氯化氢与乙烯加成反应中化学键动态转变的图像

利用从头算方法研究了氯化氢(HCl)与乙烯(CH2CH2, C2H4)的加成反应。依据分子形貌(MF)理论,计算得到了分子中一个电子所受到的作用势(PAEM)及Dpb,研究了该反应过程中化学键的形成、断裂及范德华相互作用。在分子中紧邻的两原子之间,如果两原子间的Dpb值比体系的电离能大,表明两者之间主要是成键相互作用,反之是范德华相互作用。研究表明,MF理论能够很好地描述分子间的这种范德华作用,并能够特征化化学键的形成或断裂,进而为分子间的化学键形成或断裂提供新的转变图像。     

Generation and characterization of a fairly stable triplet carbene

Most molecules are held together by covalent bonds-electron pairs jointly shared by the two atoms that are linked by the bond. Free radicals, in contrast, have at least one unpaired electron. In the case of carbon-based radicals, the carbon atom at the radical centre no longer makes four bonds with other atoms as it would do in its normal, tetravalent state. The presence of unpaired electrons renders such radicals highly reactive, so they normally occur only as transient intermediates during chemical reactions. But the discovery by Gomberg in 1900 of triphenylmethyl, the first relatively stable free radical containing a central trivalent carbon atom, illustrated that radicals with suitable geometrical and electronic structures can be stable. Compounds containing a divalent carbon atom that uses only two of its four valence electrons for bonding are usually less stable than Gomberg-type radicals with trivalent carbon. Although the role of these so-called carbenes in chemical reactions has long been postulated, they were unambiguously identified only in the 1950s. More recently, stable carbenes have been prepared, but the singlet state of these molecules, with the two nonbonding valence electrons paired, means that they are not radicals. Carbenes in the second possible electronic state, the triplet state, are radicals: the two nonbonding electrons have parallel spins and occupy different orbitals. Here we report the preparation and characterization of a triplet carbene, whose half-life of 19 minutes at room temperature shows it to be significantly more stable than previously observed triplet carbenes.     

团簇V3BP电子自旋密度的研究

基于密度泛函理论，对B3LYP/Lanl2dz赝势基组水平的V₃BP的11种优化构型通过分析其电子自旋密度分布进行稳定性的研究。结果表明：团簇V₃BP最稳定构型1^（4）外围电子自旋密度分布均匀，对称性好；内部原子间成键强弱均匀，对称性好；最不稳定构型8^（2）外围α电子和β电子交替分布且分布不均匀，对称性差且极不规整；内部原子间成键强度各不相同，同一类型键原子间成键强弱也不均匀；团簇V₃BP各优化构型的外围电子分布由α电子和β电子共同组成，且α电子所占比例较大；构型2^（4）、1^（2）、4^（2）、8^（2）内部原子间成键后均为β电子过剩，其余构型内部原子间成键后均为α电子过剩。     

非共价相互作用对中心手性的影响及中心手性的定义

建立在van't Hoff碳正四面体构型学说基础上的中心手性的概念未涉及非共价相互作用对分子中心手性的影响.可以想像,当某原子连有两个相同的原子或基团(如氢原子)时,可因非共价相互作用(如分子内氢键)的存在(如只与其中的一个氢原子形成氢键)成为不等性原子或基团.考虑到非共价相互作用对分子中心手性可能产生的影响,中心手性的概念似应为:一个原子(如C,N)的四支键分别指向四面体的各顶点,并分别键连四个不等性原子或基团(包括孤电子对)时,则该原子是手性的.     

Observation of the two-channel Kondo effect

Some of the most intriguing problems in solid-state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly correlated electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanometre-scale semiconductor 'artificial atom', and to understand and control this simple system in detail(3). Here we combine artificial atoms to create a highly correlated electron system within a nano-engineered semiconductor structure. We tune the system in situ through a quantum phase transition between two distinct states, each a version of the Kondo state, in which a bound electron interacts with surrounding mobile electrons. The boundary between these competing Kondo states is a quantum critical point-namely, the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin.     

磷,砷,锑和铋原子nd轨道在原子簇中的成键作用

对原子簇P_4,As_4,Sb_4,Sb_4~(2-)和Bi_4~(2-)中磷、砷、锑和铋原子的nd轨道参予成键作用进行了研究.通过对体系能量和在形成化学键的两原子间找到键子的几率的计算表明,磷、砷、锑和铋原子的nd轨道对这些原子簇中键的形成起着重要的作用.     

Experimental measurement of valence electron concentration of icosahedral AlPdMn quasicrystal

The valence charge density distribution for the icosahedral AlPdMn (i-AlPdMn) quasicrystal was obtained with the structure factors of the nine strongest symmetry inequivalent reflections, which were refined by using the quantitative convergent beam electron diffraction (QCBED) technique. It shows that the bonding charge is localized. The enhanced charge density in the middle of the aluminum-transition-metal (Al-TM) bond shown in the valence charge density distribution is the characteristic of covalent bonding. Assuming that the shape of an atom is a sphere with covalent radius, the number of electrons that each atom gains or loses in 55 different pseudo-Mackay clusters (PMCs) was calculated based on the obtained valence charge density distribution. It indicates that almost all the atoms lose electrons except a few Pd atoms that are in some particular shells. It also shows that the atoms of an identified element could have different valences because of chemically and/or structurally different local environments in which the atoms situate. Regardless of the topology and chemical occupancy, the number of valence electrons per atom in a cluster is close to 1.69. This strongly suggests that the pseudo-Mackay clusters are stabilized at a certain electron concentration. Biography: YU Fengmei (1965–), female, Professor, Ph.D., research direction: quasicrystal physics.     

等价式在簇合物中的应用

介绍了分子或复杂离子的等价式概念，等价式与分子或复杂离子具有相同的价电子数．等价式与分子或复杂离子的结构及它们中的每个原子的成键情况是紧密联系的，同时等价式仅与价电子有关，而与内层电子无关．在等价式中，各种键型之间存在着明确的数学关系式，这个数学关系式完全适用于任何类型的化合物，并且无任何错误之处，同时也扩大了半拓扑图式理论的应用范围．簇合物中存在的键型种类多，成键情况比较复杂．列出了主族原子簇合物和过渡金属簇合物中各种键型之间所符合的数学关系式，这样等价式概念在簇合物中得到了应用，处理结果令人满意。     

交互式CAD系统屏幕菜单的实现

本文论述的交互式CAD系统屏幕菜单的实现方法,采用了矩阵技术、初始化方式,以及菜单调用、菜单交换、菜单显示等多层次结构。程序设计思路简洁、结构紧凑、使用灵活,易于扩充,通用性强。所述方法在服装CAD系统得到了实际应用,效果甚佳。     

Allowed and forbidden transitions in artificial hydrogen and helium atoms

The strength of radiative transitions in atoms is governed by selection rules that depend on the occupation of atomic orbitals with electrons. Experiments have shown similar electron occupation of the quantized energy levels in semiconductor quantum dots--often described as artificial atoms. But unlike real atoms, the confinement potential of quantum dots is anisotropic, and the electrons can easily couple with phonons of the material. Here we report electrical pump-and-probe experiments that probe the allowed and 'forbidden' transitions between energy levels under phonon emission in quantum dots with one or two electrons (artificial hydrogen and helium atoms). The forbidden transitions are in fact allowed by higher-order processes where electrons flip their spin. We find that the relaxation time is about 200 micro s for forbidden transitions, 4 to 5 orders of magnitude longer than for allowed transitions. This indicates that the spin degree of freedom is well separated from the orbital degree of freedom, and that the total spin in the quantum dots is an excellent quantum number. This is an encouraging result for potential applications of quantum dots as basic entities for spin-based quantum information storage.     

二硼烷分子的键能和成键特性研究

用从头算方法对二硼烷分子的键能进行了量子化学计算,对其成键特性进行了讨论,计算结果表明,B—B间有一弱键生成。     

Scaling the Kondo lattice

The origin of magnetic order in metals has two extremes: an instability in a liquid of local magnetic moments interacting through conduction electrons, and a spin-density wave instability in a Fermi liquid of itinerant electrons. This dichotomy between 'local-moment' magnetism and 'itinerant-electron' magnetism is reminiscent of the valence bond/molecular orbital dichotomy present in studies of chemical bonding. The class of heavy-electron intermetallic compounds of cerium, ytterbium and various 5f elements bridges the extremes, with itinerant-electron magnetic characteristics at low temperatures that grow out of a high-temperature local-moment state. Describing this transition quantitatively has proved difficult, and one of the main unsolved problems is finding what determines the temperature scale for the evolution of this behaviour. Here we present a simple, semi-quantitative solution to this problem that provides a basic framework for interpreting the physics of heavy-electron materials and offers the prospect of a quantitative determination of the physical origin of their magnetic ordering and superconductivity. It also reveals the difference between the temperature scales that distinguish the conduction electrons' response to a single magnetic impurity and their response to a lattice of local moments, and provides an updated version of the well-known Doniach diagram.     

Overcoming lability of extremely long alkane carbon-carbon bonds through dispersion forces

Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes and the possibility of constructing hydrocarbons with extraordinarily long C-C single bonds through steric crowding. Although empirical bond distance/bond strength relationships have been established for C-C bonds (longer C-C bonds have smaller bond dissociation energies), these have no present theoretical basis. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry, as summarized by Pauling as early as 1960 and confirmed more recently. Here we report the preparation of hydrocarbons with extremely long C-C bonds (up to 1.704??), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable--noticeable decomposition occurs only above 200?°C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H???H contact surfaces, as is evident from density functional theory computations with and without inclusion of dispersion corrections.     

Si在Au(111)面的电子结构和光学性质的第一性原理研究

基于第一性原理,对Si原子在Au(111)表面和体内掺杂的电子结构和光学性质进行了比较研究。结果表明,在Au(111)体内掺杂时,Si原子与周围六个Au原子成键,p电子更局域。电荷密度分析表明,掺杂Si后,Au原子周围的电子密度明显减少,而SiAu原子之间的电子密度明显增加,电子由Au原子向SiAu原子之间转移。通过计算Mul-liken键级表明,SiAu原子之间形成共价键,表面掺杂时的SiAu键级比体内大。吸收谱的计算表明,在体内掺杂Si原子时的吸收谱明显增强。     

Hyperconjugation not steric repulsion leads to the staggered structure of ethane

Many molecules can rotate internally around one or more of their bonds so that during a full 360 degrees rotation, they will change between unstable and relatively stable conformations. Ethane is the textbook example of a molecule exhibiting such behaviour: as one of its two methyl (CH3) groups rotates once around the central carbon-carbon bond, the molecule will alternate three times between an unstable eclipsed conformation and the preferred staggered conformation. This structural preference is usually attributed to steric effects; that is, while ethane rotates towards an eclipsed structure, the electrons in C-H bonds on the different C atoms are drawing closer to each other and therefore experience increased repulsion, introducing a rotation barrier that destabilizes the eclipsed structure. Stabilization of the staggered structure through rotation-induced weakening of the central C-C bond and hyperconjugation has been considered to be involved, but evaluation of the contributions of these effects to ethane's internal rotation barrier and conformational preference remains difficult. Here we report a series of ethane structure optimizations, where successive removal of different interactions indicates that ethane's staggered conformation is the result of preferential stabilization through hyperconjugation. Removal of hyperconjugation interactions yields the eclipsed structure as the preferred conformation, whereas repulsive forces, either present or absent, have no influence on the preference for a staggered conformation.     

分子轨道成键能的计算［Ⅰ］──部分小分子分子轨道成键能的计算结果

计算了ＣＯ、ＣＯ２、ＣＯ、ＮＯ、ＮＯ２、ＮＯ等分子轨道成键能，分析了成键性质，解释了双原子分子的ＰＥＳ谱．     

线性金属配合物Cr3(dpa)4Cl2及其氧化态的Cr—Cr键的理论研究

用密度泛函理论对具有分子导线潜在应用前景的线性金属配合物Cr3(dpa)4Cl2的对称构型1,非对称构型2及其氧化态[Cr3(dpa)4Cl2]+3进行理论计算,并结合自然键轨道理论对配合物1～3的CrCr键进行分析.结果表明:(1)中性构型1和2中只形成Cr36+的dz2轨道组成的三中心三电子键,电子由两端向中间的Cr离域,电子由中间向两端的Cr离域,电子的离域更显著.由于离域和dpa-配体的螺旋盘绕作用,Cr36+的轨道中dxz和dyz轨道成份较小且绕金属轴(Z轴)发生不同程度的旋转,难以重叠形成键.(2)氧化态3中,因配体的螺旋作用,Cr的dxy轨道绕Z轴发生不同程度旋转而无法有效重叠形成键,故CrCr短键只存在1个键和2个键的三重键.(3)中性分子的HOMO轨道为非键轨道,电子云集中在两端Cr上,两端的Cr(2的长键端的Cr)是失电子形成氧化态3的中心.     

笼形水合簇中水分子对与氢键能的关系

对随机产生的一系列5¹²、5¹²6²和4³5⁶6³笼形水合簇,使用DFT/B97D/6-311++g （2d,2p）以及DFT/M052X/6-311++g （2d,2p）方法计算其能量,并讨论了这3种笼形水合簇中不同水分子对的分布与氢键的键能、笼形水合簇稳定性之间的关系。结果表明,对于3种笼形水合簇,若只考虑近邻水分子对中心水分子对的影响,则各笼形结构中只有4种氢键类型,只要确定其中1种氢键类型的个数,其他3种氢键类型的个数也可以被确定;3种笼形水合簇中氢键的平均能量和单个氢键的平均能量都随n（1221）值的增大而增加;但是在n（1221）值相同时,不同笼形水合簇中单个氢键的平均值几乎相同;当n（1221）值不同时,不同笼形水合簇的氢键网络能量出现重叠,说明笼形水合簇稳定性与4种氢键类型的分布值之间无严格对应关系。     

镧和钕胞嘧啶配合物的量子化学研究

用量子化学ＩＮＤＯ方法研究了Ｌａ和Ｎｄ胞嘧啶配合物的电子结构和化学键，结果表明稀土对配位键的主要贡献来自其５ｄ轨道，６ｓ、６ｐ次之，４ｆ轨道几乎不参与成键，配位键为共价性。胞嘧啶的成键作用不只限于配位原子，而是扩展到配体整体，电子向稀土中心迁移是依靠共轭作用。