Charge-shift bonding--a class of electron-pair bonds that emerges from valence bond theory and is supported by the electron localization function approach

This paper deals with a central paradigm of chemistry, the electron-pair bond. Valence bond (VB) theory and electron-localization function (ELF) calculations of 21 single bonds demonstrate that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the fluctuation of the electron pair density plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RE(CS), and in ELF by a depleted basin population with large variances (fluctuations). CS bonding is shown to be a fundamental mechanism that is necessary to satisfy the equilibrium condition, namely the virial ratio of the kinetic and potential energy contributions to the bond energy. The paper defines the atomic propensity and territory for CS bonding: Atoms (fragments) that are prone to CS bonding are compact electronegative and/or lone-pair-rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and involves: a) homopolar bonds of heteroatoms with zero static ionicity, b) heteropolar sigma and pi bonds of the electronegative and/or electron-pair-rich elements among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc), c) all hypercoordinate molecules. Several experimental manifestations of charge-shift bonding are discussed, such as depleted bonding density, the rarity of ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reactions as compared to hydrogen-transfers.     

Preparation of polyacrylate–polysiloxane core–shell latex particles

Crosslinked polymer seed latexes of butyl acrylate, methyl methacrylate and methacrylic acid were synthesized with ethylene glycol dimethacrylate as a crosslinking agent in a first step. Three different processes of seeded emulsion polymerization were used to prepare an outlayer of polysiloxane on the above seed latex particles: (A) direct anionic polymerization of D₄ (octamethyl tetracyclosiloxane) catalyzed by potassium hydroxide; (B) direct cationic polymerization of D₄ onto the seed catalyzed by dodecylbenzene sulfonic acid; (C) a vinyl-containing polysiloxane prepared by copolymerization of D₄ and vinyl septamethyl tetracyclosiloxane was added before the D₄ cationic polymerization. Characterization by transmission electron microscopy showed that only process C provided satisfactory results. Film hardness was measured, and the latex film from process C demonstrated the lowest hardness of all the films. The mechanism of polymerization is discussed.     

Synthesis and structure–property relationships of acrylic core–shell particle-toughened epoxy networks

The use of emulsion polymerization to prepare core–shell rubber (CSR) toughening particles with different shell thickness-to-core diameter ratios is described. The conditions leading to controlled particle size and morphology are discussed. The particle shell is crosslinked during the synthesis so that its integrity and morphology are maintained upon curing of the epoxy network. The mixing of the CSR particles with the reactive epoxy and the processing of toughened-epoxy networks are described. The characteristics of each phase and the mechanical properties of the materials are reported. The fracture parameters (K_lc, G_lc) are discussed in relation to the structure of the CSR-particles.     

Determining the temperature dependence of the emulsion particle size of polyacrylate core–shell and polystyrene by photon correlation spectroscopy

The dependence of the particle size of a polyacrylate core-shell emulsion on temperature has been investigated in the temperature range of 10–55°C by photon correlation spectroscopy. To compare, the dependence of the particle size of an aqueous suspension of a polystyrene standard on temperature has been also investigated under the same conditions. This showed that as the temperature increases, the particle size of both samples decreases, but the rate of size decrease of the polystyrene standard is larger than that of the polyacrylate core–shell emulsion. By linear regression analysis, two regression equations of both samples have been set up. Furthermore, the apparent moving activation energy has been worked out from the size–temperature data.     

Modification of D–A–π–A Configuration toward a High‐Performance Triphenylamine‐Based Sensitizer for Dye‐Sensitized Solar Cells: A Theoretical Investigation

In an attempt to shed light on how the addition of a benzothiadiazole (BTD) moiety influences the properties of dyes, a series of newly designed triphenylamine‐based sensitizers incorporating a BTD unit as an additional electron‐withdrawing group in a specific donor–acceptor–π‐acceptor architecture has been investigated. We found that different positions of the BTD unit provided significantly different responses for light absorption. Among these, it was established that the further the BTD unit is away from the donor part, the broader the absorption spectra, which is an observation that can be applied to improve light‐harvesting ability. However, when the BTD unit is connected to the anchoring group a faster, unfavorable charge recombination takes place; therefore, a thiophene unit was inserted between these two acceptors, providing redshifted absorption spectra as well as blocking unfavorable charge recombination. The results of our calculations provide valuable information and illustrate the potential benefits of using computation‐aided sensitizer design prior to further experimental synthesis.     

Selected‐Control Fabrication of Multifunctional Fluorescent–Magnetic Core–Shell and Yolk–Shell Hybrid Nanostructures

The selected‐control preparation of uniform core–shell and yolk–shell architectures, which combine the multiple functions of a superparamagnetic iron oxide (SPIO) core and europium‐doped yttrium oxide (Y₂O₃:Eu) shell in a single material with tunable fluorescence and magnetic properties, has been successfully achieved by controlling the heat‐treatment conditions. Furthermore, the shell thickness and interior cavity of SPIO@Y₂O₃:Eu core–shell and yolk–shell nanostructures can be precisely tuned. Importantly, as‐prepared SPIO@Y₂O₃:Eu yolk–shell nanocapsules (NCs) modified with amino groups as cancer‐cell fluorescence imaging agents are also demonstrated. To the best of our knowledge, this is the first report on the selected‐control fabrication of uniform SPIO@Y₂O₃:Eu core–shell nanoparticles and yolk–shell NCs. The combined magnetic manipulation and optical monitoring of magnetic–fluorescent SPIO@Y₂O₃:Eu yolk–shell NCs will open up many exciting opportunities in dual imaging for targeted delivery and thermal therapy.     

A facile synthesis of highly water-soluble, core–shell organo-silica nanoparticles with controllable size via sol–gel process

A series of highly water-soluble organo-silica nanoparticles, ranging from 2 to 10 nm in diameter, were synthesized by the cohydrolysis and copolycondensation reactions. ω-methoxy(polyethyleneoxy)propyltrimethoxysilane (PEG6-9) and hydroxymethyltriethoxysilane (HMTEOS) mixtures were catalyzed by sodium hydroxide in the presence of surfactant benzethonium chloride (BTC) with various ratios of PEG6-9/HMTEOS at room temperature. The synthesized organo-silica nanoparticles possess a core–shell structure with a core of organo-silica resulting from HMTEOS and a monolayer shell of PEG6-9. The chemo-physical characteristics of the particles were studied by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, ²⁹Si nuclear magnetic resonance (NMR), dynamic light scattering (DLS), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The molecular weight and particle size of the particles increased with increasing HMTEOS molar ratios. The richest HMTEOS composition for the water-soluble particles was found to be HMTEOS:PEG6-9 = 80:20, where the particles had a 6 nm diameter core and a 0.8 nm thick shell. We propose that these water-soluble organo-silica nanoparticles will be suitable for biomedical applications.     

Dynamics of primary charge separation in bacterial photosynthesis using the multilevel Redfield–Davies secular approach

We use Redfield's relaxation theory in the Davies formulation to study primary charge separation reactions in bacterial photosynthesis. The specific model studied is the standard one (spin‐boson), with three states for the system, harmonic oscillators for the bath, and linear ohmic system–bath coupling. The Redfield–Davies formulation, which is equivalent to the secular approximation, is Markovian, of second order in system–bath coupling, and is written for the system's density matrix. The approximation does not suffer from any negative probabilities (which appear in the original Redfield approach) and can therefore be used for long‐time processes (20 ps or more here). Our results are in line with previous studies, especially to high bath frequencies. They confirm the usefulness of the Redfield–Davies secular approach as a convenient and simple tool for studying system–bath processes. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

On the original proof by reductio ad absurdum of the Hohenberg–Kohn theorem for many‐electron Coulomb systems

It is shown that, for isolated many‐electron Coulomb systems with Coulombic external potentials, the usual reductio ad absurdum proof of the Hohenberg–Kohn theorem is unsatisfactory since the to‐be‐refuted assumption made about the one‐electron densities and the assumption about the external potentials are not compatible with the Kato cusp condition. The theorem is, however, provable by more sophisticated means, and it is shown here that the Kato cusp condition actually leads to a satisfactory proof. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Hellmann–Feynman theorem near the threshold

The features of the spectrum structure are considered for situations where some parameter λ of the N‐electron Hamiltonian reaches the threshold value η under which the discrete energy level falls into the continuous spectrum. The electron density properties are also studied. It is proved that for a sequence of the wave functions converging in energy to the lower bound of the continuous spectrum as λ approaches η the corresponding sequence of the electron densities converges to the density of the (N ? 1)‐electron ground state. The results generalize the Hellmann–Feynman theorem for the cases where only the one‐side energy derivatives exist or there is no limiting wave function. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Controllable Assembly and Separation of Colloidal Nanoparticles through a One‐Tube Synthesis Based on Density Gradient Centrifugation

Self‐assembly of gold nanoparticles into one‐dimensional (1D) nanostructures with finite primary units was achieved by introducing a thin salt (NaCl) solution layer into density gradient before centrifugation. The electrostatic interactions between Au nanoparticles would be affected and cause 1D assembly upon passing through the salt layer. A negatively charged polymer such as poly(acrylic acid) was used as an encapsulation/stabilization layer to help the formation of 1D Au assemblies, which were subsequently sorted according to unit numbers at succeeding separation zones. A centrifugal field was introduced as the external field to overcome the random Brownian motion of NPs and benefit the assembly effect. Such a facile “one‐tube synthesis” approach couples assembly and separation in one centrifuge tube by centrifuging once. The method can be tuned by changing the concentration of interference salt layer, encapsulation layer, and centrifugation rate. Furthermore, positively charged fluorescent polymers such as perylenediimide‐poly(N,N‐diethylaminoethyl methacrylate) could encapsulate the assemblies to give tunable fluorescence properties.     

Long-lived photoinduced charge separation in carbazole–pyrene–viologen system incorporated in Langmuir–Blodgett films of substituted diazacrown ethers

Diaza-18-crown-6 ethers appending two pyrenyl (Py) or two carbazolyl (Cz) groups were synthesized. These macrocyclic compounds form 1:1 host–guest complexes with methyl viologen chloride (MV²⁺), and these complexes were assembled into monolayers by Langmuir–Blodgett technique. The generated assembly involves the general structure of donor–sensitizer–acceptor (Cz–Py–MV²⁺) in space, although any of the photo- and redox-active components are not covalently bonded. Photoirradiation of the pyrenyl group resulted in the charge-separated pair Cz√⁺–Py–MV√⁺ which survived up to hours in a well anaerobic atmosphere. An electrode was fabricated by transferring the L–B film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in hydroquinone (H₂Q) solution to be ca. 168 mV when the light intensity was 218 mW/cm². This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in anodic photocurrent, and there is no net chemical change associated with the function of the cell which converts light to electricity.     

Metal Oxide Assisted Preparation of Core–Shell Beads with Dense Metal–Organic Framework Coatings for the Enhanced Extraction of Organic Pollutants

Dense and homogeneous metal–organic framework (MOF) coatings on functional bead surfaces are easily prepared by using intermediate sacrificial metal oxide coatings containing the metal precursor of the MOF. Polystyrene (PS) beads are coated with a ZnO layer to give ZnO@PS core–shell beads. The ZnO@PS beads are reactive in the presence of 2‐methylimidazole to transform part of the ZnO coating into a porous zeolitic imidazolate framework‐8 (ZIF‐8) external shell positioned above the internal ZnO precursor shell. The obtained ZIF‐8@ZnO@PS beads can be easily packed in column format for flow‐through applications, such as the solid‐phase extraction of trace priority‐listed environmental pollutants. The prepared material shows an excellent permeance to flow when packed as a column to give high enrichment factors, facile regeneration, and excellent reusability for the extraction of the pollutant bisphenol A. It also shows an outstanding performance for the simultaneous enrichment of mixtures of endocrine disrupting chemicals (bisphenol A, 4‐tert‐octylphenol and 4‐n‐nonylphenol), facilitating their analysis when present at very low levels (<1 μg L^?1) in drinking waters. For the extraction of the pollutant bisphenol A, the prepared ZIF‐8@ZnO@PS beads also show a superior extraction and preconcentration capacity to that of the PS beads used as precursors and the composite materials obtained by the direct growth of ZIF‐8 on the surface of the PS beads in the absence of metal oxide intermediate coatings.     

Experimental and Computational Studies of a Multi‐Electron Donor–Acceptor Ligand Containing the Thiazolo[5,4‐d]thiazole Core and its Incorporation into a Metal–Organic Framework

A ligand containing the thiazolo[5,4‐d]thiazole (TzTz) core (acceptor) with terminal triarylamine moieties (donors), N,N′‐(thiazolo[5,4‐d]thiazole‐2,5‐diylbis(4,1‐phenylene))bis(N‐(pyridine‐4‐yl)pyridin‐4‐amine ( 1 ), was designed as a donor–acceptor system for incorporation into electronically active metal–organic frameworks (MOFs). The capacity for the ligand to undergo multiple sequential oxidation and reduction processes was examined using UV/Vis‐near‐infrared spectroelectrochemistry (UV/Vis‐NIR SEC) in combination with DFT calculations. The delocalized nature of the highest occupied molecular orbital (HOMO) was found to inhibit charge‐transfer interactions between the terminal triarylamine moieties upon oxidation, whereas radical species localized on the TzTz core were formed upon reduction. Conversion of 1 to diamagnetic 2+ and 4+ species resulted in marked changes in the emission spectra. Incorporation of this highly delocalized multi‐electron donor–acceptor ligand into a new two‐dimensional MOF, [Zn(NO₃)₂( 1 )] ( 2 ), resulted in an inhibition of the oxidation processes, but retention of the reduction capability of 1 . Changes in the electrochemistry of 1 upon integration into 2 are broadly consistent with the geometric and electronic constraints enforced by ligation.     

Preparation and characterization of emulsifier-free core–shell interpenetrating polymer network-fluorinated polyacrylate latex particles

The emulsifier-free core–shell interpenetrating polymer network (IPN) fluorinated polyacrylate latex particles with fluorine rich in shell were prepared by emulsifier-free seeded emulsion polymerization with water as the reaction medium. The fluorinated copolymer could be fixed on the particle surface due to the formation of interpenetrating polymer network. The resultant core–shell particles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) analysis, Fourier transform infrared (FTIR) spectrometry, X-ray photoelectron spectroscopy (XPS) analysis and thermogravimetric analysis (TGA). The core–shell particles possessed very narrow monomodal particle size distributions. XPS analysis of the latex film displayed that perfluoroalkyl groups had the tendency to enrich at surface and there was a gradient concentration of fluorine in the structure of the latex film from the film–air interface to the film–glass interface. In addition, compared with the latex film of crosslinked polyacrylate prepared under the same condition, the emulsifier-free core–shell IPN-fluorinated polyacrylate latex film showed better thermal stability, higher contact angle and lower water uptake.     

Template‐Free Synthesis of Core–Shell TiO2 Microspheres Covered with High‐Energy {116}‐Facet‐Exposed N‐Doped Nanosheets and Enhanced Photocatalytic Activity under Visible Light

Core–shell TiO₂ microspheres possess a unique structure and interesting properties, and therefore, they have received much attention. The high‐energy facets of TiO₂ also are being widely studied for the high photocatalytic activities they are associated with. However, the synthesis of the core–shell structure is difficult to achieve and requires multiple‐steps and/or is expensive. Hydrofluoric acid (HF), which is highly corrosive, is usually used in the controlling high‐energy facet production. Therefore, it is still a significant challenge to develop low‐temperature, template‐free, shape‐controlled, and relative green self‐assembly routes for the formation of core–shell‐structured TiO₂ microspheres with high‐energy facets. Here, we report a template‐ and hydrofluoric acid free solvothermal self‐assembly approach to synthesize core–shell TiO₂ microspheres covered with high‐energy {116}‐facet‐exposed nanosheets, an approach in which 1,4‐butanediamine plays a key role in the formation of nanosheets with exposed {116} facets and the doping of nitrogen in situ. In the structure, nanoparticle aggregates and nanosheets with {116} high‐energy facets exposed act as core and shell, respectively. The photocatalytic activity for degradation of 2,4,6‐tribromophenol and Rhodamine B under visible irradiation and UV/Vis irradiation has been examined, and improved photocatalytic activity under visible light owing to the hierarchical core–shell structure, {116}‐plane‐oriented nanosheets, in situ N doping, and large surface areas has been found.     

Fabrication of ZIF‐8@SiO2 Core–Shell Microspheres as the Stationary Phase for High‐Performance Liquid Chromatography

The unique features of high porosity, shape selectivity, and multiple active sites make metal–organic frameworks (MOFs) promising as novel stationary phases for high‐performance liquid chromatography (HPLC). However, the wide particle size distribution and irregular shape of conventional MOFs lead to lower column efficiency of such MOF‐packed columns. Herein, the fabrication of monodisperse MOF@SiO₂ core–shell microspheres as the stationary phase for HPLC to overcome the above‐mentioned problems is reported. Zeolitic imidazolate framework 8 (ZIF‐8) was used as an example of MOFs due to its permanent porosity, uniform pore size, and exceptional chemical stability. Unique carboxyl‐modified silica spheres were used as the support to grow the ZIF‐8 shell. The fabricated monodisperse ZIF‐8@SiO₂ packed columns (5 cm long × 4.6 mm i.d.) show high column efficiency (23 000 plates m^?1 for bisphenol A) for the HPLC separation of endocrine‐disrupting chemicals (bisphenol A, β‐estradiol, and p‐(tert‐octyl)phenol) and pesticides (thiamethoxam, hexaflumuron, chlorantraniliprole, and pymetrozine) within 7 min with good relative standard deviations for 11 replicate separations of the analytes (0.01–0.39, 0.65–1.7, 0.70–1.3, and 0.17–0.91 % for retention time, peak area, peak height, and half peak width, respectively). The ZIF‐8@SiO₂ microspheres combine the advantages of the good column packing properties of the uniform monodisperse silica microspheres and the separation ability of the ZIF‐8 crystals.     

Comparative Analysis of Electron‐Density and Electron‐Localization Function for Dinuclear Manganese Complexes with Bridging Boron‐ and Carbon‐Centered Ligands

Bonding in borylene‐, carbene‐, and vinylidene‐bridged dinuclear manganese complexes [MnCp(CO)₂]₂X (X=B‐tBu, B=NMe₂, CH₂, C?CH₂) has been compared by analyses based on quantum theory of atoms in molecules (QTAIM), on the electron‐localization function (ELF), and by natural‐population analyses. All of the density functional theory based analyses agree on the absence of a significant direct Mn? Mn bond in these complexes and confirm a dominance of delocalized bonding via the bridging ligand. Interestingly, however, the topology of both charge density and ELF related to the Mn‐bridge‐Mn bonding depend qualitatively on the chosen density functional (except for the methylene‐bridged complex, which exhibits only one three‐center‐bonding attractor both in ??²ρ and in ELF). While gradient‐corrected functionals provide a picture with localized two‐center X? Mn bonding, increasing exact‐exchange admixture in hybrid functionals concentrates charge below the bridging atom and suggests a three‐center bonding situation. For example, the bridging boron ligands may be described either as substituted boranes (e.g., at BLYP or BP86 levels) or as true bridging borylenes (e.g., at BHLYP level). This dependence on the theoretical level appears to derive from a bifurcation between two different bonding situations and is discussed in terms of charge transfer between X and Mn, and in the context of self‐interaction errors exhibited by popular functionals.