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1.
We present theoretical results of size dependent structural, electronic, and optical properties of ligand‐free stoichiometric AlnAsn clusters of zinc‐blende modification. The investigation is done using a simplified parametrized linear combination of atomic orbital–density functional theory‐local density approximation–tight‐binding (LCAO–DFT–LDA–TB) method and consider clusters with n up to around 100. Initial structures have assumed as spherical parts of infinite zinc‐blende structure and then allowed to relax to the closest local‐energy‐minimum structure. We analyze the radial distributions of atoms, Mulliken populations, electronic energy levels (in particular, HOMO and LUMO), bandgap, and stability as a function of size and composition. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006  相似文献   

2.
Density functional theory was applied to a series of 13-atom Zn–Cu alloy clusters. We did a thorough search for the low-energy isomers by global optimization, plus explicit optimization of all homotops of the icosahedron. Structures of copper rich clusters tend to be compact, often icosahedra, whereas zinc rich clusters ( ${{\rm Zn}_8{\rm Cu}_{5}^{+}}$ , Zn9Cu4, Zn11Cu2) have compact copper cores surrounded by an incomplete shell of solvating Zn atoms. The icosahedral structure, low total energy, and large hardness of Zn7Cu6 indicate that it has special stability among Zn x Cu y clusters. However, Zn7Cu6 has many low lying isomers and a small cohesive energy compared to brass, which suggest that it is not stable in a broader sense.  相似文献   

3.
The Arrhenius parameters for the gas phase, unimolecular structural isomerizations of 1,1,2‐trimethylcyclopropane to three isomeric methylpentenes and two dimethylbutenes have been determined over a wide range of temperatures, 688–1124 K, using both static and shock tube reactors. For the overall loss of reactant, Ea = 63.7 (± 0.5) kcal/mol and log10 A = 15.28 (± 0.12). These values are higher by 2.6 kcal/mol and 0.7–0.8 than previously reported from experimental work or predicted from thermochemical calculations. Ea for the formation of trans‐4‐methyl‐2‐pentene is 1.5 kcal/mol higher than Ea for the formation of the cis isomer, which is identical to the Ea difference previously reported for the formation of trans‐ and cis‐2‐butene from methylcyclopropane. Substitution of methyl groups for hydrogen atoms on the cyclopropane ring is expected to weaken the C? C ring bonds, and it has been reported previously that activation energies for structural isomerizations of methylcyclopropanes do decrease substantially over the series cyclopropane > methylcyclopropane > 1,1‐ or 1,2‐dimethylcyclopropane. However, the present study shows that the trend does not continue beyond dimethylcyclopropane isomerization. Besides reductions in C? C bond energy, steric interactions may be increasingly important in determining the energy surface and conformational restrictions near the transition state in isomerizations of the more highly substituted methylcyclopropanes. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 475–482, 2006  相似文献   

4.
This paper studies the velocity distributions and diffusing behaviors of the atoms in a nano-alloy cluster. A series of ternary alloy clusters, Au50Cu25Ni25, and binary alloy clusters, Cu100−aNia are introduced in the molecular dynamics simulations. The velocity distributions of different types of atoms in both static and moving clusters are found to obey the Maxwell’s velocity distribution with the individual mass of atoms and the inner temperature of clusters. Furthermore, the velocity distribution of whole atoms of the cluster is obtained by synthesizing the velocity distributions of compositions in the cluster according to the proportions. The consistency of the atomic motions in a moving cluster is discussed by inspecting the backward velocities of atoms, which are correlated to the translational velocity and the inner temperature of the cluster. The diffusing behaviors of the atoms in a cluster are also investigated from a viewpoint of the interatomic interactions, i.e. Cu atoms enhance the activities of Ni atoms, or Ni atoms reduce that of Cu atoms in the alloy clusters.  相似文献   

5.
The focus of this study is on compound clusters and, due to the existence of many phases with different structural properties, tin-based materials have been chosen as the reference case. The clusters considered below are of two types: in the first case the clusters have the skeleton of the pure tin clusters and are doped with oxygen and aluminum atoms with composition Sn x Y y with Y = Al, O, x = 1, 10 and y = 1, 2. In the second case the clusters have a rutile lattice with a columnar or a spherical shape and a size up to 80 atoms and are doped with a number of aluminum atoms up to 20. The calculations are based on the Density Functional Theory (DFT) and the results describe the cluster structure, its binding energy and the density of states (DOS). The general indication of the calculations is that the additive coordinates outside, rather than inside, the pristine skeleton with the formation of hybrid bonds with properties similar to the ones of the host atoms. Also conspicuous effects of hybridization are observed in the electronic structure and, due to these effects, the binding energy may decrease with respect to the one in the undoped clusters.  相似文献   

6.
7.
The most stable forms of E5Li7+ (E=Ge, Sn, and Pb) have been explored by means of a stochastic search of their potential‐energy surfaces by using the gradient embedded genetic algorithm (GEGA). The preferred isomer of the Ge5Li7+ ion is a slightly distorted analogue of the D5h three‐dimensional seven‐pointed starlike structure adopted by the lighter C5Li7+ and Si5Li7+ clusters. In contrast, the preferred structures for Sn5Li7+ and Pb5Li7+ are quite different. By starting from the starlike arrangement, corresponding lowest‐energy structures are generated by migration of one of the E atoms out of the plane with the a corresponding rearrangement of the Li atoms. To understand these structural preferences, we propose a new energy decomposition analysis based on isomerizations (isomerization energy decomposition analysis (IEDA)), which enable us to extract energetic information from isomerization between structures, mainly from highly charged fragments.  相似文献   

8.
The stability, infrared spectra and electronic structures of (ZrO2)n (n=3–6) clusters have been investigated by using density‐functional theory (DFT) at B3LYP/6‐31G* level. The lowest‐energy structures have been recognized by considering a number of structural isomers for each cluster size. It is found that the lowest‐energy (ZrO2)5 cluster is the most stable among the (ZrO2)n (n=3–6) clusters. The vibration spectra of Zr? O stretching motion from terminal oxygen atom locate between 900 and 1000 cm?1, and the vibrational band of Zr? O? Zr? O four member ring is obtained at 600–700 cm?1, which are in good agreement with the experimental results. Mulliken populations and NBO charges of (ZrO2)n clusters indicate that the charge transfers occur between 4d orbital of Zr atoms and 2p orbital of O atoms. HOMO‐LUMO gaps illustrate that chemical stabilities of the lowest‐energy (ZrO2)n (n=3–6) clusters display an even‐odd alternating pattern with increasing cluster size.  相似文献   

9.
Compound clusters of fullerene molecules and transition metal atoms having the composition C60Mx and C70Mx with x = 0..150 and M ∈ {Ti, Zr, V, Y, Ta, Nb} were produced using laser vaporisation in a low-pressure inert gas aggregation cell. Intensity anomalies in the mass spectra correlate with the atomic radii of the different metals indicating the formation of complete metal layers around the central fullerene molecule. Using high laser intensities the metal-fullerene clusters can be transformed into metcars and metal-carbides. Photofragmentation spectra of preselected C60Tax indicate that the fullerene cage is destroyed for x ≥ 3.  相似文献   

10.
The structural, relative stable and electronic properties of PbnSnn (n = 2–12) alloy clusters were systematically studied using density functional theory. The isomers of PbnSnn alloy clusters were generated and determined by ab initio molecular dynamics. By comparing the calculated parameters of Pb2 dimer and Sn2 dimers with the parameters from experiments, our calculations are reasonable. With the lowest-energy structures for PbnSnn clusters, the average binding energies, fragmentation energies, second- order energy differences, vertical ionization potentials, vertical electron affinities, HOMO–LUMO gaps, and density of states were calculated and analyzed. The results indicate that the Sn atoms have a tendency to bond together, the average binding energies tend to be stable up to n = 8, Pb8Sn8 cluster is a good candidate to calculate the molecular interaction energy parameter in Wilson equation, the clusters become less chemical stable and show an insulator-to-metallic transition, 3, 6, 8 and 11 are magic numbers of PbnSnn (n = 2–12) clusters, the charges always transfer from Sn atoms to Pb atoms in PbnSnn clusters except for Pb10Sn10 cluster, and density of states of PbnSnn clusters becoming continuous and shifting toward negative with the increasing size n.  相似文献   

11.
利用密度泛函理论在B3LYP/6-311G*水平上对碱土金属叠氮化合物(MgN6)n(n=1~5)团簇各种可能构型进行了几何优化,预测了各团簇的最稳定结构。并对最稳定结构的成键特性、电荷分布、振动特性及稳定性进行理论研究。结果表明,叠氮化合物中叠氮基以直线型存在,MgN6团簇最稳定结构为直线型;(MgN6)2团簇最稳定结构为Mg2N2四元环平面结构;(MgN6)n(n=3~5)团簇最稳定结构是由2个叠氮基与2个Mg原子首先构成近似菱形,再由近似菱形延伸形成的链状结构。叠氮基中间的N原子显示正电性,两端的N原子显示负电性,且与Mg直接作用的N原子负电性更强,金属Mg原子和N原子之间形成很强的离子键。(MgN6)n(n=1~5)团簇最稳定结构的IR光谱分为4个部分,其最强振动峰均位于2209~2313cm-1,振动模式为叠氮基中N-N键的反对称伸缩振动。稳定性分析显示,(MgN6)3和(MgN6)5团簇相对于其他团簇较为稳定。  相似文献   

12.
Poly (vinylidene fluoride) (PVF2) produces thermoreversible gel in camphor when quenched to 25°C from the melt under sealed condition. The SEM micrograph of dried PVF2/camphor gel (Wequation/tex2gif-inf-3.gif= 0.25) indicates presence of fibrillar network structure and the gels at different composition shows reversible first order phase transition. The phase diagram of the gel suggest the formation of a polymer- solvent complex. The melting enthalpy gives a stoichiometric composition of the complex at Wequation/tex2gif-inf-5.gif= 0.25. This corresponds to a molar ratio of PVF2 monomer/camphor ≈ 4/5. Temperature-dependent synchroton experiments further support the conclusions derived from the phase diagram.  相似文献   

13.
The intermetallic phase SrNi2–xSb2 was synthesized by arc melting mixtures of the elements and subsequent annealing under argon, and its structure was investigated by means of both powder and single‐crystal X‐ray diffraction methods. It crystallizes with the ThCr2Si2 type (tI10, I4/mmm) and has a homogeneity range of x = 0.15(1)–0.28(1), which does not include the exact stoichiometric 1:2:2 composition. The crystal structure of the phase SrNi2–xSb2 is built from layers of edge‐sharing Ni1–xSb4 tetrahedra, which are separated by Sr atoms along the c direction. Magnetic measurements of SrNi2–xSb2 showed no superconductive transition above 1.8 K.  相似文献   

14.
Structures of the negatively charged water clusters (H2O)n with n up to 6 were optimized at the UHF/4–31 + + G** level without any geometrical restriction. There was found a chainlike structure for the trimer anion and both chainlike and cyclic geometries for the larger clusters. According to the MP2 energy estimates, all the clusters with n ≥ 3 are more stable than the combinations of separated water molecules and a free electron. The energy of the cycles is lower than that of the chains of the same size. However, the latter species are energetically closer to the neutral oligomers of the same geometry. The energy of vertical detachment of an electron from the chainlike hexamer anion is already about zero. Addition of a diffuse s function centered equidistantly between the oxygen atoms proved metastability, in particular, of the cyclic (H2O)4 structure. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 737–748, 1997  相似文献   

15.
From deuterium labelling experiments it was concluded that metastable molecular ions of ethyl methyl sulfide lose a methyl radical with the formation of both [CH3S?CH2]+ amd [CH3CH?SH]+˙ The fragmentation reactions of metastable ions generated with these structure are losses of C2H2, H2S and CH4. These reactoins and the preceding isomerizations have also been studied by means of deuterium labelling. From the results it is concluded that the three fragmentation reactions most probably occur from ions with a C? C? S skeleton. Appearance energy measurements for ions generated with the two structures above and all give rise to the same ΔHf value for these three isomeric forms. Ab initio molecular orbitals calculations confirm that these three ions fortuitously have very similar heats of formation. A potential energy diagram rationalizing the isomerizations and the principal fragmentation reaction is presented.  相似文献   

16.
We present an atom‐resolved analysis method that traces physical quantities such as the root‐mean‐square bond length fluctuation and coordination number for individual atoms as functions of temperature or time. This method is applied to explain the temperature‐dependent behaviors of three types of NiN (N=12,13,14) clusters. The detailed studies for the three types of clusters reveal characteristics as follows: (a) as the temperature increases, all three types of clusters undergo two‐stage melting, irrespective of the existence of vacancy or adatom on the icosahedral surfaces, (b) the melting of icosahedral clusters with vacancy starts with vacancy hopping, which has not been observed for any type of small clusters (N<34), (c) the melting of the icosahedral clusters with adatom (N=14) is initiated by adatom hopping, followed by the site exchange between the adatom and surface atoms. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 380–387, 2000  相似文献   

17.
The geometrical structures and thermal energies (E), enthalpies (H) and Gibbs free energies (G) of 13 isomers of 5‐chlorouracil (5ClU) in the gas and water phases were investigated using the density functional theory (DFT) method at the M06‐2X/6‐311++g(3df,3pd) level. The isomers of 5ClU can be microhydrated at different molecular target sites. The mono‐ and dihydrated forms are the most stable in both the gas and water phases, and, because of the intermolecular interactions, the hydrations lead to a degree of change in the stability trend. Two types of isomerizations were considered: the internal H—O bond rotations in which the H atom rotates 180° around the C—O bond and the intramolecular proton‐transfer reactions in which an H atom is transferred between an O atom and a neighbouring N atom. The forward and backward energy barriers for isomerizations of nonhydrated 5ClU were calculated. In addition, 16 optimized transition‐state structures for water‐mediated catalysis on isomerizations of 5ClU were investigated. The forward and backward proton‐transfer energy barriers of water‐mediated catalysis on isomerizations of 5ClU were obtained. The results indicate that the catalytic effect of two H2O molecules is much greater than that of one H2O molecule in isomerizations of 5ClU.  相似文献   

18.
Since gold clusters have mostly been studied theoretically by using DFT calculations, more accurate studies are of importance. Thus, small neutral and anionic gold clusters (Aun and Aun?, n=4–7) were investigated by means of coupled cluster with singles, doubles, and perturbative triple excitations [CCSD(T)] calculations with large basis sets, and some differences between DFT and CCSD(T) results are discussed. Interesting isomeric structures that have dangling atoms were obtained. Structures having dangling atoms appear to be stable up to n=4 for neutral gold clusters and up to n=7 for anionic clusters. The relative stabilities and electronic properties of some isomers and major structures are discussed on the basis of the CCSD(T) calculations. This accurate structure prediction of small gold clusters corresponding to experimental photoelectron spectral peaks is valuable in the field of atom‐scale materials science including nanocatalysts.  相似文献   

19.
The collective electronic response of NanKn clusters has been studied for some model structures. In their low-temperature lowest-energy structure, those clusters have all the K atoms on the surface. The collective oscillation frequencies for clusters with the K atoms segregated to the surface are red-shifted with respect to the corresponding frequencies for isomers with a very similar underlying skeleton but with the Na atoms segregated to the surface. The collective frequency varies smoothly with respect to the degree of relative segregation. These results may be useful in the analysis of the collective response of large alloy clusters and microcrystals. © 1995 John Wiley & Sons, Inc.  相似文献   

20.
Various nitrogen clusters, Nx, are selected for the present theoretical study. The number of nitrogen atoms chosen in this work varies from x = 8 to x = 32. PM3, which is known as one of the best semiempirical methods, is selected for the self-consistent molecular orbital calculations. The geometrical optimization, vibrational frequencies, and thermochemical computations are all involved for various types of molecular nitrogen clusters. The results show that all Nx's belong to the category of stable high-energy compounds. Comparison of average bond energy and delocalization energy of all cases reveals that N20(Ih symmetry) is the most stable molecule among all the nitrogen clusters studied. In addition, our results show five-membered rings are the most favored in the structures of nitrogen clusters (Nx). © 1996 John Wiley & Sons, Inc.  相似文献   

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