Long-Range van der Waals Interactions in Density Functional Theory

The early difficulties in accounting for long-range van der Waals interactions in the framework of density functional theory (DFT) have been overcome to a certain extent in recent works by several groups, and those interactions can be computed numerically. In this paper a derivation of the analytical form of the attractive van der Waals interaction between two neutral atoms with polarizabilities α₁ and α₂ at large distance R, namely E _int=−C ₆ α₁ α₂/R ⁶ is performed within the context of DFT. Use is made of the properties of the Coulomb correlation hole, and it is shown that nonlocal Coulomb correlations are responsible for long-range dispersion interactions.     

范德华作用和静电作用下的二元粒子自组装

采用简单粗粒化粒子模型,通过郎之万动力学模拟研究了具有范德华作用和静电作用的二元粒子自组装．研究发现,通过改变粒子尺寸和粒子间作用强度,二元粒子能够自发形成各种聚集结构,如球形、堆叠层状与管状结构．利用两亲性分子或两嵌段聚合物自组装理论,解释了二元粒子聚集结构的形成规律．当向溶液中加入反电荷离子时,模拟表明粒了聚集结构在相图中的分布出现了明显偏移．     

Intramolecular van der Waals Interactions Challenge Anion Binding in perthio-Bambusurils

Bambusurils (BUs) are known to be rigid cavitands that feature an extended, jigger-like conformation, and the BU[6]s strongly bind anions within their hydrophobic cavity. These features are not necessarily shared by the family of perthio-BUs. This study reveals that perthio-BUs assume a compact conformation and perthio-BU[6]s are poor anion binders, crystallizing as anion-free species from solutions containing halide salts. Computational studies show that the equatorial sulfur atoms compete against guest anions for binding with the glycoluril methine groups via strong van der Waals (vdW) attractive interactions. These competitive contacts not only account for the diminished anion-binding of perthio-BUs, but also explain their compact conformation. The semithio- and perthio-BU[4]s form linear coordination polymers with Hg^II in the solid-state regardless of their intrinsic molecular conformation. The strong involvement of sulfur atoms in intramolecular interactions differentiates the equatorial from the axial (peripheral) heteroatoms, thus offering chemoselective and regioselective transformations.     

Nonsingular van der Waals Potential and Its Contributions to Gravitational Coagulation

The coagulation and the stability of dilute colloids at high Péclet number were studied in consideration of the universal nonsingular van der Waals interactions recently developed by J. X. Lu and W. H. Marlow (1995, Phys. Rev. Lett. 74, 1724). The capture efficiency of gravitational coagulation of uncharged colloids was found to be diminished due to the effect of finite molecular size (EFMS). The gravitational coagulation stability diagram of charged colloids was also found to be shifted when the gravitational convection was much stronger than the Brownian diffusion. Copyright 2001 Academic Press.     

Direct Measurement of Repulsive van der Waals Interactions Using an Atomic Force Microscope

The forces of interaction between a flat poly(tetrafluoroethylene) (PTFE) surface and gold spheres (of radii 3–8 μm) were measured as a function of apparent surface separation for different intervening media. For air, fluorinated alkanes, and polar liquids the interaction between the surfaces was found to be attractive. With intervening liquids of low-polarity the interaction was found to be repulsive. This repulsion is attributed to a negative composite Hamaker coefficient leading to van der Waals repulsion.     

The Dielectric Function for Water and Its Application to van der Waals Forces

The dielectric response, varepsilon(ixi), for water (which is required in Lifshitz theory to calculate the van der Waals interactions in aqueous systems) is commonly constructed, in the absence of complete spectral data, by fitting a damped-harmonic-oscillator model to absorption data. Two sets of parameters for the model have been developed corresponding to different constraints: Parsegian and Weiss (J. Colloid Interface Sci., 1981, 81, 285) and Roth and Lenhoff (J. Colloid Interface Sci., 1996, 179, 637). These different representations of the dielectric response lead to significant differences in the van der Waals force calculated from Lifshitz theory. In this work, more recent and complete spectral data for water were compiled from the literature and direct integration of the Kramers-Kronig relations was used to construct a new varepsilon(ixi) for water at 298 degrees K. This approach also allows a number of different types of spectral measurements (such as infrared spectroscopy, microwave resonance techniques, and x-ray inelastic scattering) in the compilation of absorption data over a large frequency range (on the order of 8 to 10 decades in frequency). A Kramers-Kronig integration was employed to construct the real and imaginary parts of varepsilon(omega), varepsilon'(omega), and varepsilon"(omega) for water from the different spectral measurements before calculation of varepsilon(ixi) from its integral definition. The resulting new varepsilon(ixi) is intermediate between the Parsegian-Weiss and Roth-Lenhoff representations of varepsilon(ixi), does not use a model, and treats the conversion of absorption data as rigorously as possible. We believe the varepsilon(ixi) from the present work is the most reliable construction for use in van der Waals force calculations using Lifshitz theory. The extension of the varepsilon(ixi) construction to other temperatures is also discussed. Copyright 2000 Academic Press.     

A Stable Crystalline Triarylphosphine Oxide Radical Anion

Triarylphosphine oxides (Ar₃P=O) are being intensely studied as electron‐accepting (n‐type) materials. Despite the widespread application of these compounds as electron conductors, experimental data regarding the structural and electronic properties of their negatively charged states remain scarce owing to their propensity for follow‐up chemistry. Herein, a carefully designed triarylphosphine oxide scaffold is disclosed that comprises sterically demanding spirofluorenyl moieties to shield the central phosphoryl (P=O) moiety. This compound undergoes chemical one‐electron reduction to afford an exceptionally stable radical anion, which was isolated and characterized by X‐ray crystallography for the very first time. The experimental data, corroborated by computational studies, shall allow for the construction of phosphine oxide materials with enhanced stability.     

Dewetting induced by complete versus nonretarded van der Waals forces

Spontaneous rupture of some polymer films upon heating is commonplace. The very criterion for this instability is the system free energy, G(L), possessing a negative curvature. In films that are apolar with h < or = 100 nm van der Waals (vdW) interactions usually constitute a major contribution to G(L) for which the approximate form G(L) = -A/12piL(2) (where A is the Hamaker constant), ignoring retardation, has been widely used. In this work, we investigate the limits to this approximation by calculating the complete vdW interactions for popular polymer film systems in dewetting experiments including air-polystyrene-SiO2-Si, air-polystyrene-poly(methyl methacrylate)-Si, and air-poly(methyl methacrylate)-polystyrene-Si based on the theory of Dzyaloshinskii, Lifshitz, and Pitaevskii (DLP). We found that retardation effects could produce significant modifications to G(L) even when the thickness of the polymer and/or the interlayer is only 1-2 nm, contrary to conventional presumption.     

Gelation Mechanism Revealed in Organometallic Gels: Prevalence of van der Waals Interactions on Oligomerization by Coordination Chemistry

Shaping of nanomaterials is a necessary step for their inclusion in electronic devices and batteries. For this purpose, the formulation of a moldable material including these nanomaterials is desirable. Organomineral gels are a very interesting option, since the components of the nanomaterial itself form a gel without the help of a binder. As a consequence, the properties of the nanomaterial are not diluted by the binder. In this article we studied organometallic gels based on a [ZnCy₂] organometallic precursor and a primary alkyl amine which together forms spontaneously gels after few hours. We identified the main parameters controlling the gel properties monitored by rheology and NMR measurements The experiments demonstrate that the gelation time depends on the length of the alkyl chain of the amine and that the gelation mechanism derived firstly from the rigidification of the aliphatic chains of the amine, which takes precedence over the oligomerization of the inorganic backbone. This result highlights that the control of the rheological properties of organometallic gels remains mainly governed by the choice of the amine.     

Spectroscopy of DABCO-rare-gas and DABCO-DABCO van der Waals complexes

The excited electronic origin bands of several DABCO containing van der Waals complexes have been observed via (1+1) resonance enhanced multi-photon ionization. Sharp resonances with widths of about 3 cm^–1 are seen for DABCO-Rg _n=1,2,3 (Rg is Ar, Kr or Xe), for the DABCO-DABCO dimer and for DABCO-DABCO-Ar. The origins of the rare-gas complexes are blue shifted with respect to the monomer origin. Broad features originating from DABCO-Rg _n complexes with highn, appear to higher energies than the complex origins, with widths of 120 cm^–1.     

Chemical and Electrochemical Lithiation of van der Waals Tetrel-Arsenides

Lithiation of van der Waals tetrel-arsenides, GeAs and SiAs, has been investigated. Electrochemical lithiation demonstrated large initial capacities of over 950 mAh g⁻¹ accompanied by rapid fading over successive cycling in the voltage range 0.01–2 V. Limiting the voltage range to 0.5–2 V achieved more stable cycling, which was attributed to the intercalation process with lower capacities. Ex situ powder X-ray diffraction confirmed complete amorphization of the samples after lithiation, as well as recrystallization of the binary tetrel-arsenide phases after full delithiation in the voltage range 0.5–2 V. Solid-state synthetic methods produce layered phases, in which Si-As or Ge-As layers are separated by Li cations. The first layered compounds in the corresponding ternary systems were discovered, Li_0.9Ge_2.9As_3.1 and Li₃Si₇As₈, which crystallize in the Pbam (No. 55) and P2/m (No. 10) space groups, respectively. Semiconducting layered GeAs and SiAs accommodate the extra charge from Li cations through structural rearrangement in the Si-As or Ge-As layers and eventually by replacement of the tetrel dumbbells with sets of Li atoms. Ge and Si monoarsenides demonstrated high structural flexibility and a mild ability for reversible lithiation.     

The perturbation calculation of van der Waals potentials

Summary The unsymmetrized perturbation theory for interaction potentials is reformulated in such a way that the overlap and exchange effects can be taken into account in a satisfactory and conceptually simple way. This formulation, known as the generalized Heitler-London theory, its shown to be valid regardless of the ultimate limit to which the polarization approximation converges. Within the framework of this theory, the van der Waals potential of the triplet H₂(³ _u ) state is calculated and shown to be in excellent agreement with the exactab initio results. Both the exchange energy and the polarization energy are obtained from a perturbation calculation.     

Novel van der Waals Deep-UV Nonlinear Optical Materials

Van der Waals (vdW) deep-UV (DUV) nonlinear optical (NLO) crystal is an important material system recently developed. Herein, we review its concept and original intention, and then summarized the discovery process of related materials, including the role of A-site cations and the resulting two-/one-dimensional vdW DUV NLO systems. Finally, we evaluate the practical DUV NLO performance and prospected the opportunities and challenges.     

Predicting a Sharp Decline in Selectivity for Catalytic Esterification of Alcohols from van der Waals Interactions

Controlling the selectivity of catalytic reactions is a critical aspect of improving energy efficiency in the chemical industry; thus, predictive models are of key importance. Herein the performance of a heterogeneous, nanoporous Au catalyst is predicted for the complex catalytic self‐coupling of the series of C₂–C₄ alkyl alcohols, based solely on the known kinetics of the elementary steps of the catalytic cycle for methanol coupling, using scaling methods augmented by density functional theory. Notably, a sharp decrease in selectivity for ester formation with increasing molecular weight to favor the aldehyde due to van der Waals interactions of reaction intermediates with the surface was predicted and subsequently verified quantitatively by experiment. Further, the agreement between theory and experiment clearly demonstrates the efficacy of this approach for building a predictive model of catalytic behavior for a homologous set of reactants using a small set of experimental information.     

Phototransformation of stilbene in van der Waals nanocapsules

We have utilized para-hexanoylcalix[4]arene nanocapsules as hosts to carry out phototransformations of cis- and trans-stilbene. Single-crystal X-ray diffraction studies were performed to define precisely the location of encapsulated stilbenes inside the capsule and to analyze possible pathways of phototransformation. cis-Stilbene stacks as a pi-pi dimer located at the center of the capsule, whereas trans-stilbene does not form such a dimer. Irradiation of the crystalline inclusion complexes of each isomer of stilbene in the solid state leads to the appearance of the second isomer, and after prolonged photolysis, photodimerization also occurs. syn-Tetraphenylcyclobutane is formed as the major product of dimerization and its yield depends on the time and intensity of irradiation. In most cases, the single crystals of the complexes remain intact during irradiation; hence, the nanocapsules have the potential to serve as robust nanoreactors in the solid state. The confinement in the nanocapsules is sufficient to keep the reacting molecules together, although this is less restrictive than for trans-stilbene crystals, in which the molecules cannot achieve a favorable orientation for dimerization.