for Solving Crystal Structures from Powder Diffraction Data

The determination of crystal structures from single crystal diffraction data can generally be carried out routinely and straightforwardly. However, many crystalline solids can be obtained only as microcrystalline powders and are not suitable for investigation by conventional single crystal diffraction methods. In the past, this problem has limited the ability to elucidate the structural properties of such materials. For the wide range of materials in this category, there is clearly a pressing need to develop and exploit techniques that allow crystal structures to be solved from powder diffraction data. Although traditional techniques for structure solution from powder diffraction data have been applied successfully in several cases, these techniques have certain intrinsic limitations, and for the case of organic molecular crystals the challenges that must be overcome are particularly severe. For these reasons, our recent research has focused on the development and implementation of new methodologies for structure solution from powder diffraction data, leading to new “direct-space” techniques for structure solution in which a hypersurface based on the profile R-factor is searched using Monte Carlo or Genetic Algorithm techniques. This paper presents a brief overview of the problems and challenges associated with structure solution from powder diffraction data. The foundations of the techniques that we have developed are described, and illustrative examples (from the field of organic molecular crystals) are given to highlight the application of these techniques.     

How to determine structures when single crystals cannot be grown: opportunities for structure determination of molecular materials using powder diffraction data

Many crystalline solids cannot be prepared as single crystals of sufficient size and/or quality for structure determination to be carried out using single crystal X-ray diffraction techniques. In such cases, when only polycrystalline powders of a material are available, it is necessary instead to tackle structure determination using powder X-ray diffraction. This article highlights recent developments in the opportunities for determining crystal structures directly from powder diffraction data, focusing on the case of molecular solids and giving particular attention to the most challenging stage of the structure determination process, namely the structure solution stage. In particular, the direct-space strategy for structure solution is highlighted, as this approach has opened up new opportunities for the structure determination of molecular solids. The article gives an overview of the current state-of-the-art in structure determination of molecular solids from powder diffraction data. Relevant fundamental aspects of the techniques in this field are described, and examples are given to highlight the application of these techniques to determine crystal structures of molecular materials.     

Single-chain single crystals

Single-chain single crystals of isotactic polystyrene and poly(ethylene oxide) were studied by using transmission electron microscopy, high resolution electron microscopy, electron diffraction. Single-chain single crystals were prepared by spreading a dilute solution of polymers on a water surface and collecting the resulting single-chain particles on copper grids, followed by isothermal crystallization. A statistical analysis of the sizes of single-chain crystals was found to match with the known molecular weight distribution of original sample, indicating the particles to be composed of single chain. Observation of the morphology and electron diffraction gave evidence of the single crystal nature. Regular-shaped single-chain crystals were obtained after isothermal crystallization for a longer time. By close observation, several types of morphologies were found for single-chain crystals of isotactic polystyrene and poly(ethylene oxide); in addition to the conventional morphologies observed for multi-chain crystals, new morphologies were observed in both cases. The morphologies of poly(ethylene oxide) were explained according to the crystal structure and twin modes. Tent-like single-chain crystals were often observed. Because of the small size of the crystals, they can avoid collapse on the substrate. The crystalline c-axis of single-chain crystals were found to orient preferably in the direction normal to the substrate. The investigation of electron diffraction and high resolution electron microscopy revealed that the structure of the single-chain crystals of isotactic polystyrene is the same as for multi-chain crystals. A reasonable explanation is given for the unusual resistance to electron irradiation and the missing of lower-index reflections. Regular periodic stripes were found on the top surface of single-chain crystal of isotactic polystyrene with an average periodic length in accordance with (220) spacing. In addition, a statistical thermodynamics theory was developed for single-chain crystal. It is found that the equilibrium dimensions are related to molecular weight and annealing temperature, while the equilibrium melting temperature depends on molecular weight.     

Image processing and lattice determination for three-dimensional nanocrystals

Three-dimensional nanocrystals can be studied by electron diffraction using transmission cryo-electron microscopy. For molecular structure determination of proteins, such nanosized crystalline samples are out of reach for traditional single-crystal X-ray crystallography. For the study of materials that are not sensitive to the electron beam, software has been developed for determining the crystal lattice and orientation parameters. These methods require radiation-hard materials that survive careful orienting of the crystals and measuring diffraction of one and the same crystal from different, but known directions. However, as such methods can only deal with well-oriented crystalline samples, a problem exists for three-dimensional (3D) crystals of proteins and other radiation sensitive materials that do not survive careful rotational alignment in the electron microscope. Here, we discuss our newly released software AMP that can deal with nonoriented diffraction patterns, and we discuss the progress of our new preprocessing program that uses autocorrelation patterns of diffraction images for lattice determination and indexing of 3D nanocrystals.     

Effect of heat treatment on the morphology and structure of crystals,powders, and fibers of polyacrylonitrile and saran

As part of a study of chemical and physical changes accompanying the formation of carbons by the pyrolysis of polymers, conventional electron microscopy, electron diffraction, and scanning electron microscopy techniques have been used to examine structural and morphological features of polyacrylonitrile (PAN) crystals, powder, and fibers, and of Saran and poly(vinylidene chloride) (PVDC) powder. Changes accompanying the heating of these polymers in air and in nitrogen have been investigated. PAN crystals grown from propylene carbonate were similar to those obtained by Klement and Geil. When heated in air at 220°C they retained their morphology, and electron diffraction gave the same reflections as PAN. On further heating to 400°C in nitrogen the morphology was retained, but the diffraction was lost. Crystals treated in nitrogen alone at 200°C showed morphology similar to that of the polymer. PAN powders and fibers retained discernable external features of their morphology on heating to 800°C. These results are discussed with reference to changes which take place when poly(vinylidene chloride) and Saran are heated in the range 150–180°C, which results in the loss of one hydrogen chloride per monomer unit, and are subsequently carbonized at 800°C. The development of pore structure and the adsorptive properties of Saran carbons are also discussed.     

First Preparations and Characterization of Conductive Polymer Crystalline Nanoneedles

Single crystalline nanoneedles of three families of the most studied conductive organic polymers - polythiophene, polyaniline and polypyrrole - were synthesized for the first time using an interfacial polymerization process that takes place with simultaneous crystallization. As the crystal growth is concurrent with polymerization, more ordered crystal packing can be expected. Most of the bulk conducting-polymer systems studied contains regions that are inhomogeneous. Single nanocrystals of conducting polymers have not been reported, although needle-shaped bulk crystals of the quarterphenyl cation radical salt have previously been studied. The investigation of processes in a nanodomain of a single crystal is critical in ascertaining the inherent electronic properties of polymer nanoelements. The organic conductive nanoneedles were characterized using TEM, HRTEM, electron diffraction, EDS, and EPR to establish their crystal structure and composition. Scanning tunneling microscopy/spectroscopy (STM/STS) investigation were conducted to examine their electronic behaviors, leading to the discovery of a field-induced conductance switching with response times on the millisecond level. The switch voltages are in the range of 3 to 4 volts in STM experiments, consistent with the trend of the band gap of the three polymers. The organic conductive nanoneedles with nano-tip having high density of mobile electron may serve as interesting elements for nanoscale electronics.     

On the role of single regiodefects and polydispersity in regioregular poly(3-hexylthiophene): defect distribution, synthesis of defect-free chains, and a simple model for the determination of crystallinity

Identifying structure formation in semicrystalline conjugated polymers is the fundamental basis to understand electronic processes in these materials. Although correlations between physical properties, structure formation, and device parameters of regioregular, semicrystalline poly(3-hexylthiophene) (P3HT) have been established, it has remained difficult to disentangle the influence of regioregularity, polydispersity, and molecular weight. Here we show that the most commonly used synthetic protocol for the synthesis of P3HT, the living Kumada catalyst transfer polycondensation (KCTP) with Ni(dppp)Cl(2) as the catalyst, leads to regioregular chains with one single tail-to-tail (TT) defect distributed over the whole chain, in contrast to the hitherto assumed exclusive location at the chain end. NMR end-group analysis and simulations are used to quantify this effect. A series of entirely defect-free P3HT materials with different molecular weights is synthesized via new, soluble nickel initiators. Data on structure formation in defect-free P3HT, as elucidated by various calorimetric and scattering experiments, allow the development of a simple model for estimating the degree of crystallinity. We find very good agreement for predicted and experimentally determined degrees of crystallinities as high as ～70%. For Ni(dppp)Cl(2)-initiated chains comprising one distributed TT unit, the comparison of simulated crystallinities with calorimetric and optical measurements strongly suggests incorporation of the TT unit into the crystal lattice, which is accompanied by an increase in backbone torsion. Polydispersity is identified as a major parameter determining crystallinity within the molecular weight range investigated. We believe that the presented approach and results not only contribute to understanding structure formation in P3HT but are generally applicable to other semicrystalline conjugated polymers as well.     

Nanoindentation of high performance semicrystalline polymers: A case study on PEEK

Semicrystalline high performance polymers (HPPs) exhibit desirable thermomechanical properties which make them suitable materials in several industries. However, when their mechanical characteristics are investigated at nanoscale, the inhomogeneous nature of semicrystalline HPPs results in measurement of localized properties which might not represent the overall material response. In this paper, the nanomechanics of poly(ether-ether-ketone) (PEEK) as a tough HPP is scrutinized using the nanoindentation technique. It is demonstrated how surface properties of this polymer can affect the nanoindentation measurements and reveal significant anomalies compared to the nanoindentation of fully crystalline or amorphous solids. In addition, polishing and annealing procedures are introduced as simple techniques that can be used for eliminating the inhomogeneity of the mechanical response of PEEK. Lastly, the capability of depth sensing indentation for determining the distribution of crystalline and amorphous sub-regions within semicrystalline solids is scrutinized and critically discussed.     

X-ray diffraction and scanning electron microscopy studies of calcium carbonate electrodeposited on a steel surface

Calcium carbonate was deposited on a stainless steel surface with the use of an electrical potential of 10 V. The crystals formed on the surface were examined with X-ray diffraction and with scanning electron microscopy, which revealed that calcite, vaterite and amorphous calcium carbonate was formed. Two different surface active polymers were added to the solution and their effect on the crystal structure was investigated. It was found that the more hydrophilic of the two polymers promoted calcite growth and suppressed vaterite growth. The more hydrophobic polymer completely inhibited vaterite growth. Both polymers decreased the amount of crystals formed on the steel surface, the more hydrophobic polymer being the most effective. The crystal inhibition efficiency was enhanced close to the cloud point of the polymers. The results were compared with the effect of poly(acrylic acid), a commonly used antiscalant. It was found that poly(acrylic acid) was about as efficient as the more hydrophobic polymer in decreasing the amount of calcium carbonate. At higher concentrations of poly(acrylic acid), almost all of the calcium carbonate precipitated in the amorphous form.     

Crystal growth rate in ultrathin films of poly(ethylene oxide)

Many dynamical properties of polymers, including segmental relaxation and chain diffusion, exhibit anomalies in thin‐film samples. We extend the studies of thin‐film dynamics to the case of semicrystalline polymers and present a study of the crystal growth rate for thin films of poly(ethylene oxide). We used optical microscopy and quartz crystal microbalance techniques to characterize the kinetics of crystallization for films with thicknesses from 40 to 1000 nm for a range of temperatures near the melting point. A remarkable slowing down of the crystal growth is observed at all temperatures studied for films with a thickness of less than ～100 nm. The results can be used to suggest reductions of the mobility of chains at the crystal/amorphous interface. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2615–2621, 2001     

Growth and morphology of single crystals of linear aliphatic polyesters

In order to elucidate the relations between morphological habits and chemical structure of polymers, poly(ethylene sebacate), poly(hexamethylene sebacate) and poly(decamethylene 1,16-hexadecanedicarboxylate) were crystallized from dilute solutions in n-hexanol, isoamyl acetate etc., and were studied with the electron microscopy and x-ray diffraction. The crystal structure of these polyesters are tentatively determined. Morphological “regularity” and “simplicity” of the single crystals are correlated with the chemical structure of the polymers. The crystallization conditions under which “regular” and “simple” single crystals are obtained are relaxed with increase of methylene sequence length in chemical repeat unit. The Bragg extinction bands in the single crystals of poly(hexamethylene sebacate) and poly(decamethylene 1,16-hexadecanedicarboxylate) suggest nonplanar nature of these crystals. The molecular chains in the poly(ethylene sebacate) single crystals are inclined from the normal of the basal plane; the fold surface corresponds to the (001) plane.     

Evaluation of solvents’ effect on solubility,intermolecular interaction energies and habit of ascorbic acid crystals

Solubility of active pharmaceutical ingredient (API) in solvents is very important for drug development and manufacturing. Solubility data may provide further information such as thermochemical properties and intermolecular interactions that may lead to a better understanding of the formation of API crystals. In this study, solubility of ascorbic acid was determined by gravimetric method in four different commonly used polar protic solvents: water, methanol, ethanol and 2-propanol. The solubility of ascorbic acid crystal was also predicted using Conductor-like Screening Model – Realistic Solvent (COSMO-RS) approach. In this computational analysis, the generated ΔG values are based on the solubilities that were experimentally obtained to simulate the intermolecular forces. The intermolecular interaction data from COSMO-RS provide an insight into the relationship between the intermolecular interactions and its crystal habit across four different polar protic solvents. The habit of the crystals was then determined using light microscopy and scanning electron microscopy techniques, while the polymorphic form of the crystals was identified by X-ray powder diffraction and single X-ray diffraction techniques. The solubility and characterization data showed that the solvents with high polarity increased the solubility of ascorbic acid. The data also showed that different solvent polarity influenced the crystal habit, but did not change the crystal structure to form a new polymorph.     

The Elusive Structure of Levocetirizine Dihydrochloride Determined by Electron Diffraction

Levocetirizine is an orally administrated, second-generation antihistaminic active pharmaceutical ingredient that has been used to treat symptoms of allergy and long-term hives for over 25 years. Despite the wide use of this compound, its crystal structure has remained unknown. Here we report the application of 3D electron diffraction (3D ED)/Micro-crystal electron diffraction (MicroED) to determine the crystal structure of Levocetirizine dihydrochloride directly from crystalline powders that were extracted from commercially available tablets containing the compound. We also showcase the utility of dynamical refinement to unambiguously assign absolute configuration. The results highlight the immense potential of 3D ED/MicroED for structure elucidation of components of microcrystalline mixtures that obviates the need to grow large-size single crystals and the use of complementary analytical techniques, which could be important for identification as well as for primary structural characterization.     

A Robust Microfluidic Device for the Synthesis and Crystal Growth of Organometallic Polymers with Highly Organized Structures

A simple and robust microfluidic device was developed to synthesize organometallic polymers with highly organized structures. The device is compatible with organic solvents. Reactants are loaded into pairs of reservoirs connected by a 15 cm long microchannel prefilled with solvents, thus allowing long‐term counter diffusion for self‐assembly of organometallic polymers. The process can be monitored, and the resulting crystalline polymers are harvested without damage. The device was used to synthesize three insoluble silver acetylides as single crystals of X‐ray diffraction quality. Importantly, for the first time, the single‐crystal structure of silver phenylacetylide was determined. The reported approach may have wide applications, such as crystallization of membrane proteins, synthesis and crystal growth of organic, inorganic, and polymeric coordination compounds, whose single crystals cannot be obtained using traditional methods.     

Dynamic ft-ir spectroscopy of polymer films and liquid crystals

The use of continuous-scan and step-scan Fourier transform infrared (FT-IR) spectroscopic techniques to study the dynamics of the response of polymer films and liquid crystals to external perturbations is described here. The first application of dynamic stepscan FT-IR deals with the response of various polymer films to sinusoidally modulated tensile strain. In these experiments, a small amplitude sinusoidal stress is applied to a polymeric film and the transition dipole responses are monitored as a phase lag with respect to the external perturbation. The degree of deformation is small enough to cause only linear reversible responses to the sample. The main advantage of the technique is that it can provide valuable information at the molecular level that can be used to interpret the macroscopic properties of the polymeric material under investigation. Results for heterogeneous polymers include semicrystalline high density/low density polyethylene blends and the micro-phase separated copolymer Kraton^r̀ are presented. In addition, continuous-scan stroboscopic FT-IR was used to explore the submolecular (functional group) contributions to the reorientation dynamics of the liquid crystal director in response to pulsed (DC) electric fields. For the nematic liquid crystal molecules, 4-pentyl-4'-cyanobiphenyl (5CB) and 4-pentyl-4'-cyanophenylcyclohexane (5PCH), the individual response of the rigid and floppy parts was examined.     

Solid‐state polymerization of 2,3,6,7,10,11‐hexa(methacrylate) triphenylene

A new monomer, 2,3,6,7,10,11‐hexa(methacrylate) triphenylene (HMTP), and its crystals have been successfully synthesized, and the solid‐state polymerization under UV irradiation has been investigated. The photo polymerization of HMTP in solid was confirmed by the reduction of vinyl bonds in the FT‐IR and UV spectra of PHMTP in comparison with the corresponding spectra of its precursor. Thus, IR spectroscope was used to follow the polymerization of HMTP crystals under UV irradiation, and kinetic studies show a first‐order reaction with rate constant of 6.12 × 10^?3 min^?1. This value is slightly larger than that measured by the weight method. The polarizing optical microscope and X‐ray diffraction were used to study the crystal structure difference between the polymers and its monomer. The results show that the polymers' crystals obtained from photo polymerization kept the monomer crystal lattice. Because of strong overlap between the π‐electron of the triphenylene, the monomer and polymer crystals showed different fluorescence properties. All these results proved that the photo polymerization of HMTP crystals is governed by the packing structure of monomer molecules; in other words, this reaction is just lattice controlled polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1526–1534, 2005     

纤维素Ⅱ和碱纤维素Ⅱ单结晶的研究

本文用电子显微镜和电子衍射技术研究苎麻的纤维素Ⅱ和碱纤维素Ⅱ单晶。纤维素Ⅱ单晶成长条状,长约2μ,宽约0.4～0.6μ,电子衍射技术表明长条状单晶长的方向是纤维分子链的方向,故分子链不应该是折叠链结构。碱纤维Ⅱ单晶是不规则的多边形,单晶的大小为0.3～4μ,碱纤维素Ⅱ孪晶是由两块单晶重叠并错开约2.6°角度而构成的,孪晶面为(001)面。