Lattice energy estimation for inorganic ionic crystals

An empirical method based on chemical bond theory for the estimation of the lattice energy for ionic crystals has been proposed. The lattice energy contributions have been partitioned into bond dependent terms. For an individual bond, the lattice energy contribution made by it has been separated into ionic and covalent parts. Our calculated values of lattice energies agree well with available experimental and theoretical values for diverse ionic crystals. This method, which requires detailed crystallographic information and elaborate computation, might be extended and possibly yield further insights with respect to bond properties of materials.     

Structural features of Fe<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>TiSe<Subscript>2</Subscript> materials with the retrograde solubility in the solid state

Changes in the crystal structure, which are accompanied by iron intercalation and release in the layered intercalation compound TiSe₂, demonstrating the retrograde solubility in the completely solid state, are considered. Various concentration regions not exceeding, exceeding, and corresponding to the leakage limit of the overlap of titanium orbitals coordinated by iron are analyzed. It is shown that at low temperatures (below 400°С) the behavior of iron in the TiSe₂ lattice is governed by a covalent bond of iron with the lattice whereas at high temperatures (above 1000°С) iron becomes an ionic impurity. In the intermediate temperature range, iron atoms are involved in either covalent or ionic bond with the lattice. When the concentration of iron in the form of the ionic impurity increases, an increase in the cell parameter in the direction perpendicular to the layers is accompanied by a compression of the Se–Ti–Se sandwich and an increase in the van der Waals gap. When the covalent bond forms, there is a decrease in the lattice parameter in the direction perpendicular to the layers, which is accompanied by an increase in the width of the Se–Ti–Se sandwich and a decrease in the van der Waals gap.     

Unusual bond formation in aspartic protease inhibitors: a theoretical study

The origin of the formation of the weak bond N|C...O involved in an original class of aspartic protease inhibitors was investigated by means of the electron localization function (ELF) and explicitly correlated wave-function (MRCI) analysis. The distance between the electrophilic C and the nucleophilic N centers appears to be controlled directly by the polarity and proticity of the medium. In light of these investigations, an unusual dative N-C bonding picture was characterized. Formation of this bond is driven by the enhancement of the ionic contribution C(+)-O(-) induced mainly by the polarization effect of the near N lone pair, and to a lesser extent by a weak charge delocalization N-->CO. Although the main role of the solvating environment is to stabilize the ionic configuration, the protic solvent can enhance the C(+)-O(-) configuration through a slight but cumulative charge transfer towards water molecules in the short N-C distance regime. Our revisited bond scheme suggests the possible tuning of the N-CO interaction in the design of specific inhibitors.     

Density functional theory studies on the dissociation energies of metallic salts: relationship between lattice and dissociation energies

The formation and physicochemical properties of polymer electrolytes strongly depend on the lattice energy of metal salts. An indirect but efficient way to estimate the lattice energy through the relationship between the heterolytic bond dissociation and lattice energies is proposed in this work. The heterolytic bond dissociation energies for alkali metal compounds were calculated theoretically using the Density Functional Theory (DFT) of B3LYP level with 6‐311+G(d,p) and 6‐311+G(2df,p) basis sets. For transition metal compounds, the same method was employed except for using the effective core potential (ECP) of LANL2DZ and SDD on transition metals for 6‐311+G(d,p) and 6‐311+G(2df,p) calculations, respectively. The dissociation energies calculated by 6‐311+G(2df,p) basis set combined with SDD basis set were better correlated with the experimental values with average error of ca. ±1.0% than those by 6‐311+G* combined with the LANL2DZ basis set. The relationship between dissociation and lattice energies was found to be fairly linear (r>0.98). Thus, this method can be used to estimate the lattice energy of an unknown ionic compound with reasonably high accuracy. We also found that the dissociation energies of transition metal salts were relatively larger than those of alkaline metal salts for comparable ionic radii. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 827–834, 2001     

The Continuous-Addition-of-Reagent Technique in Catalytic Kinetic-Based Determinations

Abstract

A straightforward, inexpensive continuous-addition-of-reagent (CAR) system described elsewhere was applied to kinetic-catalytic analyses. The catalytic effect of copper(l1) on the oxidation of 4,4′-dihydroxybenzophenone thiosemicarbazone (DBPT) by hydrogen peroxide was exploited for this purpose. The reaction was developed by continuously adding DBPT to the reaction vessel containing the other ingredients of the catalytic reaction. The results obtained show the proposed CAR method to surpass its conventional counterpart, and its suitability for the determination of copper in serum samples.     

Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6:U4+ crystals. The 5f17s1 manifold

Excitons trapped at impurity centers in highly ionic crystals were first described by McClure and Pedrini [Phys. Rev. B 32, 8465 (1985)] as excited states consisting of a bound electron-hole pair with the hole localized on the impurity and the electron on nearby lattice sites, and a very short impurity-ligand bond length. In this work the authors present a detailed microscopic characterization of impurity-trapped excitons in U(4+)-doped Cs(2)GeF(6). Their electronic structure has been studied by means of relativistic ab initio model potential embedded cluster calculations on (UF(6))(2-) and (UF(6)Cs(8))(6+) clusters embedded in Cs(2)GeF(6), in combination with correlation methods based on multireference wave functions. The local geometry of the impurity-trapped excitons, their potential energy curves, and their multielectronic wave functions have been obtained as direct, nonempirical results of the methods. The calculated excited states appear to be significantly delocalized outside the UF(6) volume and their U-F bond length turns out to be very short, closer to that of a pentavalent uranium defect than to that of a tetravalent uranium defect. The wave functions of these excited states show a dominant U 5f(1)7s(1) configuration character. This result has never been anticipated by simpler models and reveals the unprecedented ability of diffuse orbitals of f-element impurities to act as electron traps in ionic crystals.     

Stability of two-dimensional crystalline aggregates of a protein studied by molecular dynamics

Two-dimensional protein (ferritin) aggregates with a square lattice symmetry, which were formed within a thin liquid layer on a mercury surface, were studied by molecular dynamics (MD) simulation. For the simulation, the ferritin molecule was modeled by an assembly of 49 spheres, and the intermolecular interactions were given by simple formulae. During the simulation, molecules were confined within a layer, which corresponds to the thin liquid layer. An annealing MD simulation was done starting from a random molecular configuration within the layer, and aggregates with the square lattice symmetry were also obtained. To study the stability of aggregates, dissociation processes of the aggregates were analyzed using MD simulations at room temperature. Interactions between the nearest-neighbor molecules were regarded as bonds. Mean bond energies and correlation coefficients between the bond energies were calculated from the MD trajectories. A decay profile according to the dissociation was obtained, yielding a dissociation rate constant. Buried bonds were stronger than peripheral bonds. The larger the aggregate size, the stronger the bond for each of the buried and peripheral bonds. A simple theoretical account, which is applicable to a general bonded network, was introduced to analyze the dynamics of the aggregates. © 1994 by John Wiley & Sons, Inc.     

Interpolation determination of the lattice energy of ionic crystals within the framework of stereoatomic model

A new method based on graph theory was suggested for interpolation calculation of the lattice energy U of ionic crystals. The method is based on revealing matrix correlation between the ionic radii and U values for MX compounds, where M is a metal and X is halogen, hydrogen, or chalcogen. A new formula was obtained for calculating the lattice energy solely from the ionic radii, without introduction of abitrary factors. The mean error of determining U for alkali metal halides is 0.49%. The lattice energies were calculated for a large group of inorganic substances. The accuracy of the interpolation calculation of the lattice energy of ionic crystals depends on the degree of ionicity of the bond: With an increase in the covalent contribution, the error increases.     

1-乙胺基-3-甲基咪唑四氟硼酸盐吸收CO_2的理论研究

采用密度泛函理论(DFT)对离子液体1-乙胺基-3-甲基咪唑四氟硼酸盐([NH2e-mim][BF4])吸收CO_2的反应机理进行了研究.在B3LYP/6-311++G(d,p)计算水平下,对离子液体[NH2e-mim][BF4]的结构及与CO_2反应的中间体、过渡态和产物进行了全优化,获得了优化结构的构型参数、振动频率和热力学数据.利用自然键轨道(NBO)分析了离子液体[NH2e-mim][BF4]和CO_2的自然电荷布居.计算结果表明,通过阳离子[NH2e-mim]+自偶解离产生的阳离子[NH3e-mim]2+能与阴离子[BF4]-结合形成更强的离子键.根据反应吉布斯自由能变(ΔG0—)和焓变(ΔH0—)的计算结果,判断离子液体[NH2e-mim][BF4]吸收CO_2按理论摩尔比2∶1分步进行反应,吸收过程中质子的转移需克服52.51 k J/mol的能垒.     

二苯醚的势能面与构象研究

聚苯醚是具有多元重复氧桥结构的化合物。对这类体系的构象研究是一个很有意义的课题,如最简单的聚苯醚——二苯醚,文献中已提出三种不同的构象模型。Smyth等人认为,二苯醚中两苯环都垂直于C-O-C平面,即θ=φ=90°(见图1,θ,φ分别为两个苯环与C-O-C平面的夹角)。Sutton等人根据Pauling的假设认为,二苯醚中两个苯环共平面,即θ=φ=0。这样氧原子上孤对电子可参与共轭,使分子趋于稳定。Higgasi等人利用键矩加合规则,假定C-O-C键角为118°,计算了2,4,二取代甲基二苯醚及2,4,二取代碘     

聚氨酯/聚甲基丙烯酸甲酯离子聚合体型互穿聚合物网络生成动力学研究

本文采用在聚氨酯中引入丁基二乙醇胺,形成带有叔氮原子的聚氨酯(NPU)和聚甲基丙烯酸甲酯中引入甲基丙烯酸的方法,合成了带有离子键的离子聚合体型互穿聚合物网络(IPN),动力学研究表明,离子聚合体型IPN形成速率明显低于非离子聚合体型IPN,两网络的聚合速率相互抑制,并随离子键浓度增加而显著,两网络的聚合机理互不干扰。     

SiCp/ Al复合材料与化学镀镍层结合机理研究

根据结构化学理论，用SEM, EDAX和XPS等测试手段研究了镀层和碳化硅颗粒增强铝基复合材料(SiC_p/ Al)的表面、断面形貌及界面的结合状态，分析了镀层和基体之间的结合机理。结果表明，Ni镀层与复合材料界面有良好的结合，在复合材料表面的SiC-Al界面，初期沉积物Ni按Al的晶格外延生长出现微晶层，然后吸附原子扩散迁移、碰撞结合并与界面上的SiC晶格匹配生长，在镍层中诱发了拉伸应力。镍晶格和基体粒子之间产生了键合作用，形成的键显示出共价键和离子键的混合性质。     

水溶性含C_(60)聚电解质的制备及其与重氮树脂自组装膜AFMFFM的初步研究

通过BPO引发的溶液聚合 ,合成了水溶性的星状C6 0 苯乙烯 苯乙烯磺酸钠的三元共聚物 [Star shapedC6 0 poly(St SS) ],运用自组装技术 ,在水溶液中 ,含C6 0 的三元共聚物与重氮树脂 (Diazoresin)通过正负离子间的吸附力在云母基片上交替一层一层有序地组装成固体膜 .自组装膜经紫外光幅照反应 ,通过重氮基的分解 ,层间连接的离子键转变成共价键 ,从而增加薄膜的稳定性和堆砌密度 .用原子力显微镜 摩擦力显微镜(AFM FFM)考察了C6 0 在膜中的承载作用及比较不同链结构、不同链长、不同层数自组装膜的表面形貌和微摩擦性能 .初步的研究结果显示了聚合物薄膜的微摩擦性能与聚合物的化学结构、链长和膜的层数有密切关系     

A new method of calculation of the melting temperatures of crystals of Group 1A metal halides and francium metal

A new method of calculation of melting temperatures of binary ionic crystals has been suggested. The method is based on finding a matrix relation between the ionic radii (the lattice energy U) and melting temperature of ionic crystals of the MX type, where M is a Group 1A metal, and X is a halogen. From the equation for the lattice energy U, a new equation has been derived for calculation of the melting temperature of ionic crystals with the use of only the ionic radii and the degree of bond ionicity ?: T _m = f(U, ?). The average error of determination of T _m for alkali-metal halides is 2.80%. The melting temperatures of francium halides and alkali-metal astatides (including FrAt) have been calculated. It has been shown that the accuracy of calculation of the melting temperature of ionic crystals depends on the degree of bond ionicity: the error increases with an increase in the covalent contribution. On the basis of the melting temperatures of metal halide crystals, a method has been developed for the calculation of the melting temperatures of corresponding metals. The melting temperature of francium has been calculated to be 24.861 ± 0.517°C.     

A variational biorthogonal valence bond method

The details of a simple and efficient scheme for performing variational biorthogonal valence bond calculations are presented. A variational bound on the energy functional is obtained through the use of a complete configuration expansion in a well-chosen subset of orbitals. The resultant wave functions are clearly dominated by the covalent (spin-coupled) structures, with a negligible contribution from ionic structures. The orbitals obtained compare favorably with overlap enhanced atomic orbitals obtained by other valence bond approaches. The method is illustrated by calculations on water and dioxygen difluoride. © 1994 by John Wiley & Sons, Inc.     

碳酸盐标准生成焓的计算

碳酸盐标准生成焓的计算戴长文，王振民，徐琰，戴晓弘（郑州大学化学系，郑州，４５００５２）（郑州高新技术开发区，郑州，４５０００１）关键词热力学性质，标准生成焓，碳酸盐预测无机化合物生成热的计算方法已见报道［１，２］，但引入经验参数过多，且计算偏差亦较...     

Photochemistry of a chiral salen aluminum complex in nonconventional solvents: use of imidazolium ionic liquids and chiral alcohols

The photochemistry of a chiral (salen)aluminum(III) chloride complex has been studied in nonconventional solvents, namely, two imidazolium ionic liquids differing on the hydrophobicity (hydrophilic BF(4)(-) or hydrophobic PF(6)(-) counter anions) and in chiral 2-butanols (R and S). Upon 355 nm laser excitation, the same transient absorption spectrum (with some solvatochromic shift in lambda(max)) was recorded in all cases and assigned to the (salen)Al(II) complex with radicaloid character at the metal atom. This intermediate arises from the photoinduced homolytic cleavage of the apical Al-Cl bond. The half-life of this radicaloid Al(II) species varies depending on the solvent, indicating that its reactivity is governed by the nature of the ionic liquid and also on the R or S configuration of the chiral alcohol.     

Variable trends in R-X bond dissociation energies (R = Me, Et, i-Pr, t-Bu)

[structure: see text] High level ab initio molecular orbital calculations confirm experimental indications that the effect of alkyl substituents (R = Me, Et, i-Pr, t-Bu) on R-X bond dissociation energies varies considerably according to the nature of X. A simple qualitative explanation in terms of valence-bond theory is presented, highlighting the increasing importance of the stabilization of R-X by the ionic R(+)X(-) configuration for electronegative X substituents (such as F, OH, and OCH(3)).     

Solid-state NMR spectra and long intradimer bonds in the pi-[TCNE]22- dianion

The principal (13)C chemical-shift values for the pi-[TCNE](2)(2-) dimer anion within an array of counterions have been measured to understand better the electronic structure of these atypical chemical species in several related TCNE-based structures. The structure of pi-[TCNE](2)(2-) is unusual as it contains two very long C-C bond lengths (ca. 2.9 Angstroms) between the two monomeric units and has been found to exist as a singlet state, suggestive of a (1)A(1g) (b(2u)(2)b(1g)(0)) electronic configuration. A systematic study of several oxidation states of [TCNE](n) (n = 0, 1-, 2-) was conducted to determine how the NMR chemical-shift tensor values change as a function of electronic structure and to understand the interactions that lead to spin-pairing of the monomer units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. Such theoretical methods provide information on the tensor magnitudes and orientations of their principal tensor components with respect to the molecular frame. Both theoretical and experimental ethylenic chemical-shielding tensors reveal high sensitivity in the component, delta(perpendicular), lying in the monomer molecular plane and perpendicular to the pi-electron plane. This largest shift dependence on charge density is observed to be about -111 ppm/e(-) for delta(perpendicular). The component in the molecular plane but parallel to the central C=C bond, delta(parallel), exhibits a sensitivity of approximately -43 ppm/e(-). However, the out-of-plane component delta'(perpendicular) shows a minimal dependence of -2.6 ppm/e(-) on the oxidation state (n) of [TCNE](n). These relative values support the claim that it is changes within the ethylenic pi-electrons and not the sigma-electrons that best account for the dramatic variations in bonding and shift tensors in this series of compounds. Concerning the intraion bonding, relatively weak Wiberg bond orders between the two monomeric components of the dimer correlate with the long bonds linking the two [TCNE(*)](-) monomers. The chemical-shift tensors for the cyano group, compared to the ethylene shifts, exhibit a reduced sensitivity on the TCNE oxidation state. The experimental principal chemical-shift components agree (within typical errors) with the calculated quantum mechanical shieldings used to correlate the bonding. The embedded ion model (EIM) was used to investigate the typically large electrostatic lattice potential in these ionic materials. Chemical-shielding principal values calculated with the EIM model differ from experiment by +/-3.82 ppm on average, whereas in the absence of an electrostatic field model, the experimental and theoretical results agree by +/-4.42 ppm, which is only a modest increase in error considering the overall ionic magnitudes associated with the tensor variations. Apparently, the effects of the sizable long-range electrostatic fields cancel when the shifts are computed because of lattice symmetry.