Electronic properties of divalent-metal clusters

The transition from van der Waals to metallic bonding expected to occur in divalent-metal clusters (e.g., Be _n , Mg _n , Hg _n ) as a function of cluster size is discussed. Theoretical results for several electronic properties reflecting this transition in Hg _n -clusters are briefly reviewed and compared with available experiments. The limitations of the present theory particularly concerning the role of correlations and van der Waals interactions are discussed and possible improvements are suggested.     

Calculation of the size dependence of the ionization energy and autoionization energy of Hg-clusters

To study the transition from van der Waals to metallic bonding we calculate the size dependence of the ionization energy and 5d→6p autoionization energy of Hg _n -clusters using a parametrized LCAO model. Our results are in good qualitative agreement with experiment. Comparison with experimental results suggests that electron correlations play an important role for the transition from localized (van der Waals-like) to delocalized (covalent or metallic) electronic states occuring in Hg _n atn?13–19.     

Electron impact ionization efficiency curves of van der Waals clusters

The study of van der Waals clusters is an area of growing interest and is being widely studied for a number of reasons. The measurement of the ionization efficiency (IE) curves have yielded a wealth of information by enabling ionization and appearance energies of ions to be determined which are essential for the calculation of thermochemical data. In the case of van der Waals clusters, the measurement ofIE curves enables one to determine the qualitative trends in the ionization potentials as a function of cluster size. In additionIE curves have also offered valuable insight into ionization related processes occurring in clusters. This paper will cover some of the more recent studies of Penning ionization, exciton induced decay and Coulomb explosion in van der Waals clusters through the use of electron impactIE curves.     

Electron energy loss spectroscopy of van-der-Waals clusters

A method to measure electron energy loss spectra (EELS) of clusters with a high resolution (30 meV) has been developed and has been applied to some van der Waals clusters (Ar _n , Kr _n ). Structures have been found which relate to the excitation of atoms on the surface and inside the cluster. An influence of the cluster size on the spectra has been observed.     

Specific and nonspecific interaction forces between Escherichia coli and silicon nitride, determined by poisson statistical analysis

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.     

Electronic structure of selenium- and tellurium-clusters

The vacuum-UV-photoelectron spectra (hν=10.0 eV or 8.3 eV) of selenium- and tellurium-clusters of different sizes (up to Se₂₅ and Te₈) were measured in a molecular beam by a photoion-photoelectron-coincidence experiment. Especially, the photoelectron spectra (4p-π-electrons) of the selenium-clusters converge with growing cluster size to the well known spectra of amorphous solid selenium by taking into account the dielectric constant of the bulk material. The experimental results indicate the first evidence that larger selenium aggregates are van der Waals clusters of the molecules Se₅, Se₆, Se₇ and Se₈ which are contained in the equilibrium vapour.     

A supersonic beam ion cyclotron resonance instrument for the study of van der Waals cluster ions

For the study of ionized van der Waals cluster ions an instrument is presented, which consists of a supersonic beam cluster source coupled to an ICR spectrometer with external ion source. The neutral van der Waals clusters are generated by supersonic expansion and ionized by electron impact in the external source. The cluster ions are extracted at right angle to the neutral cluster beam and fly collision-free parallel to the magnetic field direction into the differentially pumped ICR cell. For the ion transfer, an improved lens system is presented. The cluster ion transfer lens system is capable of focusing ions with energies of a few eV perpendicular to the magnetic field direction through the differential pumping orifice. The ions are injected into the ICR cell with a trap barrier pulse, ion accumulation is possible. With this system the first ICR spectra of small cluster ions of carbon dioxide are obtained.     

The bonding in the ozone molecule: From a different perspective

A new qualitative treatment of the bonding in ozone is presented. It is based upon a combination of several simple concepts: the nonparticipation of the pairs of electrons tightly held in the atomic 2s orbitals; simple overlap of the 2p orbitals to form sigma bonds; interaction of three 2p orbitals to yield bonding and nonbonding pi molecular orbitals that are populated by electron pairs; and van der Waals repulsion between the two terminal oxygen atoms forcing these atoms apart to yield the bond angle of 117° as a compromise. Both the assumptions and the resulting bonding picture are in accord with the photoelectron spectroscopic data, the results from sophisticated molecular orbital calculations, and the common physical properties of ozone.     

The chemical imagination at work in very tight places

Diamond-anvil-cell and shock-wave technologies now permit the study of matter under multimegabar pressure (that is, of several hundred GPa). The properties of matter in this pressure regime differ drastically from those known at 1 atm (about 10(5) Pa). Just how different chemistry is at high pressure and what role chemical intuition for bonding and structure can have in understanding matter at high pressure will be explored in this account. We will discuss in detail an overlapping hierarchy of responses to increased density: a) squeezing out van der Waals space (for molecular crystals); b) increasing coordination; c) decreasing the length of covalent bonds and the size of anions; and d) in an extreme regime, moving electrons off atoms and generating new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.     

Evaluation of the individual hydrogen bonding energies in N-methylacetamide chains

The individual hydrogen bonding energies in N-methylacetamide chains were evaluated at the MP2/6-31+G** level including BSSE correction and at the B3LYP/6-311++G(3df,2pd) level including BSSE and van der Waals correction. The calculation results indicate that compared with MP2 results, B3LYP calculations without van der Waals correction underestimate the individual hydrogen bonding energies about 5.4 kJ mol^?1 for both the terminal and central hydrogen bonds, whereas B3LYP calculations with van der Waals correction produce almost the same individual hydrogen bonding energies as MP2 does for those terminal hydrogen bonds, but still underestimate the individual hydrogen bonding energies about 2.5 kJ mol^?1 for the hydrogen bonds near the center. Our calculation results show that the individual hydrogen bonding energy becomes more negative (more attractive) as the chain becomes longer and that the hydrogen bonds close to the interior of the chain are stronger than those near the ends. The weakest individual hydrogen bonding energy is about ?29.0 kJ mol^?1 found in the dimer, whereas with the growth of the N-methylacetamide chain the individual hydrogen bonding energy was estimated to be as large as ?62.5 kJ mol^?1 found in the N-methylacetamide decamer, showing that there is a significant hydrogen bond cooperative effect in N-methylacetamide chains. The natural bond orbital analysis indicates that a stronger hydrogen bond corresponds to a larger positive charge for the H atom and a larger negative charge for the O atom in the N-H?O=C bond, corresponds to a stronger second-order stabilization energy between the oxygen lone pair and the N-H antibonding orbital, and corresponds to more charge transfer between the hydrogen bonded donor and acceptor molecules.     

Ion attachment to Van der Waals clusters

Mixed ionized clusters have been produced in a supersonic nozzle beam experiment by attachment of stagnant cations (i.e. NO⁺ and Xe⁺) to neutral van der Waals clusters (i.e.Ar _n ) within a Nier type ion source. This new ionization technique leads to less fragmentation than electron impact ionization and the measured cluster distributions exhibit icosahedral shell and subshell closures which have not been detected in the case of electron impact of Ar _n -clusters ionization so far. Additionally, the obtained appearance energies and metastable fractions give insight into the production mechanism and the stability of the resulting ions.     

Atomic and Ionic Radii of Elements 1–96

Atomic and cationic radii have been calculated for the first 96 elements, together with selected anionic radii. The metric adopted is the average distance from the nucleus where the electron density falls to 0.001 electrons per bohr³, following earlier work by Boyd. Our radii are derived using relativistic all‐electron density functional theory calculations, close to the basis set limit. They offer a systematic quantitative measure of the sizes of non‐interacting atoms, commonly invoked in the rationalization of chemical bonding, structure, and different properties. Remarkably, the atomic radii as defined in this way correlate well with van der Waals radii derived from crystal structures. A rationalization for trends and exceptions in those correlations is provided.     

Calculation of the electronic properties of neutral and ionized divalent-metal clusters

The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized Hg _n clusters, we determine the size dependence of the bond character and the ionization potentialI _p (n). For neutral Hg _n clusters we obtain a transition from van der Waals to covalent behaviour at the critical sizen _c ～10–20 atoms. Results forI _p (Hg _n ) withn≤20 are in good agreement with experiments, and suggest that small Hg _n ⁺ clusters can be viewed as consisting of a positive trimer core Hg ₃ ⁺ surrounded byn?3 polarized neutral atoms.     

长程范德华力导向作用下胶体凝聚的计算机模拟

采用计算机模拟方法研究了长程范德华力在胶体凝聚过程中的作用, 发现由于胶粒间的范德华力是长程力, 它对胶粒或团簇运动将产生导向作用. 与不考虑导向作用的扩散控制团簇凝聚(DLCA)模型比较, 这种导向作用不仅加速了胶体的凝聚过程, 而且形成了更致密、分形维数更大的结构体. 研究还发现, 长程范德华力导向作用对胶粒的初始浓度非常敏感, 不论是在凝聚物的结构还是凝聚速率方面, 只有在胶粒初始浓度较低时, 该导向作用效应才明显. 其可能的原因是,在胶粒初始浓度较高时, 由于胶粒布朗运动的平均自由程很短而且位阻效应大, 从而使导向作用效应未能反映出来.     

Unexpected Ge–Ge Contacts in the Two‐Dimensional Ge4Se3Te Phase and Analysis of Their Chemical Cause with the Density of Energy (DOE) Function

A hexagonal phase in the ternary Ge–Se–Te system with an approximate composition of GeSe_0.75Te_0.25 has been known since the 1960s but its structure has remained unknown. We have succeeded in growing single crystals by chemical transport as a prerequisite to solve and refine the Ge₄Se₃Te structure. It consists of layers that are held together by van der Waals type weak chalcogenide–chalcogenide interactions but also display unexpected Ge–Ge contacts, as confirmed by electron microscopy analysis. The nature of the electronic structure of Ge₄Se₃Te was characterized by chemical bonding analysis, in particular by the newly introduced density of energy (DOE) function. The Ge–Ge bonding interactions serve to hold electrons that would otherwise go into antibonding Ge–Te contacts.     

Thermodynamics of associated solutions: Henry's constants for nonpolar solutes in water

Hu, Y., Azevedo, D., Lüdecke, D. and Prausnitz, J., 1984. Thermodynamics of associated solutions: Henry's constants for nonpolar solutes in water. Fluid Phase Equilibria,17: 303–321.A systematic derivation is presented for the Helmholtz energy of a van der Waals fluid mixture whose nonideality is ascribed to both chemical and physical interactions; this derivation, applicable to all fluid densities, leads to an equation of state which contains chemical equilibrium constants in addition to the customary physical van der Waals constants a and b. Attention is given to the need for simplifying assumptions and to the variety of symplifying assumptions that can lead to useful results. A particular equation of state is used to correlate Henry's constants for nonpolar solutes in water over a wide temperature range. The correlation, however, is only partly successful, because a one-fluid van der Waals theory of mixtures is not satisfactory for mixtures containing molecules that differ appreciably in size, especially in the dilute region.     

Mode-selective broadening in low-frequency vibrational modes of trans-stilbene van der waals complexes

Both RE2PI and LIF excitation spectra have been taken of several van der Waals complexes of trans-stilbene, p-methyl-, and m-methyl-trans-stilbene. The t-stilbene spectra reveal a strongly mode-dependent broadening of the van der Waals complex transitions associated with the low-frequency vibrations 82 and 95 cm⁻¹ above the origin. The fwhm of these peaks is three times as broad as any other feature in the spectrum. The source of the broadening appears to be fast vibrational relaxation from these modes to the van der Waals modes of the complex.     

Vacuum-UV photoemission-photoion coincidence spectroscopy of neutral metal atom clusters

A newly developed photoion-photoelectron Vacuum-UV coincidence spectrometer has been coupled to a supersonic metal atom cluster beam source and has been used to investigate the electronic structure of isolated mercury clusters in the size range from 1 to 110 at several selected discrete excitation energies between 11.3 and 7.1 eV. Excitation of the van der Waals cluster Hg₁₀ at the center of the strongD _3/2-autoionization line at 10.7 eV yields a photoelectron kinetic energy distribution between 0 and 2.5 eV indicating the population of Hg₁₀ ionic states, which are also accessible by threshold ionization.     

Using the van der Waals broadening of spectral atomic lines to measure the gas temperature of an argon–helium microwave plasma at atmospheric pressure

The applications of plasmas generated with gas mixtures have become increasingly common in different scientific and technological fields. In order to understand the advantages of these discharges, for instance in chemical analysis, it is necessary to know the gas temperature (T_g, kinetic energy of the heavy particles) since it has a great influence on the atomization reactions of the molecules located in the discharge, along with the dependence of the reaction rate on this parameter. The ro-vibrational emission spectra of the molecular species are usually used to measure the gas temperature of a discharge at atmospheric pressure although under some experimental conditions, these are difficult to detect. In such cases, the gas temperature can be determined from the van der Waals broadening of the emitted atomic spectral lines related to this parameter. The method proposed is based on the van der Waals broadening taking into account two perturbers.