The S···P noncovalent interaction: diverse chalcogen bonds

The S···P interactions in the complexes of HSX (X?=?F, Cl, Br, I) with PH_nMe_3-n(n?=?0–3) have been investigated with ab initio calculations at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ//MP2/aug-cc-pVDZ level of theory. The interaction energies and structural properties of intermolecular complexes have been analyzed. Results of QTAIM analysis are dealing with expand of interactions, including pure closed-shell interactions (van der Waals interactions and chalcogen bonding, YB), partially covalent closed-shell (CS; Charge Transfer) and shared-shell interactions (SS; weak covalent bond and very strong YB) for these complexes. The energy decomposition analysis (EDA) showed that electrostatic interactions are an important contributing factor for these complexes. In considering second-order contributions, the donor-acceptor pair charge transfer (CT) is most important. These findings are consistent with the Electron Localization Function (ELF) isosurface of the complexes. In each series of HXS:PH_nMe_3-n-chalcogen bond complexes with increasing basicity of phosphines, the stability and S···P bond strength of adducts were increased so that the HXS:PMe₃ (X?=?F, Cl, Br) complexes had very strong S···P chalcogen interactions with nearly covalent characters.     

On the Importance of Gradient-Corrected Correlation for van der Waals Density Functionals

The van der Waals density functional (vdW-DF) family of exchange?Ccorrelation functionals is a promising step towards accounting for van der Waals interactions in density functional theory. This approach consists of a nonlocal correlation term in addition to semilocal generalized gradient approximation exchange and local density approximation correlation. It has proven useful for describing vdW bonded complexes but unfortunately deteriorates the prediction of solid-state properties such as bulk lattice parameters and cohesive energies, as compared to the underlying GGA functional. By considering a broad range of different condensed matter systems including weakly interacting complexes as well as strongly bonded molecules and bulk solids, we show that inclusion of gradient-corrected correlations in vdW-DF-type calculations may not only improve the accuracy for vdW bonded systems, but also amend vdW-DF deficiencies in predicting structural properties of solids. Based on this insight we construct a prototype vdW-DF which demonstrates high accuracy in describing the dispersive interactions responsible for benzene adsorption on the noble Au(111) surface.     

Dispersive forces of particle–surface interactions: direct AFM measurements and modelling

Fundamentals of particle–particle interaction are of great interest in agglomeration processes. Particle adhesion depends on dispersive forces (van der Waals force), local chemical bindings, Coulomb force and capillary attractions. Additionally, surface properties like roughness, adsorption layers and surface chemistry strongly affect adhesion forces. van der Waals interactions are poorly understood because popular ab initio force calculations for molecules like density functional theory (DFT) often do not lead to proper results. van der Waals forces are difficult to measure directly. We present direct measurements of particle–particle and particle–surface interactions in the gas phase carried out with an atomic force microscope (AFM). Special emphasis is given to a proper statistical treatment of the data. For modelling of particle adhesion, we use computer-assisted empirical potential methods. Parameters like adsorbed water and surface roughness are considered. We extract parameters for weak interactions from the Lifshitz theory and gas adsorption data. Adsorbing molecules can be used as probes to measure dispersive forces. Studying surface and particle properties combined with computer-assisted modelling is a basic requisite to reach the aim of predicting particle–particle interactions in industrial processes.     

Interactions of chemical species with acid sites in zeolites

A detailed understanding of reaction pathways in zeolite solid acids requires knowledge about the structure and thermochemistry of all chemical species which could potentially exist along the reaction coordinate. Because some high-silica zeolites, such as H-ZSM-5, consist of a collection of nearly identical, Brønsted-acid sites, equal in concentration to the Al content, the 1 : 1 stoichiometric adsorption complexes formed by various molecules when they interact with the acid sites are well defined and relatively easy to characterize using temperature-programmed desorption, microcalorimetry, and NMR. This paper briefly reviews what is known about the reactivity, thermochemistry, and structure of complexes formed by amines, pyridines, imines, alcohols, thiols, olefins, aldehydes, ketones, and nitriles. It is shown that the thermochemistry and structure of the complexes are affected by both local effects (proton transfer or hydrogen-bonding interactions) and nonlocal effects (van der Waals interactions with the zeolite cavity). However, solvation effects in zeolites are very different from that found in acidic solutions so that the use of gas-phase reference conditions provides a much better starting point for understanding adsorption complexes, and therefore acid-catalyzed reactions, in the zeolite.     

Simple Views on Metal/Oxide Interfaces: Contributions of the Long-Range Interactions to the Adhesion Energy

The image and van der Waals contributions to the metal/oxide work of adhesion are compared through the extent to which they follow the known prevalent trends, i.e. the increase in work of adhesion (a) with narrowing oxide band gap and (b) with increasing conduction electron density of the metal. The van der Waals interaction is shown to follow both trends, while the image term is suggested to be significant only for dense metals in contact with very ionic oxides. The relative contribution of these long-range interactions to the overall metal/oxide work of adhesion is found to be maximized for systems involving metals with low electronic densities and oxides with wide band gaps. At variance, high metallic electronic densities and narrow oxide gaps likely favour short-range interactions arising from charge transfer.     

Simple Views on Metal/Oxide Interfaces: Contributions of the Long-Range Interactions to the Adhesion Energy

The image and van der Waals contributions to the metal/oxide work of adhesion are compared through the extent to which they follow the known prevalent trends, i.e. the increase in work of adhesion (a) with narrowing oxide band gap and (b) with increasing conduction electron density of the metal. The van der Waals interaction is shown to follow both trends, while the image term is suggested to be significant only for dense metals in contact with very ionic oxides. The relative contribution of these long-range interactions to the overall metal/oxide work of adhesion is found to be maximized for systems involving metals with low electronic densities and oxides with wide band gaps. At variance, high metallic electronic densities and narrow oxide gaps likely favour short-range interactions arising from charge transfer.     

Interfacial Contact and Bonding in Autohesion II-Intermolecular Forces

The second stage of autohesion involves bond formation after contact is established by elastic, viscous, or viscoelastic deformation of the surfaces. The interfacial bonding for polystyrene is mainly through van der Waals intermolecular forces. Molecular repeat units across the interface interact to yield an attractive force. The intermolecular forces are derived from a 6–12 Lennard-Jones potential. The two unknown constants for this potential are determined from the equilibrium distance and comparison of the computed lattice energy with the cohesive energy.     

IGMH-based research on the intramolecular weak interaction of TKX-50

To further comprehensively study the intramolecular weak interaction of the gaseous TKX-50 molecule, the two conformations of the TKX-50 molecule were analyzed via the Independent Gradient Model based on the Hirshfeld Partition (IGMH) method based on the B3LYP/6-311 g (d,p) level for geometry optimization and for single point energy. The conclusions manifest that the weak interactions between these fragments are mainly composed of hydrogen bonds and van der Waals interactions. From the strength of inter-fragment interactions formed by contributing atomic pairs and their percentage contributions, these H bonds, together with dispersion-dominated weak interactions provided by non-direct facing atomic pairs near these H bonds, dominate the inter-fragment interaction resulting in the stability of the molecular structure. Meanwhile, the weak interactions enclosed by end-atoms of two fragments not only include the contributions provided by inter-fragment atomic pairs of two fragments but also include the contributions provided by intra-fragment atomic pairs of fragment 1. For conformation II, due to the H transfer between fragments, a pair of symmetric H bonds at the corresponding regions are extremely strong to reach the level of covalent bond while the two groups −OH at the other end become looser, resulting in conformation II own lower energy. Differences in inter-fragment interactions between two conformations were essentially brought by the stronger electron-withdrawing ability of atom O than that of atom N.     

Solid surface free energy components determination by the thin-layer wicking technique

A thin-layer wicking technique [1] and van Oss et al.'s [2] approach to interfacial free energy interactions were tested to determine the solid surface free energy components: apolar Lifshitz-van der Waals, y^l,w _y; and polar, electron donor, γ^- _y, and electron acceptor, γ⁺ _y. For this purpose, the penetration rates of n-alkanes, diiodomethane, water, formamide, toluene and chloroform were measured. For these studies, silica, used for thin-layer chromatography, and α-Al₂O₃ were used as the model solids. It was found that both are strongly polar, electron donor solids with weak electron acceptor interactions. It is concluded that van Oss, Good et al.'S [2] approach to the interfacial free energy interactions and the thin-layer wicking technique are very useful for cxplaining many interfacial phenomena taking place in the dispersed systems. However, more experimental work using different systems is needed.     

Theoretical study of interaction of thiazolidine-2-thione as an anti-thyroid agent with iodine

Quantum chemical calculations were used to investigate the interaction of tautomer forms of thiazolidine-2-thione (T2T) with iodine in the gas and solution phases. The calculations show that the planar complexes are more stable than the perpendicular complexes. Also in the perpendicular complexes, the thione form is more stable than the thiol form. The calculated thermodynamic parameters in the solution phase indicate that the stability of the formed complexes increases as the dielectric constant of the solvent is increased. This result is in good agreement with the experimental results. The NBO results predict that the LP(S) → σ*(I─I) and LP(S) → σ*(N─H) charge transfer (CT) interactions are very important in the stabilizing of the planar complexes with respect to the perpendicular complexes. In all complexes, CT takes place from T2T to I₂. Therefore, it is predicted that the donor properties of T2T are the origin of its anti-thyroid action. The atoms in molecule (AIM) analysis shows that the interactions in the planar complexes are stronger than the perpendicular complexes.     

Ag nanocrystal as a promoter for carbon nanotube-based room-temperature gas sensors

We have investigated the room-temperature sensing enhancement of Ag nanoparticles (NPs) for multiwalled carbon nanotube (MWCNT)-based gas sensors using electrical measurements, in situ infrared (IR) microspectroscopy, and density functional theory (DFT) calculations. Multiple hybrid nanosensors with structures of MWCNTs/SnO(2)/Ag and MWCNTs/Ag have been synthesized using a process that combines a simple mini-arc plasma with electrostatic force directed assembly, and characterized by electron microscopy techniques. Ag NPs were found to enhance the sensing behavior through the "electronic sensitization" mechanism. In contrast to sensors based on bare MWCNTs and MWCNTs/SnO(2), sensors with Ag NPs show not only higher sensitivity and faster response to NO(2) but also significantly enhanced sensitivity to NH(3). Our DFT calculations indicate that the increased sensitivity to NO(2) is attributed to the formation of a NO(3) complex with oxygen on the Ag surface accompanying a charge rearrangement and a net electron transfer from the hybrid to NO(2). The significant response to NH(3) is predicted to arise because NH(3) is attracted to hollow sites on the oxidized Ag surface with the H atoms pointing towards Ag atoms and electron donation from H to the hybrid sensor.     

Polarizability of coal liquefaction product distillates: Insight into intermolecular forces

A purpose of this investigation is to gain insight into the intermolecular forces present in coal liquefaction products. The mid-boiling point of distillates from coal liquefaction products reflects the intermolecular forces present. The mid-boiling point of distillates from H-Coal and Wilsonville products is a linear function of their average molar volume and average molar polarizability, which are directly related to van der Waals forces. The results presented here support, but do not prove, the conclusion that van der Waals forces are the dominant intermolecular forces in the H-Coal and Wilsonville distillates.     

Synthesis, Structural Investigation and DFT Calculations of Cadmium (II) Fluorine-Substituted β-Diketonate: A Precursor to Producing Pure Phase Nano-Sized Cadmium (II) Oxide

A novel aza-aromatic base adduct of a cadmium(II) fluorine-substituted β-diketonate, [Cd(dmp)(ttfa)₂]_n(1), (“dmp” is an abbreviation for 2,9-dimethyl-1,10-phenanthroline and “ttfa” is an abbreviation for thenoyltrifluoroacetonate) was synthesised and characterised by elemental analysis and IR, ¹H NMR and ¹³C NMR spectroscopy and studied by thermal as well as X-ray crystallography. The single-crystal structure of this complex shows that the coordination number of the Cd²⁺ ion is six with two N-donor atoms from the dmp ligand and four O-donors from ttfa. Self-assembly of this complex was mediated by CH···F–C and π–π stacking interactions. The supramolecular features of these complexes are controlled by weak directional intermolecular interactions. The structural energy of the title complex was determined by density functional theory calculations. The calculated HOMO–LUMO gap is 0.131 a.u. (3.474?eV). The CdO nano-particles were obtained by thermolysis of 1 at 180?°C with oleic acid as the surfactant. Scanning electron microscopy showed that the size of the CdO particles is ~35?nm.     

Influence of combining rules on the cavity occupancy of clathrate hydrates using van der Waals–Platteeuw-theory-based modelling

In the current study, we report an extensive series of thermodynamic calculations using continuum-level models based on the van der Waals–Platteeuw theory. The calculations are performed along the three phase hydrate–liquid water–vapor (H–L_w–V) or hydrate–ice–vapor (H–I–V) equilibrium curve for a number of gases of industrial interest (e.g., methane, ethane, propane, nitrogen and carbon dioxide). We examine the effect of deviations from the classical Lorentz–Berthelot combining rules on the hydrate equilibrium conditions as well as the cavity occupancy of the hydrates and work towards quantifying it. Hydrate equilibrium predictions have a very strong sensitivity to deviations, while cavity occupancies exhibit a weaker sensitivity. Furthermore, the sensitivity is stronger on the small cavities compared to the case of the large cavities. Model calculations are compared against experimental data for selected systems with reasonable agreement.     

Poly(di-1H,1H,2H,2H-perfluoroalkylitaconate) films: surface organisation phenomena, surface energy determinations and force of adhesion measurements

The significance of the Liftshitz/van der Waals, Lewis-acid, and Lewis-base contributors to the total surface energy of an homologous series of poly(di-1H,1H,2H,2H-perfluoroalkylitaconate)s is discussed in terms of the molecular design features and surface organisation phenomena characterising these, comb-like, polymers. Comparison of the characteristics specific to films prepared from this class of materials with those of the previously-studied homologous series of poly(perfluoroalkylacrylate)s, poly(perfluoroalkylmethacrylate)s and poly(methylpropenoxyalkyl siloxane)s suggests that, of the molecular design requirements for low-surface-energy polymers, an increase in the packing density of pendent side-chains has little effect on the contributors to surface energy but an adverse effect on the roughness of the film.     

Synthesis and characterization of novel organo-montmorillonites

Sodium Montmorillonite (NaMMT) has been modified via cation exchange reaction using three different organic cations. Basal spacings, interlamellar structure and thermal stability of these organo-montmorillonites (OMMT) clays have been characterized using wide angle X-ray diffraction (WAXD) and thermogravimetric analysis (TGA) techniques. Increase in the basal spacing due to organic modification is in good agreement with simple theoretical calculations based on van der Waals volume of the cationic ammonium ions. TGA characterization and analysis show that the amount of organic modifier in the OMMT's is in good agreement with theoretically calculated stoichiometric content expected for almost complete exchange of Na⁺ ions by organic cations. The OMMT's shows stepwise decomposition corresponding to initial weight loss from residual water desorption, followed by decomposition of the organic surfactant and the dehydroxylation of structural water of the montmorillonite layers.     

Physical interactions at carbon nanotube-polymer interface

Mechanical properties of carbon nanotube (CNT) reinforced polystyrene rod and CNT reinforced epoxy thin film were studied and the CNT-polymer interface in these composites was examined. Transmission and scanning electron microscopy examinations of CNT/polystyrene (PS) and CNT/epoxy composite showed that these polymers adhered well to CNT at the nanometer scale. Molecular mechanics simulations and elasticity calculations were used to quantify some of the important interfacial characteristics that critically control the performance of a composite material. In the absence of chemical bonding between CNT and the matrix, it is found that the non-bond interactions, consist of electrostatic and van der Waals forces, result in CNT-polymer interfacial shear stress (at 0 K) of about 138 and 186 MPa, respectively, for CNT/epoxy and CNT/PS. The high interfacial shear stress calculated, about an order of magnitude higher than micro fiber reinforced composites, is believed attributed to intimate contact between the two solid phases at the molecular scale. Simulations and calculations also showed that local non-uniformity of CNT and mismatch of the coefficients of thermal expansions between CNT and polymer matrix also promote the stress transfer ability between the two.     

Discovery of HIV Type 1 Aspartic Protease Hit Compounds through Combined Computational Approaches

A combination of computational techniques and inhibition assay experiments was employed to identify hit compounds from commercial libraries with enhanced inhibitory potency against HIV type 1 aspartic protease (HIV PR). Extensive virtual screening with the aid of reliable pharmacophore models yielded five candidate protease inhibitors. Subsequent molecular dynamics and molecular mechanics Poisson–Boltzmann surface area free‐energy calculations for the five ligand–HIV PR complexes suggested a high stability of the systems through hydrogen‐bond interactions between the ligands and the protease's flaps (Ile50/50′), as well as interactions with residues of the active site (Asp25/25′/29/29′/30/30′). Binding‐energy calculations for the three most promising compounds yielded values between ?5 and ?10 kcal mol^?1and suggested that van der Waals interactions contribute most favorably to the total energy. The predicted binding‐energy values were verified by in vitro inhibition assays, which showed promising results in the high nanomolar range. These results provide structural considerations that may guide further hit‐to‐lead optimization toward improved anti‐HIV drugs.