柔软的球状和长方体状氧化硅中空结构的制备研究

采用自组装形成的芘纳米结构作为模板,成功地制备了柔软的球状和长方体状氧化硅中空结构.当不同量的芘在十六烷基三甲基溴化铵(CTAB)溶液中自组装时,产生的自组装结构展现出明显的从球状到长方体状的形貌变化.这些结构被用作氧化硅前驱体溶胶-凝胶反应的模板,获得了球状和长方体状氧化硅/芘复合结构.通过乙醇除去模板后,生成了柔软的球状(直径约为400nm)和长方体状(长为0.5—2.5μm)的氧化硅中空结构.这些结果展现了采用有机纳米结构作为模板来合成无机中空结构的优势:合成简便、结构多样以及结构形貌的灵活可控.     

Incremental solvation of nonionized and zwitterionic glycine

Microsolvation and combined microsolvation-continuum approaches are employed in order to examine the structures and relative energies of nonionized (N) and zwitterionic (Z) glycine clusters. Bridging structures are predicted to be the global minima after 3-5 discrete water molecules are included in the calculations. Calculations incorporating electron correlation stabilize the zwitterionic structures by about 7-9 kcal/mol relative to the N structures regardless of the number of discrete water molecules considered. Continuum calculations stabilize the Z structures relative to N structures; this effect decreases as the number of discrete water molecules is increased. Eight water molecules do not appear to fully solvate glycine.     

Chemical evaluation of hypothetical uninodal zeolites

Optimized structural parameters, framework energies relative to alpha-quartz, and volumes accessible to sorption have been calculated for the systematically enumerated hypothetical uninodal zeolitic structures (structures in which all tetrahedral sites are equivalent). The structures were treated as silica polymorphs, and their energies were minimized using the GULP program with the Sanders-Catlow silica potential. Results are given for 164 structures, which include all 21 known uninodal zeolites, two known minerals (tridymite and cristobalite), and 78 unknown zeolite topologies. Twenty-three hypothetical structures were identified as chemically feasible. Complete structural information is provided, and several structures are discussed in detail. The results will assist in the design of new synthetic routes and in the identification of newly synthesized materials.     

Studies in hydrogen bonding: simulation of the crystalline solids of ice, ammonia, and ammonia hydrate

The crystal structures of ice, ammonia and ammonia hydrate have been simulated with rigid molecules using the interatomic potential function EPEN/2 and the computer program WMIN. Structural parameters were adjusted to give structures with minimum energy. The hydrogen bonding in the simulated structures is compared with that in the experimental structures.     

Calorimetric study of the native and postdenatured structures in starches with different degree of hydration

DSC studies of melting process of annealed native structures and postdenatured ones in low-amylose starches with different degrees of hydration were carried out. The starch recrystallization at different thermal treatments of the samples was studied both after the complete and partial destroy of native structures. It has been shown that native structures as well as postdenatured ones possess the ability to perfection, which is most clearly seen at the annealing at temperatures inside their melting ranges. The results obtained demonstrate that at the same duration of annealing the process of crystal perfection for secondary starch structures proceed more intensively compared to the native ones. The presence of the remained native structures in partial melt in contrast to the remained gel ones restricts the ability of the recrystallized structures to perfection.     

Electrical Conductivity of Films Formed by Few-Layer Graphene Structures

Current–voltage characteristics of few-layer graphene structures produced by electrochemical exfoliation of graphite in an electrolyte solution were measured. It was shown that few-layer graphene structures possess an electronic conductivity, which is indicative of the small degree of surface functionalization of the structures. The resistance of films formed by these structures grows with increasing relative humidity of the medium because of the shielding of fl akes of the few-layer graphene structures by a film of water.     

Evaluation of ion mobility spectroscopy for determining charge-solvated versus salt-bridge structures of protonated trimers

The cross sections of five different protonated trimers consisting of two base molecules and trifluoroacetic acid were measured by using ion mobility spectrometry. The gas-phase basicities of these five base molecules span an 8-kcal/mol range. These cross sections are compared with those determined from candidate low-energy salt-bridge and charge-solvated structures identified by using molecular mechanics calculations using three different force fields: AMBER*, MMFF, and CHARMm. With AMBER*, the charge-solvated structures are all globular and the salt-bridge structures are all linear, whereas with CHARMm, these two forms of the protonated trimers can adopt either shape. Globular structures have smaller cross sections than linear structures. Conclusions about the structure of these protonated trimers are highly dependent on the force field used to generate low-energy candidate structures. With AMBER*, all of the trimers are consistent with salt-bridge structures, whereas with MMFF the measured cross sections are more consistent with charge-solvated structures, although the assignments are ambiguous for two of the protonated trimers. Conclusions based on structures generated by using CHARMm suggest a change in structure from charge-solvated to salt-bridge structures with increasing gas-phase basicity of the constituent bases, a result that is most consistent with structural conclusions based on blackbody infrared radiative dissociation experiments for these protonated trimers and theoretical calculations on the uncharged base-acid pairs.     

液晶自组装多层级结构及其应用

液晶的分层组装与刺激响应特性使其在先进功能材料的开发与应用领域具有独特的优势.通过特定的技术手段诱导其自组装行为,可带来新奇的光学、机械、电磁等性能,进而实现一系列全新的技术应用.本文主要针对近晶相、胆甾相、蓝相这三种特殊的液晶相态,系统介绍了多形态焦锥畴结构,分层油纹,螺旋结构,双螺旋扭曲柱立方晶格等多层级结构,重点论述了材料组分优化,几何结构限制以及外场激励等条件下液晶多层级结构的大面积精细操控,回顾了其在粒子操控、表面改性、光子技术等领域的相关技术应用,并总结展望了液晶组装技术与应用的发展前景.     

The nature of base stacking: a Monte Carlo study

To elucidate the physical origin of the preference of nucleic acid bases for stacking over hydrogen bonding in water, Monte Carlo simulations were performed starting from Watson?CCrick structures of the adenine?Cthymine, adenine?Curacil and guanine?Ccytosine base pairs, as well as from the Hoogsteen adenine?Cthymine base pair, in clusters comprising 400 and 800 water molecules. The simulations employed a newly implemented Metropolis Monte Carlo algorithm based on the extended cluster approach. All simulations reached stacked structures, confirming that such structures are preferred over the hydrogen-bonded Watson?CCrick and Hoogsteen base pairs. The Monte Carlo simulations show the complete transition from hydrogen-bonded base pairs to stacked structures in the Monte Carlo framework. Analysis of the average energies shows that the preference of stacked over hydrogen-bonded structures is due to the increased water?Cbase interaction in these structures. This is corroborated by the increased number of water?Cbase hydrogen bonds in the stacked structures.     

Formation of Co-continuous Structures in Melt-Mixed Immiscible Polymer Blends

Abstract

Co-continuous structures can be regarded as the coexistence of at least two continuous structures within the same volume. Blends with co-continuous structures may combine the properties of both components in a favorable way, for example, mechanical moduli. This review article deals with the identification, characterization, and properties of co-continuous structures as well as with the development of co-continuous structures during the melt blending process. Co-continuous structures usually can be formed within a composition region about the phase inversion composition, which mainly depends on the viscosity ratio. On the other hand, co-continuous structures can be found independent of composition as intermediate stages during the initial state of morphology development and during phase inversion process in blends in which the component finally forming the dispersed phase forms the matrix in early mixing states. In addition, even at low volume fractions of one component, stable co-continuous morphologies can be created using suitable processing conditions, forming long elongated interconnected structures that do not break up because of the flow. The interfacial tension plays an important role for the stability; a lower interfacial tension leads to broader composition ranges of co-continuous structures. Another factor enhancing the formation and stability of co-continuous structure is melt yield stress of one or both components of blends. In addition, this article reviews the stability of co-continuous structures during further processing and the influence of compatibilization on the structure formation and stability. Subsequently, two models describing the co-continuous composition range are discussed.     

Structures and rearrangements of LiCl clusters

Molecular dynamics simulations are used to investigate the low-temperature structures and temperature-driven rearrangements of (LiCl)n clusters, with n ranging from 3 to 500. It is found that for n < or = 32 expanded, ring-based structures are energetically more stable than cubic (rocksalt) forms at low temperature. For n > or = 108, the cubic structures are lower in energy, but as the clusters are heated rearrangements to expanded structures occur well below the melting temperature. Hexagonal (LiCl)3 rings are a distinguishing feature of the expanded, ring-based structures. Highly asymmetric ion sizes are essential for the formation of the expanded structures. Similar transitions from more-ordered to less-ordered solid states are not found for corresponding (KCl)n clusters, which remain in the cubic structure until they melt.     

Stabilization and growth of non-native nanocrystals at low and atmospheric pressures

The stabilization and growth of nanocrystals in "non-native" structures is explored via density functional calculations. Non-native and "native" bulk structures differ in their discrete translational symmetry. Computations suggest that the lower surface energy of the non-native structures always facilitates their stabilization in the early stages of crystal growth. In the compound semiconductors considered here, the transition pathways between non-native and native structures involve planar or near-planar depolarized layers and the growth conditions have significant effects on the stabilization and growth of non-native structures. The findings of this study help in identifying heuristics for the synthesis of non-native nanocrystals.     

On the dynamics of fragment isomerization in collision-induced dissociation of peptides

The structures of peptide collision-induced dissociation (CID) product ions are investigated using ion mobility/mass spectrometry techniques combined with theoretical methods. The cross-section results are consistent with a mixture of linear and cyclic structures for both b4 and a4 fragment ions. Direct evidence for cyclic structures is essential in rationalizing the appearance of fragments with scrambled (i.e., permutated) primary structures, as the cycle may not open up where it was initially formed. It is demonstrated here that cyclic and linear a4 structures can interconvert freely as a result of collisional activation, implying that isomerization takes place prior to dissociation.     

隐藏高分子界面及生物界面分子结构的和频振动光谱研究

界面的分子结构决定界面的性质.为了以优化界面的结构来改进材料的性质,原位实时地研究界面的分子结构是很重要的.近年来和频振动光谱已发展成为一个很有效及独特的手段来研究隐藏界面的分子结构,例如液/液界面、固/液界面及固/固界面等.这篇综述讨论了和频振动光谱在研究高分子界面及生物界面等复杂界面的分子结构上的应用.具体说来,本文论述了高分子表面在水里的分子结构变化,高分子及模型粘合促进剂硅烷在界面相互作用的分子机理和隐藏的高分子/高分子及高分子/金属界面的结构.另外,此文还将介绍不同二级结构的多肽及几个有代表性的蛋白分子在界面的结构.界面在诸如化学、生物、物理、材料科学及工程和纳米技术等许多领域都很重要.发展一个独特的能原位研究隐藏界面的分子结构的技术会有力地促进这些领域的研究及跨学科研究的发展.     

Three-dimensional assembly of flower-like Au structures: the synergistic effect of macroporous structures and surface nanoarchitectures on electrocatalysis and electroanalysis

We present the fabrication of a three-dimensional (3D) assembly of flower-like Au structures via the combination of 3D macroporous Au-coated microspheres and surface nanoarchitectures using electrodeposition of nanoplate Au structures. The 3D flower-like Au structures exhibit synergistically enhanced electrocatalytic activities regarding glucose oxidation and oxygen reduction compared to those of the individual 3D macroporous and nanoplate Au structures. The 3D flower-like Au structures can also be utilized as electroanalytical platforms retaining the combined advantages of both of the 3D macroporous and nanoplate Au structures.     

Templated growth of hybrid structures at the peptide-peptide interface

This study describes the use of peptide vesicular platforms for the templated growth of fibrillar structures to craft hybrids that retain the gross morphological features of two discreet self-assembled peptides. A synthetic triskelion peptide, which results in the rapid emergence of self-assembled spherical structures, was employed as a template. Addition of either one of two different peptides, both of which form long filamentous structures when co-incubated with the triskelion solution, affords hybrids that retain the gross morphology of both the spherical and filamentous structures. It is surmised that this process is aided by hydrogen bonding and the interdigitation of aromatic residues, which leads to the growth of hybrid structures. We believe that observations concerning the surface-assisted growth of peptide fibrils and tubular structures from vesicular platforms may have ramifications for the design and development of peptide-based hybrid materials with controlled hierarchical structures.     

The stability of the closest packings of Lennard-Jones particles with vacancy sublattices at negative pressures

The stability conditions were analyzed for two-and three-dimensional closely packed crystal structures including regular vacancy sublattices. The interaction of particles was described by the Lennard-Jones potential. The limiting negative pressure at which structures lost stability was calculated for the examples considered. The enthalpies of the structures in the region of their existence were compared. It was shown that the structures with vacancy or vacancy cluster sublattices could be stabler than ideal structures under certain conditions. A defect lattice became stabler than ideal if the concentration of defects did not exceed some threshold value. The results obtained contribute to the understanding of the special features of the behavior of crystals under dilatation conditions.     

Effects of hydrophobic and dipole-dipole interactions on the conformational transitions of a model polypeptide

We studied the effects of hydrophobicity and dipole-dipole interactions between the nearest-neighbor amide planes on the secondary structures of a model polypeptide by calculating the free energy differences between different peptide structures. The free energy calculations were performed with low computational costs using the accelerated Monte Carlo simulation (umbrella sampling) method, with a bias-potential method used earlier in our accelerated molecular dynamics simulations. It was found that the hydrophobic interaction enhances the stability of alpha helices at both low and high temperatures but stabilizes beta structures only at high temperatures at which alpha helices are not stable. The nearest-neighbor dipole-dipole interaction stabilizes beta structures under all conditions, especially in the low temperature region where alpha helices are the stable structures. Our results indicate clearly that the dipole-dipole interaction between the nearest neighboring amide planes plays an important role in determining the peptide structures. Current research provides a more unified and quantitative picture for understanding the effects of different forms of interactions on polypeptide structures. In addition, the present model can be extended to describe DNA/RNA, polymer, copolymer, and other chain systems.     

Theoretical infrared spectrum of the ethanol hexamer

The hydrogen bond network of ethanol clusters is among the most complex hydrogen bond networks of molecular clusters. One of the reasons of its complexity arises from the number of possible ethanol monomers (there are three isoenergetic isomers of the ethanol monomer). This leads to difficulties in the exploration of potential energy surfaces (PESs) of ethanol clusters. In this work, we have explored the PES of the ethanol hexamer at the MP2/aug-cc-pVDZ level of theory. We have provided structures and their relative stability at 0 K and for temperatures ranging from 20 to 400 K in the gas phase. These structures are used to compute the theoretical infrared (IR) spectrum of the ethanol hexamer at the MP2/aug-cc-pVDZ level of theory. As a result, 98 different structures have been investigated, and six isomers are reported to be the most isoenergetically stable structures of the ethanol hexamer. These isomers are folded cyclic structures in which the stability is enhanced by the implication of CH⋯O interactions. Our investigations show that the PES of the ethanol hexamer is very flat, yielding several isoenergetic structures. Furthermore, we have noted that several isomers contribute to the population of the ethanol hexamer at high temperatures. As far as the IR spectroscopic study is concerned, we have found that the IR spectra of the most stable structures are in good agreement with the experiment. Considering this agreement, these structures are used to assign the experimental peaks in the CH-stretching region. We concluded that the stability of the structures of the ethanol hexamer is related both to OH⋯O hydrogen bonds and CH⋯O interactions. Overall, we have found that the IR spectrum of the ethanol hexamer, calculated from the contribution of all the possible stable structures weighted by their probability, excellently reproduce the experimental spectrum of the ethanol hexamer.