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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]

Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as ΔfH298°(HO2) = 2.94 ± 0.06 kcal mol-1 (3.64 ± 0.06 kcal mol-1 at 0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron convention is ΔfH298°(HO2+) = 264.71 ± 0.14 kcal mol-1 (265.41 ± 0.14 kcal mol-1 at 0 K), while that for the negative ion, HO2- (within the same convention), is ΔfH298°(HO2-) = −21.86 ± 0.11 kcal mol-1 (−21.22 ± 0.11 kcal mol-1 at 0 K). The related proton affinity of molecular oxygen is PA298(O2) = 100.98 ± 0.14 kcal mol-1 (99.81 ± 0.14 kcal mol-1 at 0 K), while the gas-phase acidity of H2O2 is ΔacidG298°(H2O2) = 369.08 ± 0.11 kcal mol-1, with the corresponding enthalpy of deprotonation of H2O2 of ΔacidH298°(H2O2) = 376.27 ± 0.11 kcal mol-1 (375.02 ± 0.11 kcal mol-1 at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, ΔfH298°(OH) = 8.93 ± 0.03 kcal mol-1 (8.87 ± 0.03 kcal mol-1 at 0 K), together with new and more accurate enthalpies of formation of NO, ΔfH298°(NO) = 21.76 ± 0.02 kcal mol-1 (21.64 ± 0.02 kcal mol-1 at 0 K) and NO2, ΔfH298°(NO2) = 8.12 ± 0.02 kcal mol-1 (8.79 ± 0.02 kcal mol-1 at 0 K), as well as H2O2 in the gas phase, ΔfH298°(H2O2) = −32.45 ± 0.04 kcal mol-1 (−31.01 ± 0.04 kcal mol-1 at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that the previous equilibrium constant for NO + HO2 → OH + NO2 was underestimated by a factor of ∼2, implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentally explored in the companion paper of Srinivasan et al. (next paper in this issue). [ABSTRACT FROM AUTHOR]

G-quadruplex (GQ) structures formed from guanine-rich sequences are found throughout the genome and are overrepresented in the promoter regions of some oncogenes, at the telomeric ends of eukaryotic chromosomes, and at the 5′-untranslated regions of mRNA. Interaction of small molecule ligands with GQ DNA is an area of great research interest to develop novel anticancer therapeutics and GQ sensors. In this paper we examine the interactions of TMPyP4, its isomer TMPyP2 (containing N-methyl-2-pyridyl substituents, N-Me-2Py) as well as two metal derivatives ZnTMPyP4 and CuTMPyP4 with GQs formed by dT4G4 and dT4G4T in 100 mM K+ or Na+ conditions. The DNA sequences were chosen to elucidate the effect of the 3′-T on the stabilization effect of porphyrins, binding modes, affinities, and stoichiometries determined via circular dichroism melting studies, UV-vis titrations, continuous variation analysis, and fluorescence studies. Our findings demonstrate that the stabilizing abilities of porphyrins are stronger toward (dT4G4)4 as compared to (dT4G4T)4 (ΔTm is 4.4 vs -6.4 for TMPyP4; 12.7 vs 5.7 for TMPyP2; 16.4 vs 12.1 for ZnTMPyP4; and 1.9 vs -8.4 °C for CuTMPyP4) suggesting that the 3′G-tetrad presents at least one of the binding sites. The binding affinity was determined to be moderate (Ka ∼ 106-107 μM-1) with a typical binding stoichiometry of 1:1 or 2:1 porphyrin-to-GQ. In all studies, ZnTMPyP4 emerged as a ligand superior to TMPyP4. Overall, our work contributes to clearer understanding of interactions between porphyrins and GQ DNA. [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]

This paper provides spectral characterizations of the two isomers of the 1-methylenepyrene cation, namely, the 1-pyrenemethylium and a pyrene-like isomer owing a tropylium cycle. Both are possible photodissociation products of the 1-methylpyrene cation and were proposed as potential contributors to the diffuse interstellar bands. In that respect, vibrational and electronic spectra are computed for the optimized structures at the density functional theory (DFT) and time-dependent (TD-)DFT levels. Finite temperature effects on these spectra are estimated from molecular dynamics simulations within the density functional-based tight-binding (DFTB) and TD-DFTB frameworks, these methods being first benchmarked against DFT and TD-DFT calculations. The computed spectra allow discrimination of the two isomers. When the temperature increases, bands are observed to redshift and merge. The isomerization mechanism is investigated with the metadynamics technique, a biased dynamics scheme allowing to probe reaction mechanisms with high energy barriers by investigating the free energy surface at various temperatures. Four pathways with similar barrier heights (3.5-4 eV) are found, showing that the interconversion process would only occur in interstellar clouds under photoactivation. The present study opens the way to simulations on larger methyl- and methylenePAHs of astrophysical interest and their experimental investigation. [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, a new synthetic way to modify naphthalene diimide (NDI) at "shoulder" positions is reported. The key step of the transformation is the intramolecular cyclization involving ethynyl and imidecarbonyl groups. The structure of the intermediate pyrylium cation was confirmed by X-ray crystal structural analysis. New conjugated molecules 1a-g were successfully synthesized in acceptable yields. Their absorption and fluorescence spectra were measured. Among them 1c-f are strongly emissive in solutions. Furthermore, 1b-f are also fluorescent in their solid states; in particular, 1b exhibits a typical aggregation-induced enhanced emission feature. Yellow-emissive microfibrils of 1d show potential optical waveguide behavior. HOMO/LUMO energies of 1a-f were determined based on their cyclic voltammograms. The results also reveal that HOMO/LUMO energies of these new conjugated molecules are influenced by the two flanking moieties. Notably, the thin film of 1c that is emissive shows p-type semiconducting behavior with hole mobility up to 0.0063 cm² V-1 s-1 based on the transfer and output characteristics of the OFET (organic field effect transistor). [ABSTRACT FROM AUTHOR]

Heterolysis rate constants k1 of differently substituted 1,3-diarylallyl halides and carboxylates have been determined in various solvents. The linear free energy relationship log k1 = sf(Nf + Ef) was found to predict the heterolysis rates (log k1) of 1,3-diarylallyl derivatives with a standard deviation of 0.26, corresponding to a factor of 1.82 in k1, and maximum deviation in k1 of a factor of 5. Some systematic deviations are evident, however. Thus, 1,3-diarylallyl carboxylates always react faster and 1,3-diarylallyl chlorides always react more slowly than calculated by the quoted correlation equation when both types of leaving groups were used to determine the electrofugality parameters Ef. As 1,3-diarylalyl cations are generated faster in solvolysis reactions and also react faster with nucleophiles than benzhydrylium ions of similar thermodynamic stabilities, i.e., Lewis acidities, one can conclude that the reactions involving 1,3-diarylallyl cations proceed with lower intrinsic barriers than those involving benzhydrylium ions. The electrofugality parameters Ef of 1,3-diarylallylium ions determined in this work were combined with the electrophilicity parameters E of the corresponding cations as well as with the results on ion pair dynamics reported in preceding papers for generating the full mechanistic spectrum of 1,3-diarylallyl solvolyses. [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]

Nonbonding valence electrons, better known as lone pairs, are found us all anions as well as in canons in their lower oxidation states. In this paper the properties of lone pans are analyzed using a bond valence model defined in terms of a core and-valence-shell picture, which can be reduced to the point charges of the ionic model A bond is defined in terms of the electrostatic flux linking the atoms. When the lone pairs are inactive, they are uniformly distributed around the valence shell and the ion behaves like a main group ion that has no lone pairs. In this state it can be assigned a bonding strength that obeys the valence matching rule (for stable bonds the cation and anion bonding strengths should not differ by more than a factor of 2). However, when an ion with a lone pair has a bonding strength less than half that of the counterion, it has the flexibility to form a stronger bond by converting lone pair electron density to bonding electron density in the region where the valence shells overlap. To conserve the number of lone pairs, bonding electron density elsewhere in the valence shell is converted to lone pairs. The result is the adoption of an anisotropic coordination environment with fewer bonds but bonds whose enhanced strength matches the higher bonding strength of the counterion. This analysis raises questions about the validity of a number of traditional ideas on the nature of chemical bonds It shows that lone pairs do not form dative bonds. In a neutral molecule, the base function that is often attributed to a lone pair is always accompanied by an acid function, and both functions must be simultaneously activated. Electron-pair bonds, which are found only around strongly bonding cations, normally result in the formation of molecules, explaining why the models developed to describe molecular structures are unable to give good descriptions of extended crystal structures. [ABSTRACT FROM AUTHOR]

In the present paper we report combined experimental and theoretical studies of the UV-vis-NIR spectra of the mineral compounds malachite, rosasite, and aurichalcite and of the precursor compounds for Cu/ZnO catalysts. For the copper species in the minerals the crystal field splitting and the vibronic coupling constants are estimated using the exchange charge model of the crystal field accounting for the exchange and covalence effects. On this basis the transitions responsible for the formation of the optical bands arising from the copper centers in minerals are determined and the profiles of the absorption bands corresponding to these centers are calculated. The profiles of the absorption bands calculated as a sum of bands of their respective Cu species are in quite good agreement with the experimental data. In agreement with crystal chemical considerations, the Zn ions were found to be preferentially located on the more regular, i.e., less distorted, octahedral sites in zincian malachite and rosasite, suggesting a high degree of metal ordering in these phases. This concept also applies for the mineral aurichalcite, but not for synthetic aurichalcite, which seems to exhibit a lower degree of metal ordering. The catalyst precursor was found to be a mixture of zincian malachite and a minor amount of aurichalcite. The best fit of the optical spectrum is obtained assuming a mixture of contributions from malachite (0% Zn) and rosasite (38% Zn of [Zn + Cu]), which is probably due to the intermediate Zn content of the precursor (30%). [ABSTRACT FROM AUTHOR]

The one-electron oxidation potentials [EoxNHE(H2Q)], pKa (pKa1 and pKa2) values, and bond dissociation energies (BDE1 and BDE2) of 118 important p- and o-dihydroquinones in DMSO were systematically predicted for the first time by using DFT method and the PCM cluster continuum model. The calculated results agree well with the available experimental determinations. The study shows that all the five thermodynamic parameters correlate well with the Hammett substituent parameters σp (for p-H2Q, EoxNHE(H2Q·+/H2Q) = 1.66Σσp + 0.54, pKa1 = -5.69Σσp + 16.54, pKa2 = -5.19Σσp + 23.91, BDE1 = 3.43Σσp + 82.29, BDE2 = 4.64Σσp + 67.70 and for o-H2Q, EoxNHE(H2Q·+/H2Q) = 1.85Σσp + 0.46, pKa1 = -5.53Σσp + 13.28, pKa2 = -5.24Σσp + 26.70, BDE1 = 3.54Σσp + 82.08, BDE2 = 3.82Σσp + 75.93), which hints that we can get these thermodynamic parameters as long as the structure of the hydroquinones were known. The comparisons of the calculated five thermodynamic parameters between p-hydroquinones and o-hydroquinones and the number of the phenyl ring effects on these thermodynamic parameters were also studied. At last, intramolecular hydrogen bond energies in hydroquinones at neutral, radical cation, radical, anion different state were systematically calculated and analyzed. Combined with the papers published in our group before, we will have a systematic thermodynamic picture of the transfer details between different kinds of quinones and corresponding hydroquinones, which strongly promote the fast development of the understanding and applications of quinones. [ABSTRACT FROM AUTHOR]

This paper analyses the interaction of Co+(d8, 1G, and 3F) and Co2+(d7, 4F) cations with (glycyl)nglycine (n= 1−3) oligomers. The structure, relative energies, and binding energies of the complexes formed have been theoretically determined by means of density functional methods. For all Co+complexes the ground spin state is the triplet one and the most stable structures show tricoordinated geometries. In contrast, for Co2+systems the lowest energy structures are tricoordinated (n= 1), tetracoordinated (n= 2), and pentacoordinated (n= 3). For both cobalt cations, interaction energies increase with the peptide length. Differences in the coordination properties of the ligands are discussed according to their length as well as to the electronic configuration of the metal cation, and results are compared to those previously obtained for the analogous Cu+/2+systems. The IR spectra of the most stable and low energy conformers have been simulated, and a discussion of the main vibrational features is provided. [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, the bimolecular free (unhindered) electron transfer (FET) from various trityl-containing compounds to the solvent radical cations of n-BuCl is described. In good agreement with the previously studied cases, the FET involving trityl-derived compounds results in the formation of two different types of the radical cation, which undergo the subsequent fragmentation via two alternative reaction channels. This unusual effect is caused by the intramolecular rotational motion in the ground-state molecules around the arrow-marked bond Ar-↷-X-CPh 3(Ar = aromatic moiety; X = S, O, NH, CH 2), since such oscillations are directly connected with the electron distribution within the molecule. An unhindered electron jump from the donor trityl compound to the solvent radical cation, taking place in the subfemtosecond time range, generates the solute radical cation with the inherited geometry and the electron distribution of its precursor. Among the whole variety of produced radical cations, two extreme conformer states can be distinguished, namely, a planar and a twisted state. The planar type represents the structures with minimum energy, whereas the twisted type is destabilized by the increased value of the rotational barrier in the ionized state. The difference in the energetic profiles between planar and twisted radical cations plays a crucial role in their subsequent fragmentation. The planar radical cation follows the thermodynamically favored pathway generating ArX •and Ph 3C +. A distinct part of the twisted radical cation dissociates faster than it relaxes into the more preferable planar conformation and, therefore, produces a thermodynamically unfavorable couple of products: ArX +and Ph 3C •. This fragmentation channel is exclusively caused by FET. The undertaken quantum chemical calculations enable the judgment of the energetics of the different dissociation channels of the radical cations of the trityl derivatives. [ABSTRACT FROM AUTHOR]

An ab initio quantum dynamical study is performed here to examine the complex nuclear motion underlying the first two photoelectron bands of trifluoroacetonitrile. The highly overlapping structures of the latter are found to originate from transitions to the five lowest electronic states (viz., X̃2E, Ã2A1, B̃2A2, C̃2A1, and D̃2E) of the trifluoroacetonitrile radical cation. The Jahn−Teller (JT) instability of the doubly degenerate X̃and, D̃and their pseudo-Jahn−Teller (PJT) interactions with the nondegenerate Ã, B̃, and C̃electronic states along the degenerate vibrational modes lead to multiple multidimensional conical intersections and complex nuclear trajectories through them. It is found that the JT splitting is very weak in the X̃and relatively stronger in the D̃state. However, the PJT couplings play the pivotal role in the detailed shape of the vibronic bands of the radical cation. Ultrafast nonradiative decay of electronically excited radical cation has been examined. The findings of this paper are compared with the experimental data and are also discussed in relation to those observed for the methyl cyanide radical cation. [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]

This paper describes the preparation of two tetracations 4a4+ and 4b4+ composed of di(1-azulenyl)-methylium units based on a new structural principle of a cyanine-cyanine hybrid for the design of electrochromic materials with two color changes. Di- and monocations 5a2+, 5b2+ and 6a+, 6b+ composed of di(1-azulenyl)methylium units were also prepared for the purpose of comparison. The pKR+ values of the tetracations are rather high despite their tetracationic structure, although the stability of these cations decreases with the increase of the number of the existing cation units. The cyclic voltammetry (CV) of these cations revealed the presumed multielectron redox properties. However, the tetracations did not exhibit the idealized electrochemical behavior, in which subsequent two-electron reduction was presumed as the cyanine-cyanine hybrid, probably due to the less effective electrochemical interaction among the positive charges. The scope of the creation of the novel polyelectrochromic materials taking the new structural principle is demonstrated by these examples. [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]

The hydrogen-loss channel, induced by sequential multiphoton absorption, of the vapor-phase fluorene cation was investigated using a supersonic molecular beam and a time-of-flight mass spectrometer. The fluorene cation was prepared by resonantly enhanced multiphoton ionization. The ultimate goal of this experiment is the determination of the evolution of the dissociation rate constant in a wide energy range. In this paper, we give a description of the original experimental procedure, show that the absorption process is non-Poissonian, and determine the absolute photon absorption cross section. [ABSTRACT FROM AUTHOR]

In this paper, we present the absorption properties of a series of bis-triarylamino-[2.2]paracyclophane diradical dications. The localized π−π* and the charge-transfer (CT) transitions of these dications are explained and analyzed by an exciton coupling model that also considers the photophysical properties of the “monomeric” triarylamine radical cations. Together with AM1-CISD-calculated transition moments, experimental transition moments and transition energies of the bis-triarylamine dications were used to calculate electronic couplings by a generalized Mulliken-Hush (GMH) approach. These couplings are a measure for interactions of the excited mixed-valence CT states. The modification of the diabatic states reveals similarities of the GMH three-level model and the exciton coupling model. Comparison of the two models shows that the transition moment between the excited mixed-valence states μab of the dimer equals the dipole moment difference Δ%@mt;sys@%μ%@sx@%ag%@be@%%@/hd@%%@fn;(;vis;full;auto@%m%@fnx;);vis;full@%%@/hd@%%@sxx@%%@mx@% of the ground and the excited bridge state of the corresponding monomer. [ABSTRACT FROM AUTHOR]

The G2, G3, CBS-QB3, and CBS-APNO model chemistry methods and the B3LYP, B3P86, mPW1PW, and PBE1PBE density functional theory (DFT) methods have been used to calculate ΔH° and ΔG° values for ionic clusters of the ammonium ion complexed with water and ammonia. Results for the clusters NH4+(NH3)n and NH4+(H2O)n, where n = 1−4, are reported in this paper and compared against experimental values. Agreement with the experimental values for ΔH° and ΔG° for formation of NH4+(NH3)n clusters is excellent. Comparison between experiment and theory for formation of the NH4+(H2O)n clusters is quite good considering the uncertainty in the experimental values. The four DFT methods yield excellent agreement with experiment and the model chemistry methods when the aug-cc-pVTZ basis set is used for energetic calculations and the 6-31G* basis set is used for geometries and frequencies. On the basis of these results, we predict that all ions in the lower troposphere will be saturated with at least one complete first hydration shell of water molecules. [ABSTRACT FROM AUTHOR]

Several π-complexes of cations and anions with aromatic rings have been optimized at the MP2/6-31++G** level of theory. Different aspects of the cation-π interaction have been compared to those of anion-π, including changes in the aromaticity of the ring upon complexation, charge-transfer effects using the Merz-Kollman and “atoms-in-molecules” (AIM) charges, and the contribution of correlation and dispersion energies by comparing the complexation energies computed at the HF, B3LYP, and MP2 levels of theory. In this paper, we study three aromatic systems that allow direct comparisons, free from other influences, of the cation-π versus anion-π interactions, which are the 1,3,5-trifluorobenzene (TFB), s-triazine (TAZ), and 2,5-dichloropyrazine (DCP). These compounds are able to π-interact favorably with either anions or cations because of their very small quadrupole moments. [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]

The interest over the magnitude of the conjugation effect in the allyl cation (1) and anion (2) has been revived recently by Barbour and Karty (J. Org. Chem. 2004, 69, 648–654), who derived the resonance energies of 20–22 and 17–18 kcal/mol for 1 and 2, respectively, using an empirical extrapolation approximation. This paper revisits the case by explicitly calculating the Pauling­Wheland resonance energy, which measures the stabilization from the most stable resonance structure to the delocalized energy-minimum state of a conjugated system, using our newly developed block-localized wave function (BLW) method. This BLW method has the geometrical optimization capability. The computations result in adiabatic resonance energies of 37 kcal/mol for 1 and 38 kcal/mol for 2. The significant disagreement between these values and Barbour and Karty's results originates from the neglect of structural and electronic variations in their derivation which are energy costing. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the visible/near-infrared (vis/NIR) spectra of four bis(triarylamine) radical cation mixed valence systems with varying bridge units in the framework of the generalized Mulliken-Hush theory. We outline how to apply a three-level model by using both computational AM1-CI derived as well as experimental transition moments and energies in order to extract electronic coupling matrix elements. The most important outcome is that the much simpler two-level model is a good approximation only if the adiabatic dipole moment difference between the terminal states is large compared to the transition moments associated with the bridge state. This implies that the two-level model is only applicable to mixed valence compounds in the Robin-Day class II with strongly localized redox states if qualitative correct values are desired. We demonstrate that both the spectral features and the potential energy surface of the mixed valence compounds can solely be tuned by bridge state modification reaching from asymmetrically localized to symmetrically localized and from a single minimum potential to a triple minimum potential. For the particular case of an anthracene bridge, we show that solvent induced symmetry breaking has a dramatic influence on the spectral characteristics. [ABSTRACT FROM AUTHOR]

This paper reports the application of a new general tool for the study of multicenter bonding, namely the so-called generalized population analysis, to the investigation of interesting geometry dependent variation of 3c-2e bonding in the (C…H…C) fragments of ingeniously designed organic cations I and II. This phenomenon was previously characterized by the correlation between experimental 1H NMR chemical shifts of the central hydrogen in the (C…H…C) fragment and the changes in the corresponding C-H-C bond angle. The observed values of both chemical shifts and C-H-C angles are shown herein to correlate with the calculated 3-center bond indices but the dependence displays splitting into two separate lines according to the type of corresponding cation. [ABSTRACT FROM AUTHOR]

We have previously shown that, in the case of extremely short lived chlorocarbon anions (of CCl4, CFCl3) in liquid MCH solutions, one observes the formation and decay of the solvent separated ion pairs (R+||Cl-)solv, instead of the geminate ion recombination between the solvent cation (MCH+) and the fragment anion (Cl-). For longer lived anions (CHC[inline equation] ) there was no formation of ion pairs (IPs) observable. To evaluate the correlation between IP formation and anion lifetime τ-, three new chlorocarbon solutes (RCl), hexachloroethane (Hexa), pentachloroethane (Penta), and tetrachloroethane (Tetra), were studied, for which a coarse lifetime classification from positronium studies suggested that Hexa- and Penta- should be short lived (IP formation possible) and Tetra - long lived (no IP formation). The results in this paper agree with this expectation and confirm the correlation with τ-. With anion lifetimes of 250 and 150 ns for Hexa- and Penta- at 143 K, ion pairs were observed, whereas Tetra- with τ- = 13.7 μs decayed too late to yield IPs. The solvent separated ion pairs are formed through charge transfer (CT) from MCH+ to the fragment radical R• from the anion decay: MCH+ + R· · ·Cl--η → (R+||Cl-)solv. The IP absorption is due to the CT band of (R+ ← MCH), and the stability relates to the complexing with the solvent. The efficiency η for CT reduces with time and therefore correlates to the anion lifetime: the later R• is freed, the lower η. It also correlates with the ratio of Dfast/Ddiff (competition of the high mobility approach of MCH+ (with Dfast) toward Cl- and the diffusional escape of R• (with Ddiff), away from Cl-). It is shown that η reduces by a factor of about 6 from 133 to 295 K in parallel to Dfast/Ddiff reducing from 400 to 10. It is concluded that the high mobility of the solvent cation is a requirement for positive CT from MCH+ to R•. The IP formation therefore gains importance at very low temperature; however, it loses importance at room temperature. The IP lifetime at 143 K is longest for CCl4 (τip = 111 μs), followed by Hexa (τip = 22.7 μs) and Penta (τip = 5.3 μs). If no IP is detectable, IP formation is still possible, but τip ≪ τ- (probably true for CHCl3). For all IPs so far found (list of 7 given) the IP decay rate constant kip is characterized by a very low preexponential Arrhenius factor of log A ≉ 8-10. For CCl4, Hexa, and Penta, the log A values are 9.0 ± 0.2, 8.3 ± 0.3, and 10.4 ± 0.4, respectively. Simulation of the complete mechanism is rather complex, but it is carefully analyzed with schemes of various complexity, particularily with and without the cation mechanism, related to the precursor M+* of the high mobility cation MCH+. It is shown that the details of the cationic mechanism are covered up by the strong absorption from the ion pair IP, if formed. [ABSTRACT FROM AUTHOR]

The 2-phenylethyl-N,N-dimethylamine (PENNA) radical cation offers two functional groups for a positive charge to reside, the benzyl ring and the substituted amine group. Previously published HeI photoelectron spectra (PES) and our B3LYP data of the cation ground state agree that the amine site has the lowest ionization energy (IE). In this work we present evidence that by resonant laser multiphoton ionization the electron is removed from the phenyl site. B3LYP calculations of the neutral molecule predict that by far the most stable structure is the nonsymmetric unfolded anti conformer and that no other conformer should be significantly populated. This is confirmed by the S0−S1 resonant two-photon ionization (R2PI) spectrum in which one conformer is found to be predominant. The presence of the vibrational fingerprint of the phenyl chromophore and the absolute energetic position of the R2PI spectrum clearly show the local character of the first photoexcitation. Surprisingly, the R2PI mass spectra taken via S1 resonances show strong fragmentation. The parent-to-fragment-ion ratio is about 1:10 and mostly independent of laser intensity. The metastable character of the decay excludes a fragmentation caused by cation photoabsorption. The strong dissociation directly after ionization is explained by (i) a local ionization at the phenyl chromophore, (ii) a fast charge transfer (CT) to the lower-energetic amine site, and (iii) a subsequent metastable dissociation. A first detailed analysis of the ionization process indicates that intersite R2PI ionization between local states is a one-photon two-electron process which is expected to be improbable and that a failure of local ionization only happens in cases of mixed intermediate S1 states or mixed cation states. PENNA with two possible charge sites spaced by a −CH2−CH2− bridge is an ideal model system to study the dynamics of a downhill charge transfer after local ionization to the first excited cation state as presented in a forthcoming paper. [ABSTRACT FROM AUTHOR]

Presents a paper on the reaction of halogenated cyclopropanes and nitrosyl cation with reference to the preparation of isoxazoles. Information on the pharmacological activities of isoxazole derivatives; Reference to the preparation of isoxazolines; Detailed information on the research methods used in this study.