Theoretical designs for planar tetracoordinated carbon in Cu, Ag, and Au organometallic chemistry: a new target for synthesis

In the past 30 years, substantial efforts and progress have been made in the design and synthesis of molecules containing tetracoordinated planar carbon, by overcoming the inherent preference for tetrahedral bonding. As a result, we have studied 12 organometallic molecules containing group 11 elements (i.e., M-X; M = Cu, Ag, and Au and X = I, II, III, and IV) using density functional theory to determine whether the central carbon atom exists in a planar geometry. Our theoretical findings suggest that in such M-X species, bonding interactions between the central carbon and the coinage metal ligands and between the metal ligands (i.e., metallophilic attractions) are both important in favoring planar-tetracoordinated carbon compounds over the corresponding tetrahedral structures. The compounds studied in this work are seen as excellent targets for chemical synthesis.     

Robust metal-pentagon interactions in the Th-based endohedral metallofullerenes revealed by DFT calculations

Endohedral metallofullerenes (EMFs) all feature obvious charge transfer from the metallic core to the carbon shell with the donated electrons largely accepted by the cage pentagons. In this work, a series of Th@C_2n (2n = 64-88) were thoroughly investigated by means of density functional theory calculations. Interestingly, we found that the tetravalent thorium atom mainly coordinates to three pentagonal rings with the metal–pentagon interactions independent on the distribution and distance among these pentagons. This coordination pattern is not only in sharp contrast to that of common organometallic complexes, where four pentagons are indispensable for stabilizing Th(IV), but also different from that of Ti-containing fullerenes, whose valence state highly depends on the pentagon distribution. The specificity of Th-based EMFs was rationalized by the synergetic effect of small metal ion size, low electronegativity, strong metal-cage electrostatic attractions and effective orbital overlap between the metal and cage orbitals. Our work highlights the role of cage pentagons in the Th-cage interactions, and points out the fundamental difference between EMFs and common organometallic complexes.     

Hydrotropic salt promotes anionic surfactant self-assembly into vesicles and ultralong fibers

Molecular self-assembly has become a versatile approach to create complex and functional nanoarchitectures. In this work, the self-assembly behavior of an anionic surfactant (sodium dodecylbenzene sulfonate, SDBS) and a hydrotropic salt (benzylamine hydrochloride, BzCl) in aqueous solution is investigated. Benzylamine hydrochloride is found to facilitate close packing of surfactants in the aggregates, inducing the structural transformation from SDBS micelles into unilamellar vesicles, and multilamellar vesicles. The multilamellar vesicles can transform into macroscale fibers, which are long enough to be visualized by the naked eye. Particularly, these fibers are robust enough to be conveniently separated from the surfactant solution. The combined effect of non-covalent interactions (e.g., hydrophobic effect, electrostatic attractions, and π-π interactions) is supposed to be responsible for the robustness of these self-assembled aggregates, in which π-π interactions provide the directional driving force for one-dimensional fiber formation.     

Nature of the metal-nonmetal transition in metal-ammonia solutions. I. Solvated electrons at low metal concentrations

Using a theory of polarizable fluids, we extend a variational treatment of an excess electron to the many-electron case corresponding to finite metal concentrations in metal-ammonia solutions (MAS). We evaluate dielectric, optical, and thermodynamical properties of MAS at low metal concentrations. Our semianalytical calculations based on a mean-spherical approximation correlate well with the experimental data on the concentration and temperature dependencies of the dielectric constant and the optical absorption spectrum. The properties are found to be mainly determined by the induced dipolar interactions between localized solvated electrons, which result in the two main effects: the dispersion attractions between the electrons and a sharp increase in the static dielectric constant of the solution. The first effect creates a classical phase separation for the light alkali metal solutes (Li, Na, K) below a critical temperature. The second effect leads to a dielectric instability, i.e., polarization catastrophe, which is the onset of metallization. The locus of the calculated critical concentrations is in a good agreement with the experimental phase diagram of Na-NH(3) solutions. The proposed mechanism of the metal-nonmetal transition is quite general and may occur in systems involving self-trapped quantum quasiparticles.     

Attractions, water structure, and thermodynamics of hydrophobic polymer collapse

We explore the prospects of a perturbation approach for predicting how weak attractive interactions affect collapse thermodynamics of hydrophobic polymers in water. Specifically, using molecular dynamics simulations of model polymers in explicit water, we show that the hydration structure is sensitive to the strength of the van der Waals attractions but that the hydration contribution to the potential of mean force for collapse is not. We discuss how perturbation theory ideas developed for small spherical apolar solutes need to be modified in order to account for the effect of attractions on the conformational equilibria of polymers.     

气相条件下金属离子/肽复合物电喷雾串联质谱研究

研究蛋白质与金属离子之间的相互作用的本质一直是生物学家感兴趣的课题。但蛋白结构的复杂性使得二者之间相互作用的研究难度很大,常选用氨基酸或小肽作为模型化合物进行研究^[1-4]。目前该方法的研究甩缺乏系统性。     

Inverse gas chromatographic investigation of the active sites related to CO adsorption over Rh/SiO2 catalysts in excess of hydrogen

In the present work, the novel methodology of reversed-flow gas chromatography (RF-GC) is extended in a topic of contemporary scientific and technological interest, such as the effect of hydrogen in the "topography" of the active sites related to CO adsorption. This study concerns CO adsorption on a silica-supported Rh catalyst, at 90 degrees C. The topography of the catalyst in the absence of hydrogen consists of both randomly and islands of CO bound over chemisorbed CO molecules. In contrast, under H2-rich conditions, the observed topography is almost entirely patchwise ascribed to long-range lateral attractions between adsorbate molecules. In excess of hydrogen, CO adsorption is shifted at higher lateral attractions values which correspond to weaker adsorbate-adsorbent interactions and lower surface coverage. This provides an indication of a H2-induced desorption, which can be attributed to the formation of an H-CO complex desorbing from the catalyst surface below the temperature required for CO desorption, in the absence of H2, and it may explain the well-known enhancement of the rate of selective CO oxidation in excess of hydrogen by H2.     

Advances in the coordination chemistry of nitrogen ligand complexes of coinage metals

Coinage metals nitrogen chemistry has not been studied extensively until recently. The focus of this review is the base- and halide-free complexes of the monoanionic nitrogen ligands. This review describes how minor ligand modifications can result in a drastic change in the metal–metal interactions in multinuclear compounds. Crystal structures of these complexes show individual complexes, dimers, supramolecular columnar packing or more complex supramolecular aggregates. Bulky substituents on the ligands can prevent intermolecular metal–metal interactions or the formation of supramolecular architectures. The nuclearity and metal–metal interactions in these complexes are controlled by ligand steric and electronic factors and solvent of crystallization. Many classes of nitrogen ligand coordination compounds have given rise to advances in several fundamental and applied research aspects. Recent potential applications of nitrogen ligand complexes are highlighted particularly for those complexes included in this review.     

Quantitative study of interactions between oxygen lone pair and aromatic rings: substituent effect and the importance of closeness of contact

Current models describe aromatic rings as polar groups based on the fact that benzene and hexafluorobenzene are known to have large and permanent quadrupole moments. This report describes a quantitative study of the interactions between oxygen lone pair and aromatic rings. We found that even electron-rich aromatic rings and oxygen lone pairs exhibit attractive interactions. Free energies of interactions are determined using the triptycene scaffold and the equilibrium constants were determined by low-temperature 1H NMR spectroscopy. An X-ray structure analysis for one of the model compounds confirms the close proximity between the oxygen and the center of the aromatic ring. Theoretical calculations at the MP2/aug-cc-pVTZ level corroborate the experimental results. The origin of attractive interactions was explored by using aromatic rings with a wide range of substituents. The interactions between an oxygen lone pair and an aromatic ring are attractive at van der Waals' distance even with electron-donating substituents. Electron-withdrawing groups increase the strength of the attractive interactions. The results from this study can be only partly rationalized by using the current models of aromatic system. Electrostatic-based models are consistent with the fact that stronger electron-withdrawing groups lead to stronger attractions, but fail to predict or rationalize the fact that weak attractions even exist between electron-rich arenes and oxygen lone pairs. The conclusion from this study is that aromatic rings cannot be treated as a simple quadrupolar functional group at van der Waals' distance. Dispersion forces and local dipole should also be considered.     

A quasichemical approach for protein-cluster free energies in dilute solution

Reversible formation of protein oligomers or small clusters is a key step in processes such as protein polymerization, fibril formation, and protein phase separation from dilute solution. A straightforward, statistical mechanical approach to accurately calculate cluster free energies in solution is presented using a cell-based, quasichemical (QC) approximation for the partition function of proteins in an implicit solvent. The inputs to the model are the protein potential of mean force (PMF) and the corresponding subcell degeneracies up to relatively low particle densities. The approach is tested using simple two and three dimensional lattice models in which proteins interact with either isotropic or anisotropic nearest-neighbor attractions. Comparison with direct Monte Carlo simulation shows that cluster probabilities and free energies of oligomer formation (DeltaG(i) (0)) are quantitatively predicted by the QC approach for protein volume fractions approximately 10(-2) (weight/volume concentration approximately 10 g l(-1)) and below. For small clusters, DeltaG(i) (0) depends weakly on the strength of short-ranged attractive interactions for most experimentally relevant values of the normalized osmotic second virial coefficient (b(2) (*)). For larger clusters (i"2), there is a small but non-negligible b(2) (*) dependence. The results suggest that nonspecific, hydrophobic attractions may not significantly stabilize prenuclei in processes such as non-native aggregation. Biased Monte Carlo methods are shown to accurately provide subcell degeneracies that are intractable to obtain analytically or by direct enumeration, and so offer a means to generalize the approach to mixtures and proteins with more complex PMFs.     

A Study on The Interactions Between Coordination Compounds and Supports

In this study the viewpoint is taken that the supported complexes react with active groups on the surface of the support to form new entities which may be called surface complexes.Various interactions between the support and the solute in solution of impregnation have been found,and these interactions may be divided into four calsses: (1) there is no noticeable chemical interaction between the dissolved coordination compound and the support,except the physical adsorption;(2) the metal ion of the complex is anchored on the surface through the surface group as a ligand forming a surface complex,or the support provides the central ion for the formation of a surface polynuciear complex through the bridging ligand of the complex;(3) the surface of the support provides the central ion for ligands dissociated from the complex or for the solvent molecules;(4) the metal hydroxide might be formed,when the acidity of the cation of the support is weaker than that of the metal ion of the complex in a aqueous solutio     

配合物与载体相互作用的研究

本文研究了配合物与载体的相互作用。载体浸渍于配合物的溶液时,两者间的相互作用可分成四类:(1)无明显的化学作用;(2)配合物与载体通过桥联的配体作用;(3)载体阳离子与解离的配体作用;(4)载体促进金属水合离子的水解。     

Chain conformations and phase separation in polymer solutions with varying solvent quality

Molecular dynamics simulations are used to investigate the conformations of a single polymer chain, represented by the Kremer-Grest bead-spring model, in a solution with a Lennard-Jones liquid as the solvent when the interaction strength between the polymer and solvent is varied. Results show that when the polymer-solvent interaction is unfavorable, the chain collapses as one would expect in a poor solvent. For more attractive polymer-solvent interactions, the solvent quality improves and the chain is increasingly solvated and exhibits ideal and then swollen conformations. However, as the polymer-solvent interaction strength is increased further to be more than about twice the strength of the polymer-polymer and solvent-solvent interactions, the chain exhibits an unexpected collapsing behavior. Correspondingly, for strong polymer-solvent attractions, phase separation is observed in the solutions of multiple chains. These results indicate that the solvent becomes effectively poor again at very attractive polymer-solvent interactions. Nonetheless, the mechanism of chain collapsing and phase separation in this limit differs from the case with a poor solvent rendered by unfavorable polymer-solvent interactions. In the latter, the solvent is excluded from the domain of the collapsed chains while in the former, the solvent is still present in the pervaded volume of a collapsed chain or in the polymer-rich domain that phase separates from the pure solvent. In the limit of strong polymer-solvent attractions, the solvent behaves as a glue to stick monomers together, causing a single chain to collapse and multiple chains to aggregate and phase separate.     

Iron/Brønsted Acid Catalyzed Asymmetric Hydrogenation: Mechanism and Selectivity‐Determining Interactions

Hydrogenation catalysts involving abundant base metals such as cobalt or iron are promising alternatives to precious metal systems. Despite rapid progress in this field, base metal catalysts do not yet achieve the activity and selectivity levels of their precious metal counterparts. Rational improvement of base metal complexes is facilitated by detailed knowledge about their mechanisms and selectivity‐determining factors. The mechanism for asymmetric imine hydrogenation with Knölker’s iron complex in the presence of chiral phosphoric acids is here investigated computationally at the DFT‐D level of theory, with models of up to 160 atoms. The resting state of the system is found to be an adduct between the iron complex and the deprotonated acid. Rate‐limiting H₂ splitting is followed by a stepwise hydrogenation mechanism, in which the phosphoric acid acts as the proton donor. C?H ??? O interactions between the phosphoric acid and the substrate are involved in the stereocontrol at the final hydride transfer step. Computed enantiomeric ratios show excellent agreement with experimental values, indicating that DFT‐D is able to correctly capture the selectivity‐determining interactions of this system.     

Non-covalent interactions in polysaccharide systems

[Chemical structure: see text] This paper describes the behavior of some polysaccharides with well-known chemical structures and in which the influence of cooperative secondary interactions play an important role. The roles played by hydrophobic and ionic interactions (including ionic selectivity) on polysaccharide conformation and gelation are discussed. Electrostatic attractions are also important in the complexes formed between surfactants and polyelectrolytes of opposite charge. Finally, van der Waals dipolar interactions and particularly hydrogen-bond formation are examined. The role of hydrogen bonds in solubility, conformation, and especially the local stiffness of polysaccharides, but also in polymer-polymer complexes frequently obtained with polysaccharides, is developed. Repeat unit for a number polysaccharides.     

Synthesis of pH-responsive particles with shape anisotropy

Seeded emulsion polymerization is used to produce large quantities of shape anisotropic, amphoteric particles in a size range of about 1 μm. Copolymer dicolloids (CDCs) containing pyridine groups are synthesized by swelling spherical, lightly cross-linked polystyrene seeds with a mixture of styrene and pH-responsive monomer 2-vinyl pyridine followed by secondary polymerization to contrast with their analogue homopolymer dicolloids (HDCs) where the swelling step is carried out with styrene alone. After the particles are coated with a nonionic surfactant to minimize van der Waals attractions, surface potentials and aggregation properties of dilute suspensions are studied as functions of pH and ionic strength. Compared to HDCs, which remain stable at all pH values studied (3 < pH < 9) up to an ionic strength of 5 M, the CDC particles show amphoteric behavior with strong attractions under conditions where dipolar interactions are expected to dominate.     

Water-soluble zinc porphyrins as artificial receptors for amino acids

The binding of amino acids to water-soluble zinc porphyrins in basic aqueous solution was spectrophotometrically analyzed. The amino acids were bound to the porphyrins through the coordination of the N atom with the central zinc ion. Additional attractions arise due to Coulomb interactions between the -COO(-) anion of the amino acids and the -N(CH(3))(3)(+) cation of the porphyrin substituents and due to hydrophobic interactions between the porphyrin plane and the hydrophobic substituents of the amino acids. These attractions could be explained based on the binding data. The compensatory relationships of DeltaS and DeltaH were also discussed.     

The charge capacitance of the chemical bond: Application to bonds containing metals

The charge capacitance of metal containing complexes are studied. For molecules with multiple bonding between the metal atoms it is found that the charge capacitance is correlated to the maximum bond order, natural bond order, and to some extent the effective bond order. Furthermore the charge capacitance of some methylidene metal dihydride complexes are studied. These molecules have agostic interactions of varying strength, and it is concluded that this strength is very well reflected in the charge capacitances of the systems. In accordance with the definition of agostic interactions it is therefor concluded that the charge capacitance holds information about the strength of covalent interactions. The effect therefore on the agostic interactions upon substitution of one of the hydrogen atoms with fluorine in the methylidene metal complexes is studied, and found to reduce the agostic interactions. It is also demonstrated that there is an agostic interaction in an ArCrCrAr complex. The distance dependence of the charge capacitance is also discussed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Polycarboxylate crown ethers: Synthesis,complexation, applications

Crown ethers derived from tartaric acid present a number of interesting features as receptor frameworks and offer a possibility of enhanced metal cation binding due to favorable electrostatic interactions. The synthesis of polycarboxylate crown ethers from tartaric acid is achieved by simple Williamson ether synthesis using thallous ethoxide or sodium hydride as base. Stability constants for the complexation of alkali metal and alkaline earth cations were determined by potentiometric titration. Complexation is dominated by electrostatic interactions but cooperative coordination of the cation by both the crown ether and a carboxylate group is essential to complex stability. Complexes are stable to pH 3 and the ligands can be used as simultaneous proton and metal ion buffers. The low extractibility of the complexes was applied in a membrane transport system which is a formal model of primary active transport.     

Effect of attractions on the structure of polymer solutions confined between surfaces: a density functional approach

A density functional theory is presented to study the effect of attractions on the structure of polymer solutions confined between surfaces. The polymer molecules have been modeled as a pearl necklace of freely jointed hard spheres and the solvent as hard spheres, both having Yukawa-type attractions and the mixture being confined between attractive Yukawa-type surfaces. The present theory treats the ideal gas free energy functional exactly and uses weighted density approximation for the hard chain and hard sphere contributions to the excess free energy functional. The attractive interactions are calculated using the direct correlation function obtained from the polymer reference interaction site model theory along with the mean spherical approximation closure. The theoretical predictions on the density profiles of the polymer and the solvent molecules are found to agree quite well with the Monte Carlo simulation results for varying densities, chain lengths, wall separations, and different sets of interaction potentials.