可卡因2/4位、3位手性异构与受体键合

运用从头算方法对可卡因2/4位和3位手性中心形成的8个手性异构体进行了理论模拟.通过分析所得结构参数、电性参数以及它们与实验参数IC50值的关系发现,2位甲酯基(R2)与受体的主要作用力可能为质子接受体羰基上O22原子的氢键,最大程度形成氢键的空间保证来自于2位的R构型,R2中甲基氢原子与受体的π电子(或孤对电子)间的弱相互作用可增强R2与受体的键合;受体对3位的结合部位可能存在一个体积较大的疏水袋,其允许C3上苯酯基(R1)的苯环在较大范围内旋转并通过官能团与R1发生包含静电吸引力的较强相互作用,某些官能团与R1手性匹配时可额外产生某些结合力;N8原子可能以游离碱形式与受体发生氢键作用.     

丙氨酰胺基硫脲及其阴离子配合物的手性传递

通过改变丙氨酸N端的取代基,设计合成了2种基于丙氨酸残基的酰胺基硫脲衍生物.通过X-射线单晶衍射、核磁共振波谱、吸收光谱和圆二色(CD)光谱等方法研究了2种不同取代基对硫脲分子自身及其阴离子络合物的氢键网络的影响以及由此引起的手性传递能力的不同.实验表明,当丙氨酸N端取代基为N,N-二甲基甘氨酸残基时,其羰基氧能与硫脲基团中的NH以分子内十元环氢键结合,分子呈折叠构象,实现了手性从丙氨酸残基到苯基硫脲基团的传递.阴离子加入后,该分子的硫脲基团与阴离子以双重氢键结合,分子原有的氢键网络发生改变,羰基氧与酰胺氮氢(NH)之间七元环氢键的存在将手性更多的限制于丙氨酸残基部分,手性传递至阴离子结合部位的效率极低.而N端被二甲基取代的硫脲衍生物由于分子内不同的氢键网络而表现出不同的手性传递能力:结合阴离子前,分子本身不具有折叠构象而无手性传递发生,结合阴离子后,新的氢键网络诱导了手性信号的长程传递.     

手性分子与NMR谱

目的 研究手性分子与核磁共振(NMR)谱图之间的关系,为手性化合物研究提供参考.方法 通过举例分析,阐述手性分子与NMR谱图之间的关系.如毗邻一个手性中心的亚甲基中的2个质子化学位移并非等价,-CH2-基团实际上是-CHaHb-结构.此方法可以用来确定ee%(对映体过量)值.结果 给出了应用手性衍生试剂、手性溶剂化试剂和手性位移试剂来确定ee%值的方法;此外,对手性螺环化合物的1H NMR谱图也进行了讨论.结论 对于手性分子与NMR谱图之间关系的研究,可为通过NMR谱图表征手性分子提供参考.     

水络合物

一、配位体——水分子(H_2O)的特点1、水分子中作为配位原子的氧,以不等性SP~3杂化与两个氢原子共价相连,结合成稳定的化合物——水.氧本身还有未成键的孤电子对,这是它作为配位原子必不可少的条件.2、作为配位体的水分子,是极性分子.它的固有偶极矩(D)比氧分子还大,但它的极化率(α)又比氨分子相差较大.故从水分子的偶极矩总体效应来看要比氨分子.     

螺旋手性SWCNT的尺寸对赖氨酸分子手性转变反应的限域影响

采用量子力学与分子力学组合的ONIOM方法,研究了两种构象的赖氨酸分子限域在螺旋手性单壁碳纳米管内的手性转变机理.结构分析表明:纳米管管径越小,限域在其中的赖氨酸分子骨架形变越明显,手性碳上的氢原子与氨基上氮的氮原子距离越小.势能面计算表明,两种构象的赖氨酸分子限域在SWCNT(6,4)时,旋光异构反应决速步的吉布斯自由能垒分别是194.72和170.08kJ·mol~(-1),分别由质子从手性碳向氨基氮和质子从手性碳向氨基氮与氨基上的质子向羰基氧双质子协同迁移的过渡态产生的.与裸反应的此通道决速步能垒252.6kJ·mol~(-1)相比较有显著降低.两种构象的赖氨酸分子限域在SWCNT(6,4)内旋光异构反应的表观能垒分别是160.00和178.59kJ·mol~(-1).他们限域在SWCNT(7,4)内时,旋光异构反应决速步的能垒分别是238.28和217.18kJ·mol~(-1);限域在SWCNT(8,4)内时,旋光异构反应决速步的能垒分别是253.00和250.11kJ·mol~(-1).结果表明:螺旋手性单壁碳纳米管的孔径越小,对赖氨酸分子手性转变反应的限域催化作用越好;限域在SWCNT(6,4)内的赖氨酸分子构象1更容易旋光异构.     

超分子双股螺旋链构建新策略：双重交叉卤键作用

<正>DNA双股螺旋结构的重要生命功能,如遗传信息的储存和传递,推动了仿生双股螺旋链的发展.DNA双股螺旋由单链内的共价磷酸酯键和双链间的非共价多重互补氢键维持(图1).受此启示,仿生双股螺旋链多由两条寡聚或聚合物链组成,由链间非共价相互作用,如氢键、金属配位、盐桥等维系~([1]).     

手性分子与NMR谱（英文）

目的研究手性分子与核磁共振(NMR)谱图之间的关系,为手性化合物研究提供参考。方法通过举例分析,阐述手性分子与NMR谱图之间的关系。如毗邻一个手性中心的亚甲基中的2个质子化学位移并非等价,—CH_2—基团实际上是—CH_aH_b—结构。此方法可以用来确定ee%(对映体过量)值。结果给出了应用手性衍生试剂、手性溶剂化试剂和手性位移试剂来确定ee%值的方法;此外,对手性螺环化合物的~1 H NMR谱图也进行了讨论。结论对于手性分子与NMR谱图之间关系的研究,可为通过NMR谱图表征手性分子提供参考。     

含能材料HBT中氢键相互作用的第一性原理研究

通过第一性原理研究单斜结构的5,5'-双四唑肼(HBT)晶体的能带结构和态密度,分析氢键相互作用.5,5'-双四唑肼晶体拥有4.01 eV左右的直接带隙,能量带隙较宽.通过分态密度分析碳原子和氮原子的p轨道电子和氢原子的s轨道电子的分布特征,结果表明5,5'-双四唑肼晶体内部存在分子间和分子内部的NH…N氢键相互作用.同时对氢键的Mulliken布局分布进行讨论.     

手性三唑类化合物的QSRR研究

基于手性碳原子上取代基的次序规则定义了新型手性指数(Xj),将其引入电距矢量(Mf)建立手性电距矢量(c Mf).手性电距矢量与26种手性三唑类化合物在2种纤维素类手性固定相(Chiralcel OD和ChiralcelOJ)上的3类高效液相色谱(HPLC)容量因子(ND,RD,RJ)具有良好的相关性.通过最佳变量子集回归方法,建立了它们二元或三元的QSRR模型.ND,RD和RJ的复相关系数(R2)依次为0.868,0.992和0.925,相应逐一剔除法交叉验证系数(Q2)依次为0.789,0.982和0.863,表明这些模型具有良好的预测能力与稳健性.模型显示分子中-CH3,-CH2-,CH-,-NH-和-O-等基团间的相互作用是影响手性三唑类化合物容量因子的主要因素.     

专一性溶剂化作用对S_N反应的影响

有机化学的反应中,饱和碳原子上的亲核取代(S_N)反应,作为一类重要的反应,有较系统和深入的研究,影响S_N反应有诸多的因素,如底物的结构、亲核试剂和离去基因的性质等,它们对反应的类型和反应速率有很大的影响.而S_N反应大多在液相中进行,溶剂被广泛地采用,因而溶剂的性质对S_N反应的影响也是不容忽视的.一、溶剂的分类溶剂有多种分类方法,其中按专一性溶质-溶剂相互作用来分类,是Parker根据溶剂与阴离子或阳离子的专一性相互作用,将溶剂分为质子型溶剂和非质子极性溶剂两大类,若加上第三类非质子非极性溶剂则更为完善.三者的主要区别在于溶剂的极性和形成氢键的能力.(一)质子型溶剂质子型溶剂的特点是含有能与电负性较大的元素(F、Cl、O、N等质子受体)相结合的氢原子.例如F-H、-O-H、—N—H等分子或基团中的氢原子,它作为氢键给体具有     

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。     

Direct observation of second-order atom tunnelling

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism.     

Amplification of chirality in two-dimensional enantiomorphous lattices

The concept of chirality dates back to 1848, when Pasteur manually separated left-handed from right-handed sodium ammonium tartrate crystals. Crystallization is still an important means for separating chiral molecules into their two different mirror-image isomers (enantiomers), yet remains poorly understood. For example, there are no firm rules to predict whether a particular pair of chiral partners will follow the behaviour of the vast majority of chiral molecules and crystallize together as racemic crystals, or as separate enantiomers. A somewhat simpler and more tractable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial molecular arrangement of these systems makes it easier to study the effects of specific chiral interactions; moreover, chiral assembly and recognition processes can be observed directly and with molecular resolution using scanning tunnelling microscopy. The enantioseparation of chiral molecules in two dimensions is expected to occur more readily because planar confinement excludes some bulk crystal symmetry elements and enhances chiral interactions; however, many surface structures have been found to be racemic. Here we show that the chiral hydrocarbon heptahelicene on a Cu111 surface does not undergo two-dimensional spontaneous resolution into enantiomers, but still shows enantiomorphism on a mesoscopic length scale that is readily amplified. That is, we observe formation of racemic heptahelicene domains with non-superimposable mirror-like lattice structures, with a small excess of one of the heptahelicene enantiomers suppressing the formation of one domain type. Similar to the induction of homochirality in achiral enantiomorphous monolayers by a chiral modifier, a small enantiomeric excess suffices to ensure that the entire molecular monolayer consists of domains having only one of two possible, non-superimposable, mirror-like lattice structures.     

立体电子效应控制的频哪醇重排反应

本文简要叙述了由立体电子效应控制的频哪醇重排反应。指出在频哪醇重排反应中,离去基团和迁移基团总是处在反式共平面位置,而迁移基团与它所在碳上的羟基氧上一对孤电子也处于反式共平面位置。在这对孤电子的立体电子效应作用下,促使重排基团带着一对电子从离去基团的反位上进行1,2——位移。     

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.     

Synthesis and crystal structure of tetranuclear zinc benzoate

A tetranuclear zinc benzoate Zr4O(C6H5CO2)6 was synthesized and characterized by X-ray single crystal determination. It crystallizes in cubic, space group Ia-3d. Its crystal cell is very large, α=4.10063(18)nm, V=68.953(5)nm^3 and Z=48. The structure is composed of discrete Zr4O(C6H5CO2)6 molecules. In each molecule, four zinc atoms are held together by a central oxygen atom, which results in the formation of a regular tetrahedron. All benzoate ligands coordinate to zinc atoms in a bidentate bridging mode. Each zinc atom is in a slightly distorted tetrahedral geometry, coordinated by three benzoate oxygen atoms and the central oxygen atom. The intermolecular interactions result in the formation of a three-dimensional supramolecular framework, with non-intersecting parallel channels.     

Capture of hydroxymethylene and its fast disappearance through tunnelling

Singlet carbenes exhibit a divalent carbon atom whose valence shell contains only six electrons, four involved in bonding to two other atoms and the remaining two forming a non-bonding electron pair. These features render singlet carbenes so reactive that they were long considered too short-lived for isolation and direct characterization. This view changed when it was found that attaching the divalent carbon atom to substituents that are bulky and/or able to donate electrons produces carbenes that can be isolated and stored. N-heterocyclic carbenes are such compounds now in wide use, for example as ligands in metathesis catalysis. In contrast, oxygen-donor-substituted carbenes are inherently less stable and have been less studied. The pre-eminent case is hydroxymethylene, H-C-OH; although it is the key intermediate in the high-energy chemistry of its tautomer formaldehyde, has been implicated since 1921 in the photocatalytic formation of carbohydrates, and is the parent of alkoxycarbenes that lie at the heart of transition-metal carbene chemistry, all attempts to observe this species or other alkoxycarbenes have failed. However, theoretical considerations indicate that hydroxymethylene should be isolatable. Here we report the synthesis of hydroxymethylene and its capture by matrix isolation. We unexpectedly find that H-C-OH rearranges to formaldehyde with a half-life of only 2 h at 11 K by pure hydrogen tunnelling through a large energy barrier in excess of 30 kcal mol(-1).     

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.     

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.