内嵌富勒烯[M@C_(70)](M=Li,Na,K,Be,Mg,Ca)与[Mg(H_2O)_6]~(2+)相互作用的密度泛函理论研究

使用密度泛函理论方法,对内嵌多种碱金属及碱土金属原子(M=Li,Na,K,Be,Mg,Ca)的富勒烯C70与水合Mg(II)离子之间的相互作用进行了理论研究.首先对各原子(M=Li,Na,K,Be,Mg,Ca)嵌入C70后的形成能进行了讨论,之后计算了[M@C70]各体系与[Mg(H_2O)6]2~+的相互作用能,并采用自然键轨道理论(NBO)研究了电荷转移的情况,最后进行了电子密度拓扑分析.结果表明,内嵌原子半径越大,[M@C70]各体系的热力学稳定性就越高,转移至[Mg(H_2O)6]2~+的电荷也随之增加,二者之间的相互作用属于闭壳层相互作用及共价作用.     

内嵌富勒烯[M@C70] (M=Li, Na, K, Be, Mg, Ca)与[Mg(H2O)6]2+相互作用的密度泛函理论研究

使用密度泛函理论方法, 对内嵌多种碱金属及碱土金属原子(M=Li, Na, K, Be, Mg, Ca)的富勒烯C70与水合Mg(II)离子之间的相互作用进行了理论研究. 首先对各原子(M=Li, Na, K, Be, Mg, Ca)嵌入C70后的形成能进行了讨论, 之后计算了[M@C70]各体系与[Mg(H2O)6]2+的相互作用能, 并采用自然键轨道理论(NBO)研究了电荷转移的情况, 最后进行了电子密度拓扑分析. 结果表明, 内嵌原子半径越大, [M@C70]各体系的热力学稳定性就越高, 转移至[Mg(H2O)6]2+的电荷也随之增加, 二者之间的相互作用属于闭壳层相互作用及共价作用.     

Interactions between coadsorbed species on the Rh(111) surface as a function of the separation distance between them

The interaction energies as a function of the separation distance between two molecules of CO and between CO and H or O coadsorbed on the Rh(111) surface have been calculated by the Extended Hückel Method. Weak interactions of the order of a few hundredths of eV occur between the adsorbates via the metal lattice. The interaction energy is found to change its signs as the distance between the adsorbates increases. For first neighbor occupation sites the interaction energies are positive. Second and third neighbor interaction energies between CO and CO are, however, negative, the latter being stronger than the former. The interaction energies between CO and H or O are positive for first and second order neighbors and, in general, negative for third order neighbors. The computed interaction energies allow one to explain the observed LEED patterns of adsorbed CO at low temperatures. The compression of the p(2 × 2) LEED structure of CO to the (√3 × √3)R30° structure, induced by the subsequent exposure to hydrogen or oxygen, is explained on the basis of the more favorable interaction energies when CO and H, or CO and 0 are segregated.     

内嵌复合物M@C60（M=Li,Na,K,Rb,Cs）的分子力学研究

利用分子力学方法计算了碱金属内嵌复合物Ｍ＠Ｃ６０中Ｍ与Ｃ６０之间的相互作用，考察了Ｍ在Ｃ６０笼内的平衡位置。研究表明Ｌｉ和Ｎａ的平衡位置偏离Ｃ６０分子的中心，Ｋ、Ｒｂ和ＣＳ的平衡位置在Ｃ６０分子的中心。平衡位置的确定取决于色散作用和排斥作用的大小。最后，讨论了碱金属原子进入笼内的可能机制。     

Surface temperature dependence of rotational excitation of H(2) scattered from Pd(111)

Classical dynamics simulations are performed to study rotational excitation of H(2) scattered from Pd(111), taking into account energy exchange with surface phonons through a 3D surface oscillator model. We show that dynamic trapping, identified recently in the study of dissociation dynamics, plays a prominent role. The corresponding long interaction time due to several recollisions allows an efficient energy exchange between H(2) molecules and surface phonons. This microscopic mechanism explains the puzzling experimental finding on the role of surface temperature in H(2)(nu = 0,J = 1-->3) excitation.     

B2C(1A1)和BC2(2A′)的结构与解析势能函数

采用单双取代的二次组态相互作用方法，分别选用6-311++G(d,p)和6-311G(df,pd)基组，对B₂C和BC₂分子的结构进行了优化，得到这两个分子的基态结构为C_2v和C_s，基态电子状态为¹A₁和²A′，同时还得到了它们的平衡几何结构、离解能、谐振频率和力常数. 关键词： 碳化硼 Murrell-Sorbie函数 谐振频率 势能函数     

苯酚-水团簇C_6H_5OH(H_2O)_n(n=1-6)结构与电子性质的密度泛函理论研究

利用密度泛函理论,在B3LYP/6-31+G(d, p)基组水平上对苯酚-水团簇C_6H_5OH(H_2O)_n(n=1-6)的可能构型进行全优化,得到了团簇的稳定结构;在B3LYP/6-311++G(d, p)基组水平上计算得到了各团簇构型的总能量和结合能,结果显示,在团簇尺寸较小(n≤5)时,团簇C_6H_5OH(H_2O)_n的最稳定结构为平面的环状结构,团簇尺寸较大(n5)时,团簇C_6H_5OH(H_2O)_n的最稳定结构为三维立体结构.通过对团簇结合能的二阶差分、最高占据轨道与最低空轨道之间的能隙、费米能级和电离能的分析发现,团簇C_6H_5OH(H_2O)_2的最低能量结构具有较高的稳定性,可能具有幻数结构.     

等离子体屏蔽对He2+与H原子碰撞电离微分截面的影响

应用经典径迹Monte Carlo（CTMC）方法研究了He²⁺与H原子在等离子体环境下的碰撞电离过程,计算了在5—400 keV/u的能区随等离子体屏蔽作用变化的碰撞电离总截面和一阶微分截面.等离子体中带电粒子之间的相互作用采用Debye-Hückel模型来描述.由于等离子体屏蔽效应的存在,靶中束缚态电子能级及其经典微正则分布以及入射离子与靶电子的相互作用都发生了变化,而这些变化会直接影响碰撞电离过程.研究发现,碰撞电离总截面随等离子屏蔽的增加而增大,特别是在10 keV/u以下的低能区电离截面有量级的增加.对随能量变化的一阶微分截面,在低能碰撞过程中,屏蔽作用增加,微分截面呈量级增加,高能碰撞微分截面呈倍数增加.同时,屏蔽作用导致电离电子向高能方向移动,随着碰撞能量的增加两体碰撞机制的贡献越来越大,并在较高的出射电子能量出现了一个新的峰.对无屏蔽的自由原子碰撞过程,CTMC方法计算出的电离总截面在碰撞能量大于70 keV/u的较高能区在实验误差内与实验测量结果符合很好,而在较低的能区比实验值小30%—50%. 关键词： 重粒子碰撞电离 等离子体屏蔽效应 经典径迹Monte Carlo方法 Debye-Hückel模型     

Translational heating of D(2) molecules thermally desorbed from Si(100) and Ge(100) surfaces

The translational energies of D(2) molecules thermally desorbed from the Si(100) and Ge(100) surfaces under a heating rate of 6 K/s have been measured. In contrast to the previous laser desorption study, results show a considerable translational heating; the observed translational temperature is about 3 times higher than the desorption temperature for both surfaces. This fact indicates that energy barriers for adsorption are present even in the desorption pathway. Detailed balance is applicable to the adsorption and desorption dynamics of hydrogen on the Si(100) surface.     

Interaction of H2 with Si(001)-(2 x 1): solution of the barrier puzzle

The sticking probability of H2 on Si(001) is immeasurably small at room temperature, indicating the presence of a large energy barrier to adsorption. Surprisingly, the final state energy distributions of H2 molecules desorbing from Si(001) show no signs of having traversed such a barrier, in apparent contradiction with microscopic reversibility. Here we report experimental and theoretical evidence resolving this long-standing puzzle. Adsorption and desorption proceeding along two distinct, microscopically reversible pathways can explain all observations.     

Neutron powder diffraction study of methane deuterohydrate by the maximum entropy method

To examine the nature of thermal motions of the CH₄ molecules in the methane deuterohydrate (8CH₄·46D₂O), the scattering length density distribution was observed by the maximum entropy method (MEM) using neutron powder diffraction data measured in the temperature range of 7-185 K. We drew the scattering-length density distribution as three dimensional graphic images and used the same isosurface level for all temperatures. The negative scattering length density, corresponding to the H atoms of CH₄, was observed only in the large cage. The positive scattering length density attributed to the C atom of CH₄ was observed at the center of each cage. With an increase in temperature, the negative and positive scattering length densities in the large cage disappear. The positive scattering length density remains at the center of the small cage regardless of temperature. These results strongly indicate that the motions of CH₄ depend on the cage size and geometry.     

Conformational equilibria in dihaloheptasilanes X2Si[SiMe(SiMe3)2]2 with X = F,Cl, Br,I: A Raman spectroscopic and quantum chemical DFT study

Equilibrium geometries, stabilities and vibrational wavenumbers for conformers of the dihaloheptasilanes X₂Si[SiMe(SiMe₃)₂]₂ with X = F, Cl, Br and I were calculated at the density functional B3LYP level employing 6‐311G(d) basis sets and SDD pseudopotentials for Br and I. Two spectroscopically distinct low‐energy conformers were located for all four heptasilanes with energy differences of 5.5, 4.7, 1.9 and 1.2 kJ mol⁻¹ for X = F, Cl, Br and I, respectively. Five more conformers were found for difluoroheptasilane and four for X = Cl, Br and I. They all have relative energies larger than 7.5 and up to 17 kJ mol⁻¹ and are negligibly populated at room temperature. Variable temperature solution Raman spectra (−70 to + 100 °C) in a wavenumber range typical for Si Si stretching vibrations (280‐350 cm⁻¹) confirm these results. For X = Br and I, no temperature effects at all could be observed as a very rapid inter‐conversion between the two low‐energy conformers, which is fast even on the time scale of Raman spectroscopy, occurs. For X = Cl, rapid inter‐conversion also occurs, and a third conformer could be detected at higher temperatures (50–100 °C). For X = F, intensity changes with temperature are consistent with the presence of two low‐energy conformers. Copyright © 2007 John Wiley & Sons, Ltd.     

MgB和MgB_2(~1A_1)的结构与解析势能函数

分别采用QCISD/6-311G和QCISD/6-311++G(df)方法,对MgB和MgB2分子的微观结构进行理论计算.在此计算基础上,运用多体展式理论方法,推导出MgB2分子的解析势能函数,其等值势能面图准确再现了MgB2分子的结构特征及势阱深度,并讨论了B+MgB和Mg+BB分子反应的势能面特征.这些结果可用于微观反应动力学的研究.     

Dissociation energy for C(2) loss from fullerene cations in a storage ring

We have stored positively charged fullerene ions C(+)(n) (n even, from 48 to 70 and 76), C(2+)(60) and C(2+)(70) in an electrostatic storage ring and have measured the rate of emission of neutral fragments as a function of time. In the time range of the measurements, 50 micros to a few milliseconds, the rate decreases strongly due to radiative cooling of the molecules. Using the cooling rate predicted from a dielectric model, we have extracted the dissociation energies for C(2) loss from the measurements. As expected, the energies are largest for the "magic" fullerenes, C(50), C(60), and C(70), and the value of 9.8+/-0.1 eV for C(2) loss from C(+)(60) is in reasonable agreement with theory and with other recent experiments.     

Comparison between Si(100) and Si(111) in the reaction with oxygen at high temperatures

In this Letter, both the dynamics and kinetics of the reaction of oxygen molecules on Si(100)p2 × 1 and Si(111)7 × 7 and 1× 1 surfaces are compared. In all three cases, two distinct adsorption channels were observed. For oxygen molecules with translational energies less than 0.08 eV, the initial sticking is not sensitive to the energy or the angle of incidence, but displays a high sensitivity to the surface structure. At higher energies, a second channel becomes effective. The initial sticking coefficient increases rapidly and scales with the normal component of the translational energy, but the dependence on surface structure is greatly diminished. The kinetics of SiO formation are qualitatively similar on all surfaces with slightly higher rates on Si(111).     

[(μ-CF3CO2)2Ln (μ-CF3HCO2) Al (i-Bu)2·2THF]2(Ln=Nd,Y)配合物的NMR和X-射线衍射研究

对[μ-CF₃CO₂)₂Ln(μ-CF₃HCO₂)Al(i-Bu)₂·THF]₂(Ln=Nd,Y)配合物单晶结构的X-射线分析指出,配合物具有中心对称性,配位中心由两个稀土和两个Al离子组成,稀土由两个THF和6个TFA分子配位形成畸变的三盖三棱柱结构,Al由两个TFAG和两个i-Bu配位形成四面体结构。桥连Al与两个稀土的TFA分子的羧基发生歧化加氢,其碳原子由SP²型转变为SP³型.NMR研究表明,在THF溶液中,该配合物保持了它在单晶中的配位结构,所不同的是两个i-Bu在溶液中有两种异构形成,二者间为慢交换过程。     

Bond-shift rearrangement in solid Li(3)P(7)(monoglyme)(3): a (31)P MAS NMR study.

The (31)P MAS NMR spectrum of solid Li(3)P(7)(monoglyme)(3) has been reinvestigated over a wide temperature range (-70 to +77 degrees C) and under conditions of better resolution (Larmor frequency of 162 MHz and spinning rate of approximately 30 kHz) than previously measured (121 MHz and 13 kHz). At low temperatures three spinning sideband (ssb) manifolds are observed: a singlet (centered at -45 ppm relative to 85% H(3)PO(4)) due to the apical atom (A) of the P(7)-cage trianion; a 1 : 1 : 1 triplet (at -110, -117, and -124.5 ppm) due to the negatively charged equatorial (E) atoms, and a one to two doublet (at -161 and -168.5 ppm) due to the basal (B) atoms. These results are consistent with the P(7) cage having nearly, but not perfect, C(3v) symmetry. The compound appears to be well ordered in the solid state with very little structural dispersity. On heating, the NMR lines broaden and eventually coalesce into a single ssb manifold. This behavior is ascribed to bond-shift rearrangement similar to the Cope rearrangement in bullvalene. A MAS 2D exchange experiment and a quantitative analysis of the 1D NMR lineshapes indicate that, unlike in solution where the rearrangement involves a single bond shift at a time, in the solid the process involves a succession of two bond shifts: The first leads to an intermediate species in which the rearranged P(7) cage is inverted, while in the subsequent step a second bond shift takes place that also restores the original orientation of the cage in the lattice. The overall effect of the double bond shift is equivalent to cyclic permutation of the phosphorus atoms within the five member rings of the P(7)-cage. The quantitative analysis of the dynamic lineshapes shows that this cyclic permutation proceeds at a different rate in one ring (k(d)(1)) than in the other two (k(d)(2,3)). The kinetic parameters for these processes are E(a)(1)=18.7 kJ/mol, E(a)(2,3)=58.0 kJ/mol, k(d)(1)(17 degrees C)=k(d)(2,3)(17 degrees C)=10(4) s(-1). No indications for independent threefold molecular jumps of the P(7) cage were found.     

Isotope fractionation in aqua-gas systems: Cl(2)-HCl-Cl(-), Br(2)-HBr-Br(-) and H(2)S-S(2-)

We report calculated values of isotope fractionation factors between chlorine, bromine and sulphide hydrated anions and respective gaseous compounds: hydrogen chloride, hydrogen bromide, molecular chlorine and bromine and hydrogen sulphide. For the calculation of the reduced partition function ratios (β-factors) of hydrated Cl(-), Br(-) and S(2-) anions, we used a model of a cluster composed of the considered ion surrounded by two shells of H(2)O molecules. Only the electrostatic interaction between ion and water molecules treated as electric dipoles was taken into account. The β-factors for the gaseous compounds (HCl, Cl(2), HBr, Br(2) and H(2)S) were calculated from vibrational frequencies reported by Urey and Greiff [Isotopic Exchange Equilibria, J. Am. Chem. Soc. 57, 321 (1935)] and Schauble et al. [Theoretical Estimates Equilibrium Chlorine-Isotope Fractionation, Geochim. Cosmochim. Acta 67, 3267 (2003)]. Low-temperature isotope fractionation between chlorine-hydrated anion and hydrogen chloride attains 1.55-1.68‰ (this work), which is in good agreement with experimental data (1.4-1.8‰) [Z.D. Sharp, J.D. Barnes, T.P. Fischer and M. Halick, An Experimental Determination of Chlorine Isotope Fractionation in Acid Systems and Applications to Volcanic Fumaroles, Geochim. Cosmochim. Acta 74, 264 (2010)]. The predicted isotope fractionations for hydrated bromine and HBr, Br(2) gases are very small, 1000?ln α, do not exceed 0.8‰; thus, the expected variations of bromine isotope composition in aqua-gas systems will require enhanced precision for their detection. In contrast, the sulphur isotope fractionation between H(2)S( gas ) and S(2-) attains 6.0‰ at room temperature and drops nearly linearly to 3.1‰ at 350°C.     

巴基球C60内嵌原子(Li,Na,K,Rb,Cs;F,Cl,Br,I)的稳定存在位置及其复合物的几何与电子结构

本文用量子化学EHMO/ASED方法对巴基球C60内嵌原子(A=Li,Na,K,Rb,Cs;F,Cl,Br,I)进行了计算。结果表明除Li、Na外,其它元素当置于C60笼中心附近时,复合物能量最低,最稳定;对于内嵌Li、Na原子的巴基球(Li@C60)及(Na@C60),当Li、Na位于围绕中心的一个球壳层(r～1.5A)内时最稳定。     

碱金属修饰的多孔石墨烯的储氢性能

基于第一性原理深入研究了碱金属原子(Li,Na,K)修饰的多孔石墨烯(PG)体系的储氢性能,并且通过从头算分子动力学模拟了温度对Li-PG吸附的H2分子稳定性的影响.研究结果表明,PG结构的碳环中心是碱金属原子最稳定的吸附位置,PG单胞最多可以吸附4个碱金属原子,Li原子被束缚最强,金属原子间无团聚的倾向;H2分子通过极化机制吸附在碱金属修饰的PG结构上,每个金属原子周围最多可以稳定地吸附3个H2分子;Li-PG对H2分子的吸附最强(平均吸附能为-0.246 eV/H2),Na-PG对H2分子的吸附较弱(平均吸附能为-0.129 eV/H2),K-PG对H2分子的吸附最弱(平均吸附能为-0.056 eV/H2),不适合用做储氢材料;在不考虑外界压强且温度为300 K的情况下,Li-PG结构可稳定地吸附9个H2分子,储氢量为9.25 wt.%;在400 K时,有7个吸附H2分子脱离Li-PG的束缚,在600-700 K的范围内,吸附H2分子全部脱离了Li-PG体系的束缚.