Theoretical study on the mechanism of the reaction of CH4+MgO

The reactions of (1) CH4 + MgO --> MgOH. + CH3. and (2) CH4 + MgO --> Mg + CH3OH have been studied on the singlet spin state potential energy surface at the MP2/6-311+G(2d,2p) level. These two reaction channels, both involving intermediates and transition states, have been rationalized by the structures of the species involved, natural bond orbital (NBO), and vibrational frequency analysis. We have considered two initial interacting models between CH4 and MgO: a collinear C-H approach to the O end of the MgO forming the MgOCH4 complex with C-3nu symmetry and three hydrogen atoms of the methane point to the Mg end of the MgO forming the OMgCH4 complex with C-1 symmetry. The calculations predict that reactions 1 and 2 are exothermic by 39.8 and 86.5 kJ mol(-1), respectively. Also, the former reaction proceeds more easily than the latter, and the complex HOMgCH3 is energetically preferred in the reaction of MgO + CH4.

拓扑异构酶的结构、功能与进化的结构生物信息学研究

拓扑异构酶（topoisomerase）是一类控制和修改双螺旋DNA复制和转录过程中的拓扑结构的酶，是生命活动中最重要的酶。以IA类和II类拓扑异构酶中共存的toprim以及CAP-like结构域为研究对象，对拓扑异构酶中的三大类酶的分子进化情况进行了分析。结果显示在IA类和II类酶之间序列保守性很低，但是具有两个保守的结构域，在IIB类拓扑异构酶中toprim结构域中存在着和其他toprim结构域相同的四个保守位点，而在CAP-like结构域中IA类和II类中存在较大差异，没有明显的序列保守性，IIB类和IIA类的CAP-like结构域在二级结构上非常相似。从toprim结构域系统进化研究中我们发现IIA类和皿类中toprim结构域的进化关系很近，两类酶的toprim结构域在亲缘关系上和primase较远，而以上三者和IA类的进化关系最远。CAP-like结构域的系统进化研究发现IIA类以及IA类的domain4的CAP-like结构域进化关系比较近，IIB类和他们之间关系稍微远一些，IA类的domain3和以上几个结构域的关系较远，这也与他们的二级结构上的一致性是相同的。通过分析，IIA、IIB类起源于类似IA类的古老的拓扑异构酶，'在IA类进化中经过基因复制产生了两个不同的CAP-like结构域。然后祖先拓扑异构酶发生了变化，N'端加入了ATPase结构域和DNAgyrase/Mutlsecond结构域，形成了严格依赖ATP供能的真核生物IIA类，在细菌中断开成为两个亚基的细菌中IIA类。IIB类是祖先细胞的IIA类的一个或者是两个亚单位在古细菌以及真核生物中通过复制、重组和缺失造成的，IIB类中的toprim结构域很接近IIA类，可以认为，llB类中的toPrim结构域直接由IIA类转移而来，而IIB类中的cAP一1汰e结构域较IIA类中产生更早一些，应该是由拓扑异构酶祖先中产生的二级结构为aβaaββ的CAP-like结构域直接进化而来。然而，两个结构域的基因在连接到一起时候发生了不同于一般顺序的拼接，于是nB类中两个结构域形成了不同于现在的IA类和IIA类的顺序。

Structures and physical properties of n=3 Ruddlesden-Popper compounds Ca4Mn3-xNbxO10 (0 <= x <= 0.2)

The Ruddlesden-Popper series of compounds Ca4Mn3-xNbxO10(x = 0-0.2) have been prepared by solid-state methods. Structural, magnetic, electrical, and magnetoresistive studies were performed on the compounds. Nb doping caused increases in both unit cell volume and octahedral distortion. The magnetization measurements indicated that the doped samples displayed ferromagnetism-like behavior, which could be explained by the double-exchange interaction between Mn4+ and Mn3+ induced by the charge-compensation effect.

Selective hydrogenation of nitrobenzene to aniline in dense phase carbon dioxide over Ni/gamma-Al2O3: Significance of molecular interactions

The selective hydrogenation of nitrobenzene (NB) over Ni/gamma-Al2O3 Catalysts Was investigated using different media of dense phase CO2, ethanol, and n-hexane. In dense phase CO2, the total rate of NB hydrogenation was larger than that in organic solvents under similar reaction conditions; the selectivity to the desired product, aniline, was almost 100% over the whole conversion range of 0-100%. The phase behavior of the reactant mixture in/under dense phase CO2 was examined at reaction conditions. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions Of CO2 with the following reactant and reaction intermediates: NB, nitrosobenzene (NSB), and N-phenylhydroxylamine (PHA). Dense phase CO2 strongly interacts with NB, NSB, and PHA, modifying the reactivity of each species and contributing to positive effects on the reaction rate and the selectivity to aniline. A possible reaction pathway for the hydrogenation of NB in/under dense phase CO2 over Ni/gamma-Al2O3 is also proposed.

Copolymerization of ethylene with norbornene catalyzed by cationic rare earth metal fluorenyl functionalized N-heterocyclic carbene complexes

Rare earth metal bis(alkyl) complexes attached by fluorenyl modified N-heterocyclic carbene (NHC) (Flu-NHC)Ln(CH2SiMe3)(2) (Flu-NHC = (C13H8CH2CH2(NCHCCHN)C6H2Me3-2,4,6); Ln = Sc (2a); Y (2b); Ho (2c); Lu (2d)), ((tBu)Flu-NHC)Ln(CH2SiMe3)(2) ((tBu)Flu-NHC = 2,7-(Bu2C13H6CH2CH2)-Bu-t(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (1a); Lu (1d)) and attached by indenyl modified N-heterocyclic carbene (Ind-NHC)Ln(CH2SiMe3)(2) (Ind-NHC = C9H6CH2CH2(NCHCCHN)C6H2Me3-2,4,6; Ln = Sc (3a); Lu (3d)), under the activation of (AlBu3)-Bu-i and [Ph3C][B(C6F5)(4)], showed varied catalytic activities toward homo- and copolymerization of ethylene and norbornene. Among which the scandium complexes, in spite of ligand type, exhibited medium to high catalytic activity for ethylene polymerization (10(5) g mol(Sc)(-1) h(-1) atm(-1)), but all were almost inert to norbornene polymerization. Remarkably, higher activity was found for the copolymerization of ethylene and norbornene when using Sc based catalytic systems, which reached up to 5 x 10(6) g mol(Sc)(-1) h(-1) atm(-1) with 2a. The composition of the isolated copolymer was varying from random to alternating according to the feed ratio of the two monomers (r(E) = 4.1, r(NB) = 0.013).

Phase behavior of rare earth manganites

Among complex oxides containing rare earth and manganese BaLn(2)Mn(2)O(7)( Ln = rare earth) with the layered perovskite type and Ln(2)(Mn, M)O-7 with pyrochlore-related structure were studied since these compounds show many kinds of phases and unique phase transitions. In BaLn(2)Mn(2)O(7) there appear many phases, depending on the synthetic conditions for each rare earth. The tetragonal phase of so-called Ruddlesden-Popper type is the fundamental structure and many kinds of deformed modification of this structure are obtained. For BaEu2Mn2O7 at least five phases have been identified from the results of X-ray diffraction analysis with the space group P4(2)/mnm, Fmmm, Immm and A2/m in addition to the fundamental tetragonal I4/mmm phase. In the pyrochlore-related type compounds, Ln(2)Mn(2-x)M(x)O(7)(M = Ta, Nb, W etc), there also appear several phases With different crystal structures. With regard to every rare earth, Ln(2)MnTaO(7) phase is stable only for excess Ta and can be obtained under high oxygen partial pressure process. This group has trigonal structure with zirkelite type ( P3(1)21 space group).

A Monte Carlo simulation for the micellization of ABC 3-miktoarm star terpolymers in a selective solvent

We have used Monte Carlo simulation to study the micellization of ABC 3-miktoarm star terpolymers in a selective solvent (good to A segment, bad to B and C segments). The simulation results reveal that the self-assembled morphology is determined by the block length, molecular architecture, terpolymer concentration and insolubility of insoluble block in the solvent. In dilute solution, symmetric terpolymers (N-B = N-C = 30) tend to aggregate into a novel wormlike pearl-necklace structure linked by an alternating arrangement of B and C spheres, whereas the asymmetric terpolymers (NB = 10, NC = 50) are likely to aggregate into spherical or cylindrical micelles (formed by C blocks) connected with some small B spheres, when the concentration of terpolymer is relatively low (chain number is 100). However, when the concentration of terpolymer is relatively high (chain number is 250), the symmetric terpolymers tend to aggregate into a netlike structure linked by an alternation of B and C spheres, whereas the asymmetric terpolymers are likely to aggregate into wormlike micelles (formed by C blocks) connected with some of small spheres (formed by B blocks). Moreover, when the insolubility of insoluble block in the solvent is weak, the insoluble blocks aggregate into some incompact micelles.

Determination of rare and rare earth elements in soils and sediments by ICP-MS using Ti(OH)(4)-Fe(OH)(3) co-precipitation preconcentration

A method was developed for the determination of trace and ultratrace amounts of REE. Cd. In. Tl. Th. Nb, Ta. Zr and Hf in soils and sediments. With NaOH-Na2O2 as the flux. Ti(OH)(4)-Fe(OH)(3) co-precipitation as the preconcentration technique and inductively coupled plasma mass spectrometry (ICP-MS) for measurement, the whole procedure was concise and suitable for batch analysis of multi-element solutions. An investigation was carried out of the Ti(OH)(4)-Fe(OH)(3) co-precipitation system, and the results obtained showed that the natural situation of Ti tightly coexisting with Nb. Ta, Zr and Hf in geological samples plays a very important role in the complete co-precipitation of the four elements. The accuracy of this procedure was established using six Chinese soil and sediment certified reference materials (GSS and GSD). and the relative errors between the found and certified values were mostly below 10%.

Ti-Al系金属间化合物的价电子结构及其合金化行为

由固体和分子经验电子理论(EET)分析了Ti-Al系金属间化合物合金化前后的价电子结构,再计算了该系合金各相的均匀变形因子α和解理能Gc 值,据此分析了合金化对该系金属间化合物脆性的影响。结果表明,常量合金化元素Nb使Ti3Al基合金的α2相无序化,增加了韧的第二相,减弱了Ti-Ti共价键,综合导致α和Gc 值增大,使Ti3Al的脆性有本质改善;微量合金化元素Mn减弱了TiAl基合金的Al-Al共价键,并诱发孪生,使α和Gc 有所提高,在一定程度上降低了TiAl的脆性;TiAl3极低的α和Gc导致恶劣的本征脆性,Mn也难以改变其脆性本质

The role of Li-O bonds in calculations of nonlinear optical coefficients of LiXO3-type complex crystals

The second-order nonlinear optical (NLO) tenser coefficients of LiXO3 (X = I; Nb or Ta) type complex crystals have been calculated using the chemical bond theory of complex crystals. Contributions of each type of bond to the total second-order NLO coefficient d(ij) and the linear susceptibility X are quantitatively determined. All tensor values thus calculated are in good agreement with experimental data. The Li-O bonds are found to be an important group in the contributions to the total NLO tenser coefficient, especially for those in LiNbO3 and LiTaO3. The importance of Li-O bonds depends on the environment of Li atom in these crystals.

Bond-charge calculation of nonlinear optical susceptibilities of LiXO3 type complex crystals

Second order nonlinear optical (NLO) tensor coefficients of LiXO3 (X = I, Nb, Ta) type crystals have been evaluated on the basis of the dielectric theory of complex crystals and the modified bond charge model. The current method is capable of calculating single bond contributions to the total second order NLO susceptibility. The tenser values thus calculated agree well with experimental data. By introducing the subformula equation and the concept of the effective charge of one valence electron, we are able to successfully treat such complex crystals as LiXO3 type compounds. In addition, the bond charge expression is modified to a more reasonable form for complex crystals. (C) 1998 Elsevier Science B.V.

Eu~(3+)激活的LaNb(Ta)复合卤氧化物的合成和光谱特性

采用真空气体输运技术制备了Eu0 .0 6La0 .94 M2 O6X(M =Nb ,Ta ;X =Cl,Br) ,测定了它们的晶体结构及光谱 ,研究了其光谱性质与晶体结构的关系 ,不同金属阳离子和卤素配体对铕光谱性质的影响及其变化规律。

The effect of stoichiometry on nonlinear optical properties of LiNbO3

From the chemical bond viewpoint, second-order nonlinear optical (NLO) tensor coefficients of LiNbO3 have been investigated. The single-bond contributions to the second-order NLO susceptibility and the linear susceptibility were determined. The tensor values thus calculated are in good agreement with experimental data. Based on theoretical results of LiNbO3 with Li/Nb = 1, we also have calculated linear and nonlinear optical properties of nonstoichiometric samples with Li/Nb < 1. In the calculation, we find that the Li-O bond is an important type of chemical bond in these LiNbO3 samples, which have large NLO contributions to the total nonlinearities. The refractive indices and second-order NLO tensor coefficients have been determined as a function of the stoichiometry.

液/液界面电化学分析在双水相萃取中的应用

用循环伏安法(CV)研究了聚乙二醇(PEG)-6000/磷酸钾(K_3PO_4)双水相体系中各种组分对麦迪霉素推动质子在水/硝基苯(W/NB)界面上转移行为的影响.用液/液界面电化学分析技术检测了麦迪霉素在双水相体系中的分配情况,发现了麦迪霉素在双水相体系中的选择性分配,其分配系数大于44.测定的线性范围为50～800μg/mL,回收率大于93%.

Cupric (II)-2,2 '-Biquinoline Transfer Across the Liquid/Liquid Interface and Kinetics of Complexation

Cupric (II) ion transfer across the water/nitrobenzene interface facilitated by 2,2'-biqulnoline was investigated by cyclic voltammtry. Transfer process was controlled by diffusion. At [BQ](NB)>>[Cu2+](W), transfer ion was found to be 1:2 Cu2+-biquinoline