Partial Positive Charge Research Articles

The modulation of ethane‐selective adsorption performance in series of bimetal PCN‐250 metal–organic frameworks: Impact of metal composition

AbstractThe developments of high‐performance adsorbents have attracted ever‐increasing attention in the separation of ethane/ethylene (C2H6/C2H4) mixtures. In this work, a series of ultra‐stable C2H6‐selective metal–organic frameworks (MOFs), PCN‐250, and its bimetal version PCN‐250(Fe2M2+, M2+ = Ni2+, Co2+, Zn2+, Mn2+), were synthesized to investigate the influence of metal compositions on the C2H6‐selective performance. Among the five materials, at 298 K and 100 kPa, PCN‐250(Fe2Co) showed the largest C2H6 uptake (6.21 mmol/g) while PCN‐250(Fe2Zn) exhibited the highest C2H6/C2H4 ideal adsorption solution theory (IAST) selectivity (1.70). The molecular simulation revealed that the introduced metal ions have different influences on the atomic partial charge within the framework which varied the framework adsorption affinities toward C2H6/C2H4 molecules. Moreover, the research on PCN‐250(Fe2Zn) suggested that the increased positive partial charge of metal sites together with a decreased charge density of benzene ring (BR) sites from the organic ligands will be beneficial to improve the desired C2H6 selectivity.

Electronic Structure of Superoxidized Radical Cationic Dodecaborate-Based Clusters.

The expanding field of boron clusters has attracted continuous theoretical efforts to understand their diverse structures and unique bonding. We recently discovered a new reversible redox event of B12(O-3-methylbutyl)12 in which the superoxidized radical cationic form [B12(O-3-methylbutyl)12]•+ was identified and isolated for the first time. Herein, comprehensive (TD-)DFT studies in tandem with electrochemical experiments were employed to demonstrate the generality of the reported behavior across perfunctionalized B12(OR)12 clusters (R = aryl or alkyl). While the spin density of radical cationic clusters is delocalized in the core region, the oxidation brings about notable gains of positive partial charges on the supporting groups whose electronics can readily tune the redox potential of the 0/•+ couple. The underlying changes of frontier orbitals were elucidated, and the resulting [B12(OR)12]•+ species manifest a general diagnostic absorption as a consequence of mixed local/charge-transfer excitations.

Argon Adsorption on Cationic Gold Clusters Aun+ (n ≤ 20).

The interaction of Aun+ (n ≤ 20) clusters with Ar is investigated by combining mass spectrometric experiments and density functional theory calculations. We show that the inert Ar atom forms relatively strong bonds with Aun+. The strength of the bond strongly varies with the cluster size and is governed by a fine interplay between geometry and electronic structure. The chemical bond between Aun+ and Ar involves electron transfer from Ar to Au, and a stronger interaction is found when the Au adsorption site has a higher positive partial charge, which depends on the cluster geometry. Au15+ is a peculiar cluster size, which stands out for its much stronger interaction with Ar than its neighbors, signaled by a higher abundance in mass spectra and a larger Ar adsorption energy. This is shown to be a consequence of a low-coordinated Au adsorption site in Au15+, which possesses a large positive partial charge.

Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12 X11 ]- (X=F, Cl, Br, I, CN).

Electrophilic anions of type [B12X11]− posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B12X11]− with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ‐donors and (ii) CO/N2 as σ‐donor‐π‐acceptors. Temperature‐dependent formation of [B12X11NG]− indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B12X11]− and steric effects. The binding of CO and N2 to [B12X11]− is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B12X11CO]− and [B12X11N2]− were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies νCO and νN2 , respectively. Observed shifts of νCO and νN2 are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π‐backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B12X11]− reactivety dependent on the substituent X and provides first systematic data on π‐backdonation from delocalized σ‐electron systems of closo‐borate anions.

Removal of triclosan by CTAB-modified zeolite-rich tuff from aqueous solutions

AbstractTriclosan (TCS) adsorption behavior by a modified zeolite with Cetyl Trimethyl Ammonium Bromide (CTAB) was evaluated factoring in pH, contact time, and TCS initial concentration in a batch system. Natural clinoptilolite-type zeolite from Sonora, Mexico was conditioned with a sodium chloride solution, and, subsequently, modified with CTAB. All the zeolites were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS), and the Zero Point Charge (pHZPC). It was found that the morphological and structural properties of natural zeolite were not affected after treatment with cationic surfactant. Furthermore, adsorption process efficiency was enhanced by the presence of CTAB, obtaining TCS maximum adsorption capacity at an optimal pH of 9.0. In this context, the positively charged surface of the modified zeolite and the anionic triclosan species present were decisive. Kinetics data were well adjusted to a pseudo-second order model with a TCS adsorption capacity of 1.430 ± 0.051 mg g-1 at an equilibrium time of 18 h. Isotherm results were best adjusted to the Langmuir model with a qmax = 2.027 mg g-1 using an initial Co concentration of 18.0 mg L-1, and reaching an equilibrium Ce concentration of 0.559 mg L-1. The mechanism for the adsorption of TCS by CTAB-modified zeolite was proposed to be electrostatic attractions between the group of partial positive charge of CTAB and the anionic species of triclosan. Consequently, CTAB-modified zeolites could be used as effective adsorbents for triclosan removal.

N-Arylimide Molecular Balances: A Comprehensive Platform for Studying Aromatic Interactions in Solution.

Noncovalent interactions of aromatic surfaces play a key role in many biological processes and in determining the properties and utility of synthetic materials, sensors, and catalysts. However, the study of aromatic interactions has been challenging because these interactions are usually very weak and their trends are modulated by many factors such as structural, electronic, steric, and solvent effects. Recently, N-arylimide molecular balances have emerged as highly versatile and effective platforms for studying aromatic interactions in solution. These molecular balances can accurately measure weak noncovalent interactions in solution via their influence on the folded-unfolded conformational equilibrium. The structure (i.e., size, shape, π-conjugation, and substitution) and nature (i.e., element, charge, and polarity) of the π-surfaces and interacting groups can be readily varied, enabling the study of a wide range of aromatic interactions. These include aromatic stacking, heterocyclic aromatic stacking, and alkyl-π, chalcogen-π, silver-π, halogen-π, substituent-π, and solvent-π interactions. The ability to measure a diverse array of aromatic interactions within a single model system provides a unique perspective and insights as the interaction energies, stability trends, and solvent effects for different types of interactions can be directly compared. Some broad conclusions that have emerged from this comprehensive analysis include: (1) The strongest aromatic interactions involve groups with positive charges such as pyridinium and metal ions which interact with the electrostatically negative π-face of the aromatic surface via cation-π or metal-π interactions. Attractive electrostatic interactions can also form between aromatic surfaces and groups with partial positive charges. (2) Electrostatic interactions involving aromatic surfaces can be switched from repulsive to attractive using electron-withdrawing substituents or heterocycles. These electrostatic trends appear to span many types of aromatic interactions involving a polar group interacting with a π-surface such as halogen-π, chalcogen-π, and carbonyl-π. (3) Nonpolar groups form weak but measurable stabilizing interactions with aromatic surfaces in organic solvents due to favorable dispersion and/or solvophobic effects. A good predictor of the interaction strength is provided by the change in solvent-accessible surface area. (4) Solvent effects modulate the aromatic interactions in the forms of solvophobic effects and competitive solvation, which can be modeled using solvent cohesion density and specific solvent-solute interactions.

Dissecting the electric quadrupolar and polarization effects operating in halogen bonding through electron density analysis with a focus on bromine.

The form of the electron density change (or difference) is usable as a kind of fingerprint of the electronic structural origin or mechanism that gives rise to intermolecular interactions. Here, this method is applied to halogen-bonding brominated systems to dissect the electric quadrupolar effect (arising from the anisotropic distribution of the valence electrons and intrinsic to the s2px 2py 2pz electronic configuration) and the polarization effect (induced by a partial negative charge of the halogen-bond accepting atom). It is shown that a suitable location of the "extra point" for placing a partial positive charge to represent the former is crucial and is clearly found from the electron density difference from the spherically isotropic Br- ion, while the latter consists of the dipolar polarization of the Br atom and the delocalized polarization of the whole molecule. A practical way for application to molecular dynamics simulations, etc., to represent these two factors is discussed.

Isomerization of Epoxides into Allylic Alcohols Catalyzed by Supported Au Nanoparticles on TiO2: A Mechanistic Insight.

Based on primary and β-secondary isotope effects, it is established that the isomerization of epoxides into allylic alcohols catalyzed by supported Au nanoparticles proceeds via an unsymmetrical concerted transition state in which there is partial positive charge developing on the C-O carbon atom progressing C-H abstraction. A carbocationic intermediate can be formed only in cases of sufficient stabilization of the positive charge by appropriate substituents.

N8− Polynitrogen Stabilized on Nitrogen-Doped Carbon Nanotubes as an Efficient Electrocatalyst for Oxygen Reduction Reaction

In this work, non-traditional metal-free polynitrogen chain N8− deposited on a nitrogen-doped carbon nanotubes (PN-NCNT) catalyst was successfully synthesized by a facile cyclic voltammetry (CV) approach, which was further tested in an oxygen reduction reaction (ORR). The formation of PN on NCNT was confirmed by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy. Partial positive charge of carbon within NCNT facilitated electron transfer and accordingly induced the formation of more PN species compared to CNT substrate as determined by temperature-programmed decomposition (TPD). Rotating disk electrode (RDE) measurements suggested that a higher current density was achieved over PN-NCNT than that on PN-CNT catalyst, which can be attributed to formation of the larger amount of N8− on NCNT. Kinetic study suggested a four-electron pathway mechanism over PN-NCNT. Moreover, it showed long stability and good methanol tolerance, which indicates its great potential application. This work provides insights on designing and synthesizing non-traditional metal-free catalysts for ORR in fuel cells.

2H-1,2,3-Triazole-chalcones as novel cytotoxic agents against prostate cancer

Prostate cancer is an important cause of death in the male population and for which there is no satisfactory chemotherapy. Herein a new series of chalcone hybrids containing 2H-1,2,3-triazole core as the ring B has been synthesized and evaluated in vitro against PC-3 prostate cancer cell line. Compounds 4a, 4c and 4e significantly reduced cell viability and showed IC50 of 28.55, 15.64 and 25.56 µM, respectively. The structure-activity relationship supported by computational chemistry points that the polarity of the molecular surface area should have some relevance to the efficiency of the compounds, in particular the ratio of the partial positive charge sites and the total molecular surface area exposed to the cell environment.

Nitroaromatic Compounds to Induce a Partial Positive Charge on the Silver Nanoparticle Surface for Facilitated Transport Membranes for Olefin/Paraffin Separation

Two nitroaromatic compounds (NACs), 1,2-dinitrobenzene (DNB) and 3,4-dinitrotoluene (DNT), were utilized as polarizers to induce a partial positive charge on the silver nanoparticle (Ag NP) surface with the help of the strong electron withdrawing property of the nitro group. Membranes containing surface-charged Ag NPs dispersed in poly(N-vinyl pyrrolidone) showed remarkably high separation performance for propylene/propane mixtures. The mixed-gas selectivity was higher than 170, which is the highest among polymer-based membranes ever reported including composite membranes, and the propylene permeance was about 0.9 gas permeance unit (GPU). This high separation performance is due to the facilitated olefin transport, where Ag NPs with a positive charge, as an olefin carrier, interact specifically and reversibly with propylene, but not with propane. Therefore, it was found that the facilitated olefin transport yielding extremely high separation performance is strongly associated with the binding energy of silver atoms, which is a measure of the positive charge density on the Ag NP surface.

Unusual and Conventional Dative Bond Formation by s2 Lone Pair Donation from Alkaline Earth Metal Atoms to BH3, AlH3, and GaH3.

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns2 electron pair on an alkaline earth atom (Mg or Ca) into a vacant n'p orbital on the group 13 atom of BH3, AlH3, or GaH3. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH3, MgAlH3, MgGaH3, and CaBH3 are found to be much as one would expect for datively bound molecules, CaAlH3 and CaGaH3 were found to have very unusual geometries in that their Al-H or Ga-H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M'-MH3 species lie in the 0.7-1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH3, AlH3, and GaH3 groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.

Driving Force for Exciton Dissociation in Organic Solar Cells: The Influence of Donor and Acceptor Relative Orientation

The presence of a favorable driving force for electron transfer (ΔG) at the donor/acceptor (D/A) interface is fundamental to overcome the exciton binding energy and improve the efficiency of organic photovoltaic devices (OPVs). Using density functional theory (DFT), time-dependent DFT, and classical molecular dynamics, here we systematically investigate the dependence of ΔG on the molecular orientation between the donor and the acceptor in the dimer. This study was performed by considering the model configuration of D/A species of four donor A–D–A molecules named DRCN­(4–7)­T and C60 acceptor. We identified two configurations that gave the highest negative values of ΔG (which favors exciton dissociation): (i) face-on orientation when the fullerene is located at the central thiophene ring of the DRCN­(4–7)­T molecule. In this situation, the high degree of hybridization between the highest occupied molecular orbital of the acceptor and the lowest unoccupied molecular orbital of the donor stabilizes the energy of the lowest charge transfer state (CT1). (ii) Edge-on orientation when C60 is close to the positive partial charges of the D molecule. In this configuration, the fullerene further polarizes the D molecule which enhances the dimers’ dipole moment. The electrostatic potential produced by this dipole decreases then the energy of the CT1 state. Molecular dynamics calculations performed to simulate the deposition of DRCN5T/C60 blends showed that dimers with a very negative driving force (as identified by our DFT calculations) are indeed present in the film. We also found a tweezers-like effect produced by the lateral alkyl groups attached to the central thiophene ring of the DRCN5T molecule. Those lateral groups can capture the fullerenes and guide them toward the center of the molecule during the solvent evaporation. This process would favor the formation of face-on dimers that tend to have a negative ΔG. The comparison of our data with experimental results measured in DRCN­(4–7)­T suggests that there is a correlation between negative values of ΔG (averaged over some D/A configurations) and the performance of the corresponding solar cell. The fine-tuning of the D/A configuration can make the difference to decrease the germinated recombination and improve the efficiency of OPVs.

Experimental and Theoretical Insights to Physicochemical Properties of Aqueous Solutions of 1, 2-Ethanediol and 1, 2, 3-Propanetriol at Different Temperatures

Water is designated as “Universal solvent” due to its physical and chemical attributes. Water becomes attracted to different types of molecules due to the polar arrangement of oxygen and hydrogen atoms having partial negative and positive charges. The density (ρ), ultrasonic velocity (U), viscosity (η) and refractive index (nD) of the aqueous solutions of 1, 2-ethanediol / 1, 2, 3-propanetriol have been determined at three different temperatures. Derived properties such molar volume (V), isentropic compressibility (ᵝs), acoustic impedance (Z) and free length (Lf) have been calculated using density, ultrasonic velocity and refractive index values at the measured temperatures. To evaluate various, inter and intra-molecular associations present in the systems, the excess properties (VE, ᵝsE, ZE, Lf E, ηE and ∆nD) are estimated. The excess values obtained experimentally have been fitted to the Redlich -Kister polynomial equation. Multiple linear regression and mixing rules have been adopted to calculate the viscosity and refractive index for the aqueous solutions at 298.15, 308.15 and 318.15K.

Graphene aerogel nanoparticles for in-situ loading/pH sensitive releasing anticancer drugs

Free polymer graphene aerogel nanoparticles (GA NPs) were synthesized by using reduction/aggregation of graphene oxide (GO) sheets in the presence of vitamin C (as a biocompatible reductant agent) at a low temperature (40 °C), followed by an effective sonication. Synthesis of GA NPs in doxorubicin hydrochloride (DOX)-containing solution results in the simultaneous synthesis and drug loading with higher performance (than that of the separately synthesized and loaded samples). To investigate the mechanism of loading and the capability of GA NPs in the loading of other drug structures, two groups of ionized (DOX, Amikacin sulfate and, d-glucosamine hydrochloride) and non-ionized (Paclitaxel (PTX)) drugs were examined. Furthermore, the relationship between the bipolar level of DOX solution (contributing to H-bonding of DOX and GO) and the amount of DOX loading was investigated. The DOX showed higher loading (>3 times) than PTX, as anticancer drugs. Since both DOX and PTX possess aromatic structures, the higher loading of DOX was assigned to its positive partial charge and ionized nature. Accordingly, other drugs (having positive partial charge and ionized nature, but no aromatic structure) such as Amikacin sulfate and d-glucosamine hydrochloride presented higher loading than PTX. These results indicated that although the π-π interactions induced by aromatic structures are important in drug loading, the electrostatic interaction of ionized drugs with GO (especially through H-bonding) is the dominant mechanism. DOX-loaded GANPs showed high pH-sensitive release (equivalent to the carrier weight) after 5 days, which can indicate benefits in tumor cell acidic microenvironments in-vivo.

Quantitative structure-activity relationship in the photodegradation of azo dyes

The photolysis characteristics of azo dyes are critically important in environmental pollution control, dye-sensitized solar cells, and dyeing-related industries. However, there is still lack of quantitative relationship between the structures of azo dyes and their photolysis characteristics. To address this issue, the photolysis of 22 azo dyes were conducted side by side at three pH (4.0, 6.0, 9.0). The obtained pseudo-first order photodegradation rate constants (k1) were processed with meta-analysis. Statistically, the hydrazone tautomer had a smaller excitation energy and was easier to undergo photolysis than the azo tautomer. The ortho-substituted sulfonate groups had an obvious protective effect on the photostability of azo dyes. The softness (s), the most positive and negative partial charge on a carbon atom (qC+, qC−) were found to be crucial descriptors in the establishment of QSAR models for the photostability of azo dyes. The QSAR model at pH 9.0 was robust for predicting the photostability of azo dyes under UV irradiation. N2-purging experiments and quantum chemical computation verified that the cleavage of azo bond was not a result of direct photolysis but was caused by the attack of photoinduced reactive oxygen species. The results here are helpful for the design of more stable azo dyes or the selection of suitable approaches for the treatment of dye-contaminated water bodies.

Evaluation of the silver species nature in Ag-ITQ2 zeolites by the CO oxidation reaction

Abstract Silver containing zeolites are widely used in heterogeneous catalysis and their catalytic performance is strongly dependent on the nature of the metal species supported on the zeolites. In this work, Ag-ITQ2 zeolites have been prepared by different methods: incipient wetness impregnation, ion exchange and preformed Ag-nanoclusters supported on the zeolite. The samples have been pretreated with oxygen or hydrogen previously to the catalytic test in the CO oxidation reaction. The results allow to correlate the nature of the Ag-species with its catalytic activity in this reaction as metallic silver is active for the CO oxidation but cationic silver does not show significant activity. It is shown that Ag0 is formed on the catalysts prepared by impregnation even without a reduction treatment. On the contrary, Ag-nanoclusters form stable partially charged silver species that only can be reduced to Ag0 nanoparticles under severe thermal conditions. Silver species formed in the catalysts prepared by ion exchange also present a partial positive charge. We concluded that this reaction is a useful test for monitoring the nature and evolution of silver species in the zeolite and to determine the stability of the silver nanoclusters.

Comparative study of the structure and dynamics of water confined between nickel nanosheets and bulk water, a study using reactive force fields

In this study, the structure and dynamics of water molecules confined between nickel surfaces and its comparison with water in bulk are investigated. To this end, molecular dynamics approach accompanied by reactive force field is used to study the characteristics of the confined water. The results showed that water molecules near the enclosing surfaces change in terms of structure and dynamics. The water molecules form a two-layer structure close to the nickel surface, and the results of the partial charge distribution give an electric double layer at the interface, with a positive partial charge of nickel atoms at surface against the negative partial charge of the first layer of water molecules. In addition, intense stratification effects near the nickel surfaces are visible. The radial distribution functions showed that, on average, the hydrogen atoms of the water molecules in the near-surface layers are closer to the nickel than the oxygen atoms, and also because of the repulsive effects induced by the interaction with the surfaces, water molecules would be found at distances further than 2 Å. Different orientations of the water molecules relative to the nickel surface were calculated using orientational ordering. This calculation shows the character of heterogeneous water molecules in confined state.

Hydroxyl Functionalized Pyridinium Ionic Liquids: Experimental and Theoretical Study on Physicochemical and Electrochemical Properties.

Structurally modified hydroxyl functionalized pyridinium ionic liquids (ILs), liquid at room temperature, were synthesized and characterized. Alkylated N-(2-hydroxyethyl)-pyridinium ILs were prepared from alkylpyridines via corresponding bromide salts by N-alkylation (65–93%) and final anion exchange (75–96%). Pyridinium-alkylation strongly influenced the IL physicochemical and electrochemical properties. Experimental values for the ILs physicochemical properties (density, viscosity, conductivity, and thermal decomposition temperature), were in good agreement with corresponding predicted values obtained by theoretical calculations. The pyridinium ILs have electrochemical window of 3.0–5.4 V and were thermally stable up to 405°C. The IL viscosity and density were measured over a wide temperature range (25–80°C). Pyridine alkyl-substitution strongly affected the partial positive charge on the nitrogen atom of the pyridinium cations, as shown by charge distribution calculations. On-going studies on Mg complexes of the new ILs demonstrate promising properties for high current density electrodeposition of magnesium.

Prediction of Solubility Parameters of Light Gases in Glassy Polymers on the Basis of Simulation of a Short Segment of a Polymer Chain

A method is developed for predicting the sorption parameters of polymers (the infinite dilution solubility coefficient of gases S and the Henry law equilibrium constant kD of double sorption models) of most light gases in glassy polymers on the basis of a computer model of a polymer chain segment. From the molecular dynamic model of conformations of a short (about 200–600 atoms) segment of the polymer chain, the dependences of a number of geometric indices, such as the accessible surface area and the accessible surface area with a positive partial charge, on the probe radius (in fact, on the effective radius of penetrant or sorbate) are calculated. The linear approximation coefficients of these dependences are used as explanatory variables in multiple linear regression. Significant variables and their weights in regression are determined on the basis of experimental measurements from the Database of Physicochemical Properties of Polymers of the Laboratory of Membrane Gas Separation at the Institute of Petrochemical Synthesis, Russian Academy of Sciences. By using a more accurate algorithm for calculating the areas of macromolecules, the shortcomings of the first version of this method are eliminated. For S, the coefficient of regression determination (adjusted R2) on the test sample is equal to 0.8 (this corresponds to an average relative error of 77% in predicting S, which is considered good accuracy), and for kD, the coefficient of determination is 0.81 and the average relative error is 47%, which is also a good result considering large (up to decimal order) errors of the experimental measurements of S and kD.