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1.
用分子动力学模拟方法 (MD)研究了 3 0 0K时钙钛矿型CaSiO3 ,从高压到负压的解压缩过程 .MD模拟获得的P V关系与实验数据相近 ,与已报道的MD模拟数据基本一致 ,所得体积模量也在实验数据分布范围内 .减压缩和重压缩的MD模拟数据与实验结果相似 .钙钛矿型CaSiO3 解压缩成非晶态时 ,存在两个结构破坏阶段 :破坏硅氧八面体和破坏钙氧二十面体 .当钙氧二十面体被破坏后 ,重压缩不能得到钙钛矿型结构 .只要钙氧二十面体未被破坏 ,重压缩可恢复钙钛矿型结构 .本研究得到的结果尚未见相关报道 .由MD模拟数据计算了CaSiO3 系统的红外光谱 ,分析这些数据可知钙钛矿型CaSiO3 解压缩非晶化是一个二阶软模相变 .研究表明钙钛矿型CaSiO3 结构存在一个等容的亚稳极限 ,其解压缩非晶化是一个受动力学控制的亚稳状态 .  相似文献   

2.
采用X射线衍射(XRD)和高分辨透射电镜(HRTEM)对Al-Fe-Ce非晶合金的局域结构进行了研究,分析了二十面体短程序对非晶合金的形成及热稳定性的影响.Al-Fe-Ce非晶合金中的二十面体短程序均匀弥散分布在非晶基体中.Al颗粒被二十面体化学短程序紧紧包围,二十面体化学短程序阻碍了Al颗粒的长大,面心立方铝(fcc-Al)的形核需要二十面体结构的分解及溶于其中的元素的扩散.二十面体结构单元的存在和均匀分布有利于增强非晶合金的玻璃形成能力和非晶相的热稳定性.  相似文献   

3.
Ni3Al合金液态与非晶中的原子团簇   总被引:1,自引:0,他引:1  
采用常温常压分子动力学模拟技术,模拟了液态Ni3Al中原子团簇在快速凝固条件下的演变过程,模型采用的是TB(tight binding)作用势.用偶分布函数、键对和多面体等结构参数来描述快速凝固条件下团簇种类和数量的变化,并将团簇结构可视化.在2 000 K下,液态Ni3Al中团簇数量较少,且都是由缺陷二十面体构成;在4×1013 K•s-1的冷速下,团簇的数量随温度的降低不断增加,且出现完整二十面体团簇,体系最终形成了由二十面体和缺陷二十面体团簇网络所组成的非晶结构.  相似文献   

4.
近日,上海交通大学材料科学与工程学院陈明伟教授领衔的国际研究团队在非晶合金原子结构的研究方面取得突破性进展,首次在实验上表征了非晶中重要结构单元二十面体团簇的原子空间构型,并证明二十面体原子团簇的几何不稳定性是非晶形成的结构起源。专家认为,该研究是非晶结构研究上取得的又一重大进展,将推动对非晶中很多基本科学难题,如玻璃转变问题、非晶形变机制等的研究,为探索新型非晶材料提供指南。相关研究成果日前在线发表于《科学》杂志。非晶态材料是一类远离平衡态、结构无序的刚性固体物质,具有许多特异物理、化学性质,并在  相似文献   

5.
王维宴  卢本卓  陈慰祖  王存新 《化学学报》2002,60(12):2129-2134
用分子动力学(MD)模拟方法设计了两个模拟时间为600ps的对比计算机模拟 实验,研究了R6态的胰岛素六聚体在水溶液中的稳定性以及苯酚和锌离子对结构稳 定性的影响。通过对MD模拟所得到的轨迹的分析发现,在维系胰岛素六聚体稳定性 的因素中,静电相互作用和氢键起着主要的作用。对于包含锌离子和苯酚的体系。 胰岛素六聚体的稳定性得到了增强;对不含锌离子和苯酚的关系,胰岛素六聚体的 稳定性明显减弱,在这种情况下,胰岛素六聚体还表现出解聚的倾向。这些模拟结 果与实验观测结果相吻合。  相似文献   

6.
许多病毒具有二十面体对称结构,二十面体病毒衣壳结构的模拟构筑已成为物质结构的研究目标之一.介绍了二十面体病毒衣壳结构的几种模型,对各种模型的特点及应用范围进行了归纳,并对衣壳结构模拟构筑的意义进行了简要介绍.  相似文献   

7.
LaBO3(B=Fe,Co)中氧的迁移与光催化反应活性   总被引:10,自引:0,他引:10  
以柠檬酸法合成的钙钛矿型复合氧化物LaBO3(B=Fe,Co)为催化剂,对水溶性染料进行光催化降解,实验结果表明,其光催化活性与钙钛矿型结构中氧空位沿BO6八面体棱边以曲线而非直线的迁移机制有关.在光催化氧化过程中,光生电子首先被表面氧空位束缚,再与表面的吸附氧反应生成超氧基(O2-·)而加速对染料分子的降解.钙钛矿型复合氧化物中的氧空位是由氧的迁移产生的,它可以作为电子的陷阱而捕俘电子,并作为氧的吸附中心而提高催化剂表面的吸附氧量.  相似文献   

8.
采用EDTA-柠檬酸联合络合法合成了钙钛矿型金属氧化物SrCo0.8Fe0.2O3-δ(SCF182)粉体。通过XRD和ESEM研究了前驱体溶液不同的pH值对合成的SCF182粉体的晶体结构和微观形貌的影响。并采用固定床实验研究前驱体溶液pH值对SCF182的氧吸附/脱附性能的影响。结果表明,pH值对SCF182晶胞结构和参数影响不大,但影响SCF182的晶粒粒径和微观形貌,当pH值为8的弱碱性条件下时,能够合成蓬松多孔网状纳米结构的钙钛矿粉体。固定床实验显示,前驱体溶液的pH值影响合成的粉体SCF182的氧吸附速率;且SCF182的氧脱附性能随pH值的增大先升高后降低,pH值为8时氧脱附量达到峰值为42.2 mg/g。  相似文献   

9.
二十面体准晶对非晶形成影响的模拟   总被引:2,自引:2,他引:0  
采用分子动力学模拟技术,以液态金属Ni为例,研究了在不同冷却条件下形成晶体及非晶的过程.模拟采用镶嵌原子法(EAM)作用势,得到了不同温度、不同冷却速度下Ni的径向分布函数以及原子组态变化的重要信息,利用键对分析技术探讨了二十面体准晶对非晶形成的影响.  相似文献   

10.
A_2BO_4型类钙钛矿材料因其独特的物化性能,可应用于催化、固体氧化物燃料电池等领域。本文简要介绍了A_2BO_4型类钙钛矿材料的结构和非化学计量氧(δ)的研究方法,对材料的δ分子模拟研究进展进行了综述,重点介绍了材料非化学计量氧迁移、输运机制的分子模拟研究现状,包括未掺杂纯相、A位掺杂和B位掺杂A_2BO_4型类钙钛矿三类材料,同时也对该领域的发展趋势进行了展望。  相似文献   

11.
The structure of the hydrated calcium(II) ion in aqueous solution has been studied by means of extended X-ray absorption fine structure spectroscopy (EXAFS), large-angle X-ray scattering (LAXS), and molecular dynamics (MD) methods. The EXAFS data displayed a broad and asymmetric distribution of the Ca-O bond distances with the centroid at 2.46(2) A. LAXS studies on four aqueous calcium halide solutions (1.5-2 mol dm(-)(3)) gave a mean Ca-O bond distance of 2.46(1) A. This is consistent with a hydration number of 8 determined from correlations between mean distances and coordination numbers from crystal structures. The LAXS studies showed a second coordination sphere with a mean Ca.O(II) distance of 4.58(5) A, and for the hydrated halide ions the distances Cl.O 3.25(1) A, Br.O 3.36(1) A, and I.O 3.61(1) A were obtained. Molecular dynamics simulations of CaCl(2)(aq) were performed using three different Ca(2+)-OH(2) pair potentials. The potential from the GROMOS program gave results in agreement with experiments, i.e., a coordination number of 8 and an average Ca-O distance of 2.46 A, and was used for further comparisons. Theoretical EXAFS oscillations were computed for individual MD snapshots and showed very large variations, though the simulated average spectrum from 2000 snapshots gave satisfactory agreement with the experimental EXAFS spectra. The effect of thermal motions of the coordinated atoms is inherent in the MD simulation method. Thermal disorder parameters evaluated from simulated spatial atom distribution functions of the oxygen atoms coordinated to the calcium ion were in close agreement with those from the current LAXS and EXAFS analyses. The combined results are consistent with a root-mean-square displacement from the mean Ca-O distance of 0.09(2) A in aqueous solution at 300 K.  相似文献   

12.
The rotational isomeric states (RIS) of glycerol at infinite dilution have been characterized in the aqueous phase via a 1 micros conventional molecular dynamics (MD) simulation, a 40 ns enhanced sampling replica exchange molecular dynamics (REMD) simulation, and a reevaluation of the experimental NMR data. The MD and REMD simulations employed the GLYCAM06/AMBER force field with explicit treatment of solvation. The shorter time scale of the REMD sampling method gave rise to RIS and theoretical scalar 3J(HH) coupling constants that were comparable to those from the much longer traditional MD simulation. The 3J(HH) coupling constants computed from the MD methods were in excellent agreement with those observed experimentally. Despite the agreement between the computed and the experimental J-values, there were variations between the rotamer populations computed directly from the MD data and those derived from the experimental NMR data. The experimentally derived populations were determined utilizing limiting J-values from an analysis of NMR data from substituted ethane molecules and may not be completely appropriate for application in more complex molecules, such as glycerol. Here, new limiting J-values have been derived via a combined MD and quantum mechanical approach and were used to decompose the experimental 3J(HH) coupling constants into population distributions for the glycerol RIS.  相似文献   

13.
Raman scattering and x-ray diffraction studies of CaSnO(3) perovskite were performed under high-pressure conditions. This high-pressure study was motivated by a recent theoretical study predicting a phase transition in CaSnO(3) from GdFeO(3)-type perovskite to CaIrO(3)-type structure occurred at 12 GPa. Despite no obvious structure change up to a pressure of 26 GPa based on the x-ray diffraction data, high pressure Raman measurements revealed that some Raman modes disappeared upon compression; either merging into neighboring bands or vanishing. The signals for these Raman peaks were recovered during decompression. The measured pressure derivative of Raman shift (?ν∕?P) of CaSnO(3) ranged from ~1.29 to ~4.35, up to 20 GPa. Due to the lack of lattice dynamic study for CaSnO(3) perovskite, the mode symmetry for CaSnO(3) was tentatively assigned based on the empirical relation among Ca-bearing perovskites. The pressure derivative of the Raman shifts was found to be related to their mode vibrations: modes related to Ca and O shifts had a strong pressure dependence compared with those associated with oxygen octahedral rotation.  相似文献   

14.
The results of classical molecular dynamics (MD) simulations and Raman spectroscopy studies of neat liquid N-methylacetamide (NMA), the simplest model system relevant to the peptides, are reported as a function of temperature and pressure. The MD simulations predict that near ambient conditions, the molecules form a hydrogen bond network consisting primarily of linear chains. Both the links between molecules within the hydrogen-bonded chains and the associations between chains are stabilized by weak methyl-donated "improper" hydrogen bonds. The three-dimensional structural motifs observed in the liquid show some similarity to protein beta-sheets. The temperature and pressure dependence of the hydrogen bond network, as probed by the mode frequency of the experimentally determined amide-I Raman band, blue shifts on heating and red shifts under compression, respectively, suggesting weakened and enhanced hydrogen bonding in response to temperature and pressure increases. Disruption of the hydrogen-bonding network is clearly observed in the simulation data as temperature is increased, whereas the improper hydrogen bonding is enhanced under compression to reduce the energetic cost of increasing the packing fraction. Because of the neglect of polarizability in the molecular model, the computed dielectric constant is underestimated compared to the experimental value, indicating that the simulation may underestimate dipolar coupling in the liquid.  相似文献   

15.
16.
The increasing importance of hydrogenase enzymes in the new energy research field has led us to examine the structure and dynamics of potential hydrogenase mimics, based on a ferrocene-peptide scaffold, using molecular dynamics (MD) simulations. To enable this MD study, a molecular mechanics force field for ferrocene-bearing peptides was developed and implemented in the CHARMM simulation package, thus extending the usefulness of the package into peptide-bioorganometallic chemistry. Using the automated frequency-matching method (AFMM), optimized intramolecular force-field parameters were generated through quantum chemical reference normal modes. The partial charges for ferrocene were derived by fitting point charges to quantum-chemically computed electrostatic potentials. The force field was tested against experimental X-ray crystal structures of dipeptide derivatives of ferrocene-1,1'-dicarboxylic acid. The calculations reproduce accurately the molecular geometries, including the characteristic C2-symmetrical intramolecular hydrogen-bonding pattern, that were stable over 0.1 micros MD simulations. The crystal packing properties of ferrocene-1-(D)alanine-(D)proline-1'-(D)alanine-(D)proline were also accurately reproduced. The lattice parameters of this crystal were conserved during a 0.1 micros MD simulation and match the experimental values almost exactly. Simulations of the peptides in dichloromethane are also in good agreement with experimental NMR and circular dichroism (CD) data in solution. The developed force field was used to perform MD simulations on novel, as yet unsynthesized peptide fragments that surround the active site of [Ni-Fe] hydrogenase. The results of this simulation lead us to propose an improved design for synthetic peptide-based hydrogenase models. The presented MD simulation results of metallocenes thereby provide a convincing validation of our proposal to use ferrocene-peptides as minimal enzyme mimics.  相似文献   

17.
Sodium niobate (NaNbO(3)) has a particularly complex phase diagram, with a series of phase transitions as a function of temperature and pressure, and even at room temperature a number of different structural variations have been suggested. Recent work has demonstrated that bulk powders of NaNbO(3), prepared using a variety of synthetic approaches, contain a mixture of perovskite phases; the commonly reported Pbcm phase and a second, polar phase tentatively identified as belonging to space group P2(1)ma. The two phases exhibit very similar (23)Na MAS NMR spectra, although high-resolution MQMAS spectra were able to distinguish between them. Here, we investigate whether different perovskite polymorphs can be distinguished and/or identified using a variety of (93)Nb NMR methods, including MAS, MQMAS and wideline experiments. We compare the experimental results obtained for these more common perovskite materials to those for the metastable ilmenite polymorph of NaNbO(3). Our experimental results are supported by first-principles calculations of NMR parameters using a planewave pseudopotential approach. The calculated NMR parameters appear very different for each of the phases investigated, but high forces on the atoms indicate many of the structural models derived from diffraction require optimisation of the atomic coordinates. After geometry optimisation, most of these perovskite phases exhibit very similar NMR parameters, in contrast to recent work where it was suggested that (93)Nb provides a useful tool for distinguishing NaNbO(3) polymorphs. Finally, we consider the origin of the quadrupolar coupling in these materials, and its dependence on the deviation from ideality of the NbO(6) octahedra.  相似文献   

18.
A mechanical separation process in a de-oiling hydrocyclone is described in which disperse oil droplets are separated from a continuous water phase. This separation process is influenced by droplet breakage and coalescence. Based on experimental data and simulation results in a stirred tank, a modified breakage model, which can be applied to droplet breakage in the de-oiling hydrocyclone, is developed. Then, a simulation model is developed coupling the numerical solution of the flow field in the hydrocyclone based on computational fluid dynamics (CFD) with population balances. The homogenous discrete method and the inhomogeneous discrete method are applied for solving the population balance model (PBM). The investigations show that the numerical results obtained by the simulation model coupled with the modified PBM using the inhomogeneous discrete method are in good accordance with experimental data under a high flow rate. According to this simulation model, the effect of three different inlet designs on the separation efficiency of the de-oiling hydrocyclone has been discussed. The results indicate that the separation efficiency of the de-oiling hydrocyclone can be improved with an appropriate inlet design.  相似文献   

19.
The absolute binding free energies and binding enthalpies of twelve host–guest systems in the SAMPL5 blind challenge were computed using our attach-pull-release (APR) approach. This method has previously shown good correlations between experimental and calculated binding data in retrospective studies of cucurbit[7]uril (CB7) and β-cyclodextrin (βCD) systems. In the present work, the computed binding free energies for host octa acid (OA or OAH) and tetra-endo-methyl octa-acid (TEMOA or OAMe) with guests are in good agreement with prospective experimental data, with a coefficient of determination (R2) of 0.8 and root-mean-squared error of 1.7 kcal/mol using the TIP3P water model. The binding enthalpy calculations achieve moderate correlations, with R2 of 0.5 and RMSE of 2.5 kcal/mol, for TIP3P water. Calculations using the newly developed OPC water model also show good performance. Furthermore, the present calculations semi-quantitatively capture the experimental trend of enthalpy-entropy compensation observed, and successfully predict guests with the strongest and weakest binding affinity. The most populated binding poses of all twelve systems, based on clustering analysis of 750 ns molecular dynamics (MD) trajectories, were extracted and analyzed. Computational methods using MD simulations and explicit solvent models in a rigorous statistical thermodynamic framework, like APR, can generate reasonable predictions of binding thermodynamics. Especially with continuing improvement in simulation force fields, such methods hold the promise of making substantial contributions to hit identification and lead optimization in the drug discovery process.  相似文献   

20.
采用分子动力学模拟方法构建了氢化丁腈橡胶的分子链结构并计算出不同压缩比下的均方回转半径,同时对应力弛豫进行了理论预测并与压缩应力弛豫实验结果进行了对比.研究发现,均方回转半径随着压力的增大而逐渐降低,随着温度的增大而逐渐增大;在不同压力下,应力均随着时间的增大而逐渐降低而且当时间达到一定程度后应力降低逐渐平缓;计算结果表明压力增大或低温下均方回转半径小、弛豫时间长,体系越表现出弹性特征,此结果表明可用均方回转半径的变化来定量描述橡胶材料体系的应力弛豫变化,分子模拟得到的应力弛豫规律与实验结果有较好吻合.  相似文献   

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