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This study is devoted to the characterization of ion exchange inside a microsphere of carboxylic resin. It aims at describing the kinetics of this exchange reaction which is known to be controlled by interdiffusion in the particle. The fractional attainment of equilibrium function of time depends on the concentration of the cations in the resin which can be modelized by the Nernst-Planck equation. A powerful approach for the numerical resolution of this equation is introduced in this paper. This modeling is based on the work of Helfferich but involves an implicit numerical scheme which reduces the computational cost. Knowing the diffusion coefficients of the cations in the resin and the radius of the spherical exchanger, the kinetics can be hence completely determined. When those diffusion parameters are missing, they can be deduced by fitting experimental data of fractional attainment of equilibrium. An efficient optimization tool coupled with the implicit resolution has been developed for this purpose. A monovalent/trivalent cation exchange had been experimentally characterized for a carboxylic resin. Diffusion coefficients and concentration profiles in the resin were then deduced through this new model.

It has long been recognized that cationic nanoparticles induce cell membrane permeability. Recently, it has been found that cationic nanoparticles induce the formation and/or growth of nanoscale holes in supported lipid bilayers. In this paper, we show that noncytotoxic concentrations of cationic nanoparticles induce 30-2000 pA currents in 293A (human embryonic kidney) and KB (human epidermoid carcinoma) cells, consistent with a nanoscale defect such as a single hole or group of holes in the cell membrane ranging from 1 to 350 nm(2) in total area. Other forms of nanoscale defects, including the nanoparticle porating agents adsorbing onto or intercalating into the lipid bilayer, are also consistent; although the size of the defect must increase to account for any reduction in ion conduction, as compared to a water channel. An individual defect forming event takes 1-100 ms, while membrane resealing may occur over tens of seconds. Patch-clamp data provide direct evidence for the formation of nanoscale defects in living cell membranes. The cationic polymer data are compared and contrasted with patch-clamp data obtained for an amphiphilic phenylene ethynylene antimicrobial oligomer (AMO-3), a small molecule that is proposed to make well-defined 3.4 nm holes in lipid bilayers. Here, we observe data that are consistent with AMO-3 making approximately 3 nm holes in living cell membranes.

The binding of dicationic Hoechst 33258 (ligand) to DNA was characterized by means of the fluorescence spectra, fluorescence intensity titration, time-resolved fluorescence decay, light scattering, circular dichroism, and fluorescence thermal denaturation measurements, and two binding modes were distinguished by the experimental results. Type 1 binding has the stoichiometry of one ligand to more than 12 base pairs, and it is defined as quasi-minor groove binding which has the typical prolonged fluorescence lifetime of about 4.4 ns. In type 1 binding, planar conformation of the ligand is favorable. Type 2 binding with phosphate to ligand ratio (P/L) < 2.5 has the stoichiometry of one ligand to two phosphates. It is defined as a highly dense and orderly stacked binding with DNA backbone as the template. Electrostatic interactions between doubly protonated ligands and negatively charged DNA backbone play a predominant role in the type 2 binding mode. The characteristics of this type of binding result in a twisted conformation of the ligand that has a fluorescence lifetime of less than 1 ns. The results also indicate that the binding is in a cooperative manner primarily by stacking of the aromatic rings of the neighboring ligands. Type 1 binding is only observed for double-stranded DNA (dsDNA) with affinity constant of 1.83 x 10(7) M-1. In the type 2 binding mode, the binding affinity constants are 4.9 x 10(6) and 4.3 x 10(6) M-1 for dsDNA and single-stranded DNA (ssDNA), respectively. The type 2 binding is base pair independent while the type 1 binding is base pair related. The experiments described in this paper revealed that the dication bindings are different from the monocation bindings reported by previous study. The dication binding leads to stronger aggregation at low ligand concentration and results in orderly arrangements of the ligands along DNA chains. Furthermore the dication binding is demonstrated to be beneficial for enhancing the DNA's stability.

This paper reports thermodynamic data for the transfer of calixarene derivatives and their metal-ion complexes in dipolar aprotic solvents. These data are used to assess the effect of solvation of these compounds on the selective complexation shown by these macrocycles for soft metal cations in different media. Thus, solubilities and derived Gibbs energies of solution of 5,11,17,23-tetra-tert-butyl[25,27-bis(hydroxyl)-26,28-bis(ethylthioethoxy)]calix(4)arene, 1, and 5,11,17,23-tetra-tert-butyl-[25,27-bis(ethylenethanoate)-26,28-bis(ethylthioethoxy)]-calix(4)arene, 2, in various solvents at 298.15 K are reported. Solvation of these ligands in one medium relative to another is analyzed from their standard transfer Gibbs energies using acetonitrile as the reference solvent. These data are combined with transfer enthalpies (derived from standard solution enthalpies obtained calorimetrically) to calculate the corresponding entropies of transfer of these calix(4)arene derivatives from acetonitrile to methanol and N,N-dimethylformamide. As far as the metal-ion salts (silver and mercury) in their free and complex forms are concerned, standard solution enthalpies were determined in acetonitrile, methanol, and N,N-dimethylformamide. These data are used to derive their transfer enthalpies from one medium to another. It is concluded that the extent of complexation of these macrocycles with soft metal cations is controlled by not only the solvation changes that the free cation undergoes in moving from one medium to another but also those for the ligand and its complex cation in these solvents.

The interaction of a calix(4)arene derivative, namely 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra[2-(4-pyridyl)methoxy]calix(4)arene, 1a, and its monomeric component, p-tert-butylphenoxy-4-pyridine, 1b, with metal cations has been investigated in acetonitrile and methanol. (1)H NMR measurements carried out in CD(3)CN show the primary role played by the pyridyl nitrogens in their complexation with metal cations. Conductance measurements demonstrated that for all cations (except mercury) the composition of the metal ion complexes of 1a is 1:1 (ligand:metal cation). However, 1a hosts two mercury cations per unit of ligand. For the monomer 1b, complexes of 2:1 (ligand:metal cation) stoichiometries are formed with the exception of Pb(2+) (1:1 composition). The thermodynamics of complexation of these systems are reported in acetonitrile. Data in methanol are limited to stability constant values for mercury(II) and these ligands. This paper demonstrates for the first time that thermodynamic data for the complexation of the monomeric component of the ligand and metal cations contribute significantly to the interpretation of systems involving cation-calixarene interactions in solution.

Recently, Kurnikov et al. (J. Phys. Chem. B 2002, 106, 7) have shown that solvation of DNA duplexes destabilizes holes of sizes larger than three base pairs. In this paper, we consider the effects of solvation and internal reorganization on the hole charge distribution in sequences 5'-X-GG-Y-3'. Radical cation states in DNA are found to be localized to a single guanine site independent of the nature of adjacent base pairs.

In this paper, we study the collapse of a mixed insoluble monolayer formed by a cationic matrix, dioctadecyl-dimethylammonium bromide (DOMA), and a tetra-anionic porphyrin, tetrakis(4-sulfonatophenyl)porphyrin (TSPP), in a molar ratio TSPP/DOMA = 1:4. During the collapse of this system, we visualized the formation of circular domains consisting exclusively of trilayer, although the domains coalescence was not observed. The coexistence of trilayer and monolayer at the final step of the collapse cannot be interpreted exclusively in terms of a thermodynamic phase equilibrium, intervening as an additional factor the anisotropic line tension of the domain. A high line tension implies a high resistance to the domain deformation, and the anisotropy of the line tension implies the lack of coalescence between these domains, which has been experimentally observed by Brewster angle microscopy for us. Under these circumstances, the domains of collapsed material could enclose monolayer regions where the local surface pressure drops thus stopping the collapse process. The collapse of the TSPP/DOMA system is reversible, that is, the return of the three-dimensional material to the monolayer fits into a simple kinetics according to the nucleation−growth-collision theory. As for the collapse, the reverse process is also affected by the line tension of the domains. This paper relates the high line tension and the anisotropic line tension of a given domains with the reversible nature of the collapse process. [ABSTRACT FROM AUTHOR]

Description of the local microscopic structure in ionic liquids (ILs) is a prerequisite to obtain a comprehensive understanding of the influence of the nature of ions on the properties of ILs. The local structure is mainly determined by the spatial arrangement of the nearest neighboring ions. Therefore, the main interaction patterns in ILs, such as cation-anion H-bond-like motifs, cation-cation alkyl tail aggregation, and ring stacking, were considered within the framework of the nearest-neighbor approach with respect to each particular interaction site. We employed classical molecular dynamics (MD) simulations to study in detail the spatial, radial, and orientational relative distribution of ions in a set of imidazolium-based ILs, in which the 1-butyl-3-methylimidazolium (C4mim+) cation is coupled with the acetate (OAc-), chloride (Cl-), tetrafluoroborate (BF4 -), hexafluorophosphate (PF6 -), trifluoromethanesulfonate (TfO-), or bis(trifluoromethanesulfonyl)amide (TFSA-) anion. It was established that several structural properties are strongly anion-specific, while some can be treated as universally applicable to ILs, regardless of the nature of the anion. Namely, strongly basic anions, such as OAc- and Cl-, prefer to be located in the imidazolium ring plane next to the C-H2/4-5 sites. By contrast, the other four bulky and weakly coordinating anions tend to occupy positions above/below the plane. Similarly, the H-bond-like interactions involving the H² site are found to be particularly enhanced in comparison with the ones at H4-5 in the case of asymmetric and/or more basic anions (C4mimOAc, C4mimCl, C4mimTfO, and C4mimTFSA), in accordance with recent spectroscopic and theoretical findings. Other IL-specific details related to the multiple H-bond-like binding and cation stacking issues are also discussed in this paper. The secondary H-bonding of anions with the alkyl hydrogen atoms of cations as well as the cation-cation alkyl chain aggregation turned out to be poorly sensitive to the nature of the anion. [ABSTRACT FROM AUTHOR]

Despite strong electrostatic repulsion,like-charged ions in aqueoussolution can effectively attract each other via ion–water interactions.In this paper we investigate such an effective interaction of like-chargedions in water by using the 3D-RISM-SCF method (i.e., electronic structuretheory combined with three-dimensional integral equation theory formolecular solvents). Free energy profiles are calculated at the CCSD(T)level for a series of molecular ions including guanidinium (Gdm+), alkyl-substituted ammonium, and aromatic amine cations.Polarizable continuum model (PCM) and mean-field QM/MM free energycalculations are also performed for comparison. The results show thatthe stability of like-charged ion pairs in aqueous solution is determinedby a very subtle balance between interionic interactions (includingdispersion and π-stacking interactions) and ionic solvation/hydrophobiceffects and that the Gdm+ion has a rather favorable characterfor like-charge association among all the cations studied. Furthermore,we investigate the like-charge pairing in Arg-Ala-Arg and Lys-Ala-Lystripeptides in water and show that the Arg-Arg pair has a contactfree-energy minimum of about −6 kcal/mol. This result indicatesthat arginine pairing observed on protein surfaces and interfacesis stabilized considerably by solvation effects. [ABSTRACT FROM AUTHOR]

Supportedionic liquid membranes (SILMs) are promising tools for the separationof carbon dioxide from other gases. In this paper, new imidazolium,pyrrolidinium, piperidinium, and morpholinium ionic liquids with atriethylene glycol side chain and tosylate anions, as well as theirsymmetrical dicationic analogues, have been synthesized and incorporatedinto SILMs. The selectivities for CO2/N2andCO2/CH4separations have been measured. Theselectivities exhibited by the dicationic ionic liquids are up totwo times higher than the values of the corresponding monocationicionic liquids. Quantum chemical calculations have been used to investigatethe difference in the interaction of carbon dioxide with monocationicand dicationic ionic liquids. The reason for the increased gas separationselectivity of the dicationic ionic liquids is two-fold: (1) a decreasein permeance of nitrogen and methane through the ionic liquid layer,presumably due to their less favorable interactions with the gases,while the permeance of carbon dioxide is reduced much less; (2) anincrease in the number of interaction sites for the interactions withthe quadrupolar carbon dioxide molecules in the dicationic ionic liquids,compared to the monocationic analogues. [ABSTRACT FROM AUTHOR]

In this paper we report the characterization of the dichloromethane(DCM)/water interface in terms of density profile, width, and surfacestructure. The use of steered molecular dynamics (SMD) to study thetransfer of the K+cation from the organic layer to thewater layer is also described. The corresponding free energy is insemiquantitative agreement with published experimental and theoreticalresults. The transference of the K+cation from the waterlayer toward the DCM layer occurs with concomitant water transportas a water microdroplet that detaches itself from the water layerafter ca. 16 Å of penetration into the organic layer by breakingthe thin water thread that unites both. Complexation of the watermicrodroplet by a polyethylene-glycol type podand induces the lossof water molecules from the water microdroplet to bulk DCM and, eventually,to the water layer. [ABSTRACT FROM AUTHOR]

The detailed oxidative decomposition mechanism of propylene carbonate (PC) in the lithium ion battery is investigated using density functional theory (DFT) at the level of B3LYP/6-311++G(d), both in the gas phase and in solvent. The calculated results indicate that PC is initially oxidized on the cathode to a radical cation intermediate, PC•+, and then decomposes through three pathways, generating carbon dioxide CO2and radical cations. These radical cations prefer to be reduced on the anode or by gaining one electron from PC, forming propanal, acetone, or relevant radicals. The radicals terminate by forming final products, including trans-2-ethyl-4-methyl-1,3-dioxolane, cis-2-ethyl-4-methyl-1,3-dioxolane, and 2,5-dimethyl-1,4-dioxane. Among all the products, acetone is most easily formed. The calculations in this paper give detailed explanations of the experimental findings that have been reported in the literature and clarify the role of intermediate propylene oxide in PC decomposition. Propylene oxide is one of the important intermediates. As propylene oxide is formed, it isomerizes forming a more stabile product, acetone. [ABSTRACT FROM AUTHOR]

In this paper we have corroborated the usefulness of spectroscopic techniques, such as UV−visible, in the study and thermodynamic characterization of the thermal unfolding of catalase as a function of the concentration and alkyl chain length of n-alkyltrimethylammonium bromides (CnTAB, n8, 10, and 12). For this reason, a thermodynamic model was used which included experimental data corresponding to the pre- and posttransition into the observable transition. It has been found that n-alkyltrimethylammonium bromides play two opposite roles in the folding and stability of catalase. They act as a structure stabilizer at a low molar concentration and as a destabilizer at a higher concentration. The maximum of the unfolding temperature has been found to decrease with the alkyl chain. The reason for this difference has been suggested to be the side chains involved. In the presence of C8TAB and C10TAB, Gibbs energies of unfolding (G(T)) decrease with concentration, whereas for C12TAB an increase has been observed. These findings can be explained by the fact that when differences in the hydrophobic nature of the surfactants exist, different pathways of unfolding may occur. Also, the presence of surfactants has been observed to affect the cold denaturation of catalase. Thermodynamic results suggest that the thermal denaturation of catalase in the presence of n-alkyltrimethylammonium bromides is a perfect transition between two states. [ABSTRACT FROM AUTHOR]

In this paper we report the results of the synthesis and structural, transport, and magnetic characterization of pure La0.5Sr1.5MnO4and B-site lightly doped samples, i.e. La0.5Sr1.5Mn0.95B0.05O4, where B Ru, Co, and Ni. The choice was made in order to probe the charge ordering/orbital ordering ground state of the monolayered La0.5Sr1.5MnO4manganite as a consequence of the cation doping. It is shown that even a light doping is successful in suppressing the charge and orbital order found in pure La0.5Sr1.5MnO4. No long-range magnetic order has been detected in any of the doped samples but the setup of a spin-glass state with a common freezing temperature (∼22 K). Structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the orbital character of the egelectrons along with the overall cation size. [ABSTRACT FROM AUTHOR]

This paper presents the results of a high-resolution neutron diffraction and magnetometry investigation on the optimally doped (x 0.3) La1.4Sr0.8Ca0.8Mn2O7bilayered manganite. In particular, two samples with different oxygen contents have been studied to put in prominence the role of the Mn average valence states at fixed cation concentration. The results show, for the first time, the absence of long-range magnetic order in this optimally doped manganite when the A-site of the structure is doped with equal proportions of isovalent Ca and Sr. This holds for both samples, which present different lattice effects with T, thus suggesting the primary role of cation disorder as the source of the lack of long-range order. The presence, for both samples, of defined insulating- to metallic-like transitions suggests that the transport properties are not linked to the evolution of long-range order and that two-dimensional spin ordering in the layers of the perovskite blocks may be sufficient to “assist” the hole hopping. A possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions. [ABSTRACT FROM AUTHOR]

A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication, nT2, n 6−50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in some cases, also at BLYP/6-31G(d), was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2and 50T2, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n6−12) were studied at the CASSCF(m,m)/6-31G(d) (m 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations. [ABSTRACT FROM AUTHOR]

To explore the coordination number (around the cation) dependence of the nonlinear optical (NLO) properties in alkalides, this paper studies the structures and large NLO responses of model alkalides, Li(NH3)nNa (n 1−4). At the MP2/aug-cc-pVDZ level, the structural characteristic is determined to be that the Li−Na distance increases (from 3.030 to 4.646 Å) with the increasing of the number of NH3(nfrom 1 to 4). Results show that Li(NH3)nNa (n 1−4) have considerably large first hyperpolarizabilities (0). Especially, a prominent coordination number dependence of the 0value is found as follows:  0 13 669 (n 1) < 26 840 (n 2) < 39 764 (n 3) < 77 921 au (n 4) at the MP2 level. With the same coordination number (four N atoms) of Lications, the 0value (77 921 au) of this “small” inorganic molecule Li(NH3)nNa is over five times larger than that of the “big” organic molecule LiCalix4pyrrole-Na (14 772 au). This indicates that the 0value is strongly related to the flexibility of the complexant. Obviously, the flexibility of (NH3)4is much greater than that of the cuplike shaped Calix4pyrrole. This work suggests that two important factors should be taken into account to enhance the first hyperpolarizability of alkalide, i.e., the coordination number around the cation and the flexibility of the complexant. [ABSTRACT FROM AUTHOR]

This paper describes a quantum chemical study of the electronic structure of thienylene vinylene oligomers ranging in size from two thienylene rings (2TV) to 12TV. The geometries of the TV oligomers in the ground state, the lowest triplet state, and the singly and doubly oxidized states were optimized using density functional theory calculations. The electronic absorption spectra were obtained from configuration interaction calculations with an INDO/s reference wave function. Comparison with experimental data shows that the agreement is satisfactory, except for the triplet-triplet absorption spectra. For closed shell systems (ground state and doubly occupied state), the spectra were also calculated by time dependent density functional theory (TDDFT). TDDFT considerably underestimates the neutral singlet-singlet excitation energies for longer chains. The nature of the excited states for the TV radical cations was found to be more similar to that of thiophenes than to that of phenylene vinylenes, indicating that the sulfur atom has a marked influence on the π-electron system. For the (singlet) absorption spectra of doubly oxidized TVs, the results from TDDFT calculations are surprisingly good; they are also good for long chains. TDDFT calculations for doubly charged TVs also confirm the existence of a second, weak absorption band as has been found experimentally. [ABSTRACT FROM AUTHOR]