零维到三维结构中三核铜自旋阻挫簇单元的磁构关系和反对称交换

量子自旋液体是最近几年刚被人们证实除铁磁体、反铁磁体之外的第三种磁性类型,因其有望解释高温超导的运行机制、改变计算机硬盘信息存储方式而在物理、材料等领域备受关注。自旋阻挫作为量子自旋液体的最小单元可能是解开量子自旋液体诸多问题的钥匙,所以在磁学、电学研究领域再一次成为人们研究的热点。基于文献报道的三核铜配合物[Cu3(μ3-OH)(μ-OPz)3(NO3)2(H2O)2]·CH3OH(1),我们合成了三维金属有机框架配合物{[Ag(HOPz)Cu3(μ3-OH)(NO3)3(OPz)2Ag(NO3)]·6H2O}n(2)(HOPz=甲基(2-吡嗪基)酮肟),并从自旋阻挫的角度对二者磁性质进行对比和详细分析。磁化率数据表明自旋间有很强的反铁磁相互作用和反对称交换。通过包含各向同性和反对称交换的哈密顿算符对两者磁学数据进行拟合并研究其磁构关系,所获最佳拟合参数为:配合物1:Jav=-426 cm^-1,g⊥=1.83,g∥=2.00;配合物2:Jav=-401 cm^-1,g⊥=1.85,g∥=2.00。     

Extended Open-Chain Polyenides as Versatile Delocalized Anion Ligands for Metal Chain Clusters

Although small cyclic- and open-chain unsaturated hydrocarbon anions such as cyclopentadienide and open-chain pentadienide are used as the strongly electron-donating auxiliary ligands for metal complexes, more extended π-conjugated unsaturated hydrocarbon anions have rarely been used in coordination chemistry, despite their potential ability to serve as the multiply bridging π-ligands for metal clusters. This work reports isolation of metal chain clusters bearing the multi-dentate, open-chain extended unsaturated hydrocarbon anion ligands. The extended open-chain π-conjugated polyenyl ligands could effectively stabilize oxidized palladium chains, including an unprecedented [Pd₄]⁴⁺ chain.     

Structures and stability of Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>C and Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>N clusters

We report results of the atomic and electronic structures of Al₇C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is found to be the one in which carbon atom occupies an interstitial position in Al₇ cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al₇C^- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al₇C^- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al₇N which has the same number of valence electrons as in Al₇C^- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al₇N)₂ to study interaction between magic clusters. Received 28 July 2001     

Multiple Window and Cluster Size Scan Procedures

Researchers apply scan statistics to test for unusually large clusters of events within a time window of specified length w, or alternatively an unusually small window w that contains a specified number of events. In some cases, the researcher is interested in testing for a range of specified window lengths, or a set of several specified number of events k (cluster sizes). In this paper, we derive accurate approximations for the joint distributions of scan statistics for a range of values of w, or of k, that can be used to set an experiment-wide level of significance that takes into account the multiple comparisons involved. We use these methods to compare different ways of choosing the window sizes for the different cluster sizes. One special case is a multiple comparison procedure based on a generalized likelihood ratio test (GLRT) for a range of window sizes. We compare the power of the GLRT with another method for allocating the window sizes. We find that the GLRT is sensitive for very small window sizes at the expense of moderate and larger window sizes. We illustrate these results on two examples, one involving clustering of translocation breakpoints in DNA, and the other involving disease clusters.     

Hydroxylation-induced modifications of the Al2O3/NiAl(0 0 1) surface at low water vapour pressure

STM, STS, LEED and XPS data for crystalline θ-Al₂O₃ and non-crystalline Al₂O₃ ultra-thin films grown on NiAl(0 0 1) at 1025 K and exposed to water vapour at low pressure (1 × 10⁻⁷-1 × 10⁻⁵ mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O_1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al³⁺ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlO_xOH_y with x + y ∼ 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H₂O) = 1 × 10⁻⁷ mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified.     

Reorientational relaxation and rotational–translational coupling in water clusters in a d.c. external electric field

Molecular dynamics simulations have been carried out for small water clusters (N=16, 32, 64) in a d.c. electric field at T=200 K. It was shown that for relatively weak fields, there was a significant decrease of reorientational and structural relaxation times for all cluster sizes examined. Regarding the molecular reorientational motions, in the strong field regime, a decoupling of tumbling and spinning librations was observed. Reorientational relaxation times of the dipole and vector joining the two hydrogen atoms were found to follow different relaxation laws, with the former decreasing and the latter increasing with electric field increase. These trends were qualitatively explained by invoking the Debye model with field-dependent friction for dipole librations and the symmetric double-well for spinning rotations on a plane perpendicular to the field axis. Finally, the interdependence of the reorientation on the translational modes of the cluster was indicated, with the translationally slow molecules being rotationally slow as well and vice versa.     

用遗传算法研究Con(n=3-60)团簇的结构特性

我们采用半经验的Gupta多体势结合遗传算法,系统地研究了Con(n=3-60)团簇的几何结构特性.我们发现在钴团簇的生长中存在一个类fcc构型与类Ih构型之间的竞争,从n=39开始,钴团簇呈现出明显的Ih生长模式.Con(n=3-60)团簇的幻数为13,19,23,38,55,结合钴团簇的平均最近邻原子间距和平均配位数,分析了钴团簇幻数序列存在的原因.我们发现团簇内部原子具有增强团簇对称性和加强团簇稳定性的显著作用.     

Au-analyte adducts resulting from single massive gold cluster impacts

Utilizing Au₄₀₀⁴⁺ primary ions produces large molecular ion yields, some in excess of unity, with minimal surface damage. A surprising observation is the occurrence of Au-analyte adducts as part of the ejecta desorbed by a single Au-cluster impact. We present data that demonstrate that Au and Au-adducts as secondary ions (e.g., AuCN⁻, AuGly⁻ and AuCsI⁻) are the result of the interaction between a single primary ion, Au₄₀₀⁴⁺ and the target atoms.     

Barrier-free intermolecular proton transfer in the uracil-glycine complex induced by excess electron attachment

The photoelectron spectra (PES) of anions of uracil-glycine and uracil-phenylalanine complexes reveal broad features with maxima at 1.8 and 2.0 eV. The results of ab initio density functional B3LYP and second order M?ller-Plesset theory calculations indicate that the excess electron occupies a π^* orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of glycine to the O8 atom of uracil. As a result, the four most stable structures of the anion of uracil-glycine complex can be characterized as the neutral radical of hydrogenated uracil solvated by the anion of deprotonated glycine. The similarity between the PES spectra for the uracil complexes with glycine and phenylalanine suggests that the BFPT is also operative in the case of the latter anionic species. The BFPT to the O8 atom of uracil may be related to the damage of nucleic acid bases by low energy electrons because the O8 atom is involved in a hydrogen bond with adenine in the standard Watson-Crick pairing scheme. Received 6 April 2002 Published online 13 September 2002     

激光溅射Ni等离子体与气相CH3OH团簇的反应

用激光溅射 分子束技术研究了气相中Ni的等离子体与甲醇分子团簇的反应 .观察到Ni+ (CH3 OH) n、NiO+ (CH3 OH) n、H+ (CH3 OH) n、H3 O+ (CH3 OH) n 四个种类的团簇正离子和CH3 O-(CH3 OH) n(n≤ 2 5 )一种团簇负离子 .详细考察了激光烧蚀等离子体作用于脉冲分子束的不同位置时 ,对团簇产物种类和团簇尺寸大小的影响 .发现NiO+ (CH3 OH) n 是由Ni+ (CH3 OH) n 团簇内的脱甲烷反应生成的 ,而H+ (CH3 OH) n、H3 O+ (CH3 OH) n主要是激光等离子体中的电子与甲醇团簇碰撞电离产生的