A theoretical study of MgH2 ambient and high-pressure phases using NQCC parameters

Quadrupolar parameters of nuclei can be used as a tool to understand the electronic structure of the compounds. Magnesium hydride (MgH₂) is a potential hydrogen storage material due to its outstanding hydrogen capacity, however, its high thermodynamic stability is unfavorable for dehydrogenation processes. Understanding the bonding nature of Mg and H is essential for improving its dehydrogenation performance. In this work the charge density distribution in MgH₂ is studied. For this purpose, using calculated NQCCs of hydrogen atoms, the electronic structure of α-MgH₂ with several high pressure forms of MgH₂ were compared. The results show that in the high pressure phases (β, γ, and δ) some hydrogens have very small NQCC and therefore these hydrogens form weaker bond with Mg. In other words, easier condition for dehydrogenation in pressure-induced forms is expected. The electric field gradient (EFG) at the site of quadrupolar nuclei were calculated to obtain NQCC parameters using Gaussian 03 at B3LYP/6-31G level of theory. The selected level and basis set give the rather acceptable qualitative NQCCs of hydrogen atoms.     

Density functional theory and Hartree–Fock-density functional theory calculations of O, S, and Ge quadrupole coupling constants

The performance of several density functional theory and Hartree–Fock density functional theory methods in conjunction with Pople type bases for the calculation of ¹⁷O, ³³S, and ⁷³Ge quadrupole coupling constants in gaseous state molecules was investigated.Assessment of the several models was made by linear regression analysis of the calculated gradient of the molecular electric field versus the experimental nuclear quadrupole coupling constant (NQCCs). Calculations for oxygen on six molecules with the B3LYP/6-311++G(3df,3p) model yield a residual standard deviation of 0.057 MHz (1.4%); for sulfur on 12 molecules with the B3LYP/6-311G(3df,3p) model, 0.42 MHz (1.8%); and for germanium on nine molecules with the B3P86/6-311G(2d) model, 0.83 MHz (1.0%).In the case of germyl acetylene, our calculations indicate that the experimental NQCC reported some time ago by Thomas and Laurie [J. Chem. Phys. 44 (1966) 2602] was incorrectly assigned with respect to algebraic sign.Predictions are made of the ¹⁷O and ³³S NQCCs in furan, 4H-pyran-4-one, 4H-pyran-4-thione, and 4H-thiapyran-4-thione; and of the ⁷³Ge NQCC in germyl bromide.     

Quantum chemical studies on antimalarial of artemisinin (qinghaosu) derivatives

In this paper theoretical studies were performed on artemisinin (qinghaosu) derivatives with semiempirical quantum chemical methods AMI and PM3. The antimalarial activity -logC has an obvious correlation with the net charge of C(16) and bond orders of bonds O(1)-C(10), O(2)-C(6), O(1)-O(2) and O(5)-C(16). According to the calculation results, we derived structure-activity relationship, presented the probable pharmacophore of qinghaosu derivatives and the interaction fashion between the drugs and the plasmodium receptor.     

Investigation of the muffin-tin approximations for potential and charge density in the MS-Xα method by means of calculations of H2O

By detailed analysis of results for H₂O it is shown that both approximations, the muffin-tin approximation of the potential as well as the muffin-tin approximation of the charge density, severely influence the results. Their effects are of the same order of magnitude. Good results for molecular total energies are achieved by roughly compensating the effects from the two approximations, not by minimizing them. The total energy changes drastically when radius or position of the outer sphere are varied. Equilibrium distances strongly depend on the choice of the atomic sphere radii and always are calculated too large, not due to the charge density approximation, but due to the potential approximation. In order to lay hold of angular properties, the sphere radii have to be chosen in a completely different way than for total energies.     

Limitations of the molecular multipole expansion treatment of electrostatic interactions for C-H...O and O-H...O hydrogen bonds and application of a general charge density approach

A molecular multipole expansion treatment (up to hexadecapole) is examined for its accuracy in describing hydrogen-bond electrostatic interactions, with particular reference to explaining the differences between blue-shifted C-H...O and red-shifted O-H...O bonds. In interactions of H2O and CH4 with point charges at hydrogen-bonding distances, we find that the molecular multipole treatment not only fails to reproduce ab initio energies but also forces on OH or CH bonds, and therefore cannot properly account for the electrostatic component of the interaction. A treatment based on a molecule's permanent charge density and its derivatives and the charge density and its derivatives induced by an external multipole distribution is in full accord with ab initio results, as shown by application to models of the H2O-H2O and CH4-FH systems. Such a charge density approach provides a fundamental basis for understanding the importance of interaction forces in initiating structural change and thereby altering molecular properties.     

Water monomer interaction with gold nanoclusters from van der Waals density functional theory

We investigate the interaction between water molecules and gold nanoclusters Au(n) through a systematic density functional theory study within both the generalized gradient approximation and the nonlocal van der Waals (vdW) density functional theory. Both planar (n = 6-12) and three-dimensional (3D) clusters (n = 17-20) are studied. We find that applying vdW density functional theory leads to an increase in the Au-Au bond length and a decrease in the cohesive energy for all clusters studied. We classify water adsorption on nanoclusters according to the corner, edge, and surface adsorption geometries. In both corner and edge adsorptions, water molecule approaches the cluster through the O atom. For planar clusters, surface adsorption occurs in a O-up/H-down geometry with water plane oriented nearly perpendicular to the cluster. For 3D clusters, water instead favors a near-flat surface adsorption geometry with the water O atom sitting nearly atop a surface Au atom, in agreement with previous study on bulk surfaces. Including vdW interaction increases the adsorption energy for the weak surface adsorption but reduces the adsorption energy for the strong corner adsorption due to increased water-cluster bond length. By analyzing the adsorption induced charge rearrangement through Bader's charge partitioning and electron density difference and the orbital interaction through the projected density of states, we conclude that the bonding between water and gold nanocluster is determined by an interplay between electrostatic interaction and covalent interaction involving both the water lone-pair and in-plane orbitals and the gold 5d and 6s orbitals. Including vdW interaction does not change qualitatively the physical picture but does change quantitatively the adsorption structure due to the fluxionality of gold nanoclusters.     

不同电极上血红素对青蒿素的电催化还原

 在含20%乙醇的Britton-Robinson缓冲液介质(pH=7.2)中,采用循环伏安法在玻碳电极和银电极上比较了血红素对青蒿素还原的催化作用. 由于血红素和青蒿素加合物的形成及血红素中Fe2+的催化作用,青蒿素在玻碳电极和银电极上的还原过电位分别降低了0.32和0.09 V,还原活化能分别降低了62.1和17.6 kJ/mol. 还比较了血红素和配合物EDTA-Fe3+对青蒿素的催化还原效果,结果表明,EDTA-Fe2+的催化作用远低于血红素. 进一步证实了血红素在青蒿素的药理研究中起着关键作用.     

Defect dynamics observed by NMR of quadrupolar nuclei in gallium nitride

Hexagonal and cubic polytypes of bulk gallium nitride powders are characterized by 69,71Ga and 14N MAS NMR at 11.7 T. The (corrected) 71Ga chemical shifts are 333.0 and 357.5 ppm, respectively; the corresponding 14N chemical shifts are -301.8 and -297.0 ppm (all shifts referenced to 1 M gallium nitrate). The 69,71Ga nuclear quadrupole coupling constants (NQCC) in the hexagonal form are axially symmetric and agree with previous single-crystal determinations. The 71Ga MAS NMR satellite pattern envelope of the cubic form has a large Gaussian half-height width of 297 kHz, due to nonzero NQCC values induced by defects. The 14N MAS NMR spinning sideband pattern of the cubic form has a Lorentzian envelope half-height width of 17.5 kHz for the same reason. A sample containing both phases shows an unexpected marked loss of the 71Ga MAS NMR satellite transition intensity expected for the hexagonal phase. Static 71Ga-selective Hahn spin-echo measurements at the perpendicular edge of the powder pattern for the hexagonal form in this sample show a large reduction in T2, especially at higher temperatures. The partial destruction of both spin-echoes and rotational echoes is due to a chemical-exchange type process involving sites having different NQCC values.     

Temperature-dependent Raman study of pure and silver nanoparticles dispersed N-(4-n-heptyloxybenzylidene)-4’-n-butylaniline (7O.4)

Temperature-dependent micro-Raman study of C-H in-plane bending mode of aromatic rings, C-N and C=N stretching of linking group (-C(H)=N) and C=C stretching of rings of pure and silver nanoparticles dispersed (0.5% and 1% by weight) Schiff’s base liquid crystal (LC) compound, N-(4-n-heptyloxybenzylidene)-4’-n-butylaniline (7O.4) in 500–2250 cm^?1 region has been done. The change in Raman spectral parameters (peak position and linewidth) at crystal–smecticG (K–smG) and smecticG–smecticC (smG–smC) gives the evidence of charge shift at phase transition which is associated with changes in orientation and vibrational freedom of the molecules. The peak position of the Raman bands shows blue shift for 0.5 wt% dispersed sample, whereas it shows red shift for 1 wt% dispersed sample. The blue and red shifts of the Raman bands indicate an increase and decrease in the charge density, respectively. The optimised structure and theoretical room temperature Raman spectra of 7O.4 were obtained using density functional theory. The vibrational assignment using potential energy distribution is reported using vibrational energy distribution analysis (VEDA).     

O2在MgO(001)完整和缺陷表面上的吸附

在密度泛函理论的框架下， 采用嵌入点电荷簇模型研究了O2在MgO(001)完整和缺陷表面上的吸附.用电荷自洽的方法确定了点电荷的值.计算结果表明， O2倾向吸附在低配位的角Mg2＋端.并且发现， 当O2为平躺吸附时，键长有较大的拉伸，将有利于O2的解离.同时，分别计算了使用裸簇和嵌入表观±2.0 e点电荷簇模型时的吸附能，并与采用电荷自洽方法的计算值进行了比较.结果表明，电荷自洽方法更能有效反映簇周围的环境，得到的计算结果能够较好地与实验值吻合.最后，分别计算了不同吸附情况下O2的振动频率.     

Magnetization-induced double-layer capacitance enhancement in active carbon/Fe3O4 nanocomposites

The effects of magnetic fields on electrochemical processes have made a great impact on both theoretical and practical significances in im-proving capacitor performance. In this study, active carbon/Fe...     

醚的初始暗氧化反应机理的研究

有机化合物的空气氧化是一个历史悠久的研究课题。早在1851年Schonbein[1]就注意到醚在空气中会被氧化产生过氧化物。本世纪初,Clover和Milas分析了醚的空气氧化产物,并且对醚的氧化机理,进行了研究。     

Magnetization-induced double-layer capacitance enhancement in active carbon/Fe_3O_4 nanocomposites

The effects of magnetic fields on electrochemical processes have made a great impact on both theoretical and practical significances in improving capacitor performance. In this study, active carbon/Fe3O4-NPs nanocomposites(AC/Fe3O4-NPs) were synthesized using a facile hydrothermal method and ultrasonic technique. Transmission electron micrographs(TEM) showed that Fe3O4nanoparticles(Fe3O4-NPs) grew along the edge of AC. AC/Fe3O4-NPs nanocomposites were further used as an electrochemical electrode, and its electrochemical performance was tested under magnetization and non-magnetization conditions, respectively, in a three-electrode electrochemical device. Micro-magnetic field could improve the electric double-layer capacitance, reduce the charge transfer resistance, and enhance the discharge performance. The capacitance enhancement of magnetized electrode was increased by 33.1% at the current density of 1 A/g, and the energy density was improved to 15.97 Wh/kg, due to the addition of magnetic particles.     

How accurate is Poisson-Boltzmann theory for monovalent ions near highly charged interfaces?

Surface sensitive synchrotron X-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion (Cs+) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H3O+ to the phosphate group, leading to proton-transfer back to the phosphate group and to a reduced surface charge. The increase of Cs+ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating possible deviations from Poisson-Boltzmann theory.     

掺铝Co_3O_4的制备及其电容性能研究

以KOH作沉淀剂制备掺杂Al(Ⅲ)的Co3O4粉体.X射线衍射显示,掺Al的Co3O4不改变其晶型结构.循环伏安、恒流充放电等测试表明,化学掺Al后的Co3O4电极,其比容量提高,当Co(Ⅱ)与Al(Ⅲ)的摩尔比为1∶0.05时,在0~400 mV的电位区间内,5 mA/cm2电流密度下,单电极放电比容量可达518.07 F/g.     

Infrared intensity analysis of hydroxyl stretching modes in carboxylic acid dimers by means of the charge–charge flux–dipole flux model

The Charge‑Charge Flux‑Dipole Flux (CCFDF) model in terms of multipoles from the quantum theory of atoms in molecules (QTAIM) was used to investigate the variations in infrared intensities of hydroxyl (O H) stretching modes during the dimerization of carboxylic acids. The hydrogen bond formation in these systems results into bathochromic shifts of vibrational frequencies for all the O H stretching modes along with huge infrared intensity increments for some of them. These bands become more intense on dimerization due mainly to changes in the cross-term contribution between charge and charge flux. In addition, interaction energies for the pair of atoms directly involved in individual hydrogen bonds (O…H) are linearly correlated to electron densities at their bond critical points (BCPs). Therefore, the hydrogen bonds between the carbonyl group (CO) of acetic acid and the hydroxyl group of halogenated monomers show the largest electron density values at their BCPs. The formation of these intermolecular interactions is also accompanied by ionic character enhancements of O H bonds and electron density decrements at their BCPs. We finally noticed that the hydrogen atom belonging to the hydroxyl group loses electronic charge, while the oxygen from the CO end becomes more negatively charged during dimerization. © 2019 Wiley Periodicals, Inc.     

Elucidation of the natural artemisinin decomposition route upon iron interaction: a fine electronic redistribution promotes reactivity

The conformational analysis of artemisinin by molecular dynamics and quantum chemistry calculations revealed the existence of seven energy minima with specific interconversion pathways. Among the seven conformers, only , and were able to undergo bond rearrangements upon Fe(2+) interaction. These rearrangements were due to a peculiar puckering of the trioxane ring that brings its three oxygen atoms in an ideal geometrical position for interacting with Fe(2+) ions, promoting an electronic redistribution in the molecule. A rapid molecule rearrangement led to a stable energy minimum structure with an additional ring that is similar to a plant metabolite. Our results suggest an alternative pathway for generating toxic radical chemical species for the malaria parasite, where artemisinin is not toxic by itself but rather is an intermediate for molecular partners that generate radical structures deleterious for the parasite proteins, after electron transfers from the Fe(2+)/artemisinin complex.     

含中氮茚有机太阳能电池染料敏化剂的分子设计

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)方法研究了9个新的中氮茚[3, 4, 5-ab]异吲哚(INI)为给体的染料敏化剂性质.对影响电池效率的光捕获效率、电子注入、染料再生和电荷复合等重要因素与D5和D9染料进行了对比.计算表明,设计的INI系列敏化剂在440-500 nm内有最大吸收峰,表现出明显的电荷分离特征, INI2具有比D9染料更高的最大理论短路电流. Fukui反应指数计算指出INI2的亲核加成最易实现.染料分子在二氧化钛(101)面吸附计算表明,染料INI2以间接注入途径实现电子注入.综合计算结果,中氮茚INI染料有希望作为性能优良的染料敏化剂而得到应用.     

脉冲激光沉积LiNi0.5Mn0.5O2薄膜正极性能

高能量密度、功率密度和高温度稳定性的全固态薄膜锂离子电池是微电子器件的理想电源.开发新型的大比容量正极薄膜材料是解决问题的关键之一.与LiCoO2正极相比,层状结构的LiNi0.5Mn0.5O2有更高的可逆比容量和结构稳定性.本文应用脉冲激光沉积法制备LiNi0.5Mn0.5O2沉积薄膜,研究了衬底材料、温度对薄膜的微观结构、表面形貌及组分的影响.由LiNi0.5Mn0.5O2电极组装半电池,研究了薄膜的电化学性能与晶体结构、表面形貌及组分间的关系,表征了LiNi0.5Mn0.5O2沉积薄膜于不同充电截止电压的循环稳定性及倍率性能,并讨论了LiNi0.5Mn0.5O2薄膜的结构特点.     

Photoelectrochemical oxidation of methanol on oxide nanosheets

Photoelectrochemical oxidation of alcohol on various nanosheet electrodes such as Nb6O17, Ca2Nb3O10, Ti(0.91)O2, Ti4O9, and MnO2 system host layers were measured to evaluate the photocatalysis of water photolysis with alcohol as a sacrificial agent. The nanosheet electrodes were prepared by the layer-by-layer (LBL) method, using electrostatic principles. The highest photooxidation current density was observed in methanol solution for Nb6O17 and Ca2Nb3O10 nanosheets, while the density was lower for Ti(0.91)O2, Ti4O9, and MnO2 nanosheets in decreasing order. The rank in the photocurrent density was in agreement with that in the photocatalytic activity, which means that the degree of photooxidation of the alcohol determines the activity of the alcohol in the water photolysis process. The photocurrent was independent of the number of nanosheet layers on the electrode, indicating that only the mono-nanosheet layer attached directly on a substrate acts as a photoelectrocatalyst and that the interlayer space is not important. Consequently, higher photooxidation current on the Nb6O17 mono-nanosheet layer means that the charge separation of electron and hole under illumination is very large and that the hole-capturing process by CH3OH is very quick compared with the surface recombination on the Nb6O17 nanosheet. The adsorption of a transition metal cation on the nanosheet acted as the surface recombination center, because the photocurrent decreased after the adsorption. The photocatalytic mechanism has been discussed in detail in terms of various photoelectrochemical behaviors.