Stabilization of the Alleged 'Bishomoaromatic' Bicyclo[3.2.l]octa-2,6-dienyl Anion by Counterion Interactions and by Hyperconjugation

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution.

Influence of a Counterion on the Ion Atmosphere of an Anion: A Molecular Dynamics Study of LiX and CsX (X = F-, Cl-, I-) in Methanol

We report molecular dynamics (MD) simulations to explore the influence of a counterion on the structure and dynamics of cationic and anionic solvation shells for various ions in methanol at 298 K. We show that the variation in ionic size of either the cation or the anion in an ion pair influences the solvation structure of the other ion as well as the diffusivity in an electrolyte solution of methanol. The extent of ionic association between the cation and its counteranion of different ionic sizes has been investigated by analyzing the radial distribution functions (RDFs) and the orientation of methanol molecules in the first solvation shell (FSS) of ions. It is shown that the methanol in the FSS of the anion as well the cation exhibit quite different radial and orientational structures as compared to methanol which lie in the FSS of either the anion or the cation but not both. We find that the coordination number (CN) of F-, Cr-, and I- ions decreases with increasing size of the anion which is contrary to the trend reported for the anions in H2O. The mean residence time (MRT) of methanol molecules in the FSS of ions has been calculated using the stable states picture (SSP) approach. It is seen that the ion-counterion interaction has a considerable influence on the MRT of methanol molecules in the FSS of ions. We also discuss the stability order of the ion-counterion using the potentials of mean force (PMFs) for ion pairs with ions of different sizes. The PMF plots reveal that the Li+-F- pair (small-small) is highly stable and the Li+-I- pair is least stable (small-large) in electrolyte solutions.

Tunable wettability by counterion exchange at the surface of electrostatic self-assembled multilayers

A novel method to tune surface wettability rapidly and reversibly has been developed by ion exchange of the counterions at the surface of a multilayer film assembled via electrostatic interaction.

Incorporation of Nanoparticles into Polyelectrolyte Multilayers via Counterion Exchange and in situ Reduction

We report a general method for incorporation of nanoparticles into polyelectrolyte multilayer (PEM) thin films by utilizing the excess charges and associated counterions present in the PEMs. Silver ions were introduced directly into multilayers assembled from poly(diallyldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS), (PDDA/PSS)(n), by a rapid ion exchange process, which were then converted into silver nanoparticles via in situ reduction to create composite thin films. The size and the content of the nanoparticles in the film call be tuned by adjusting the ionic strength in the polyelectrolyte solutions used for the assembly. Spatial control over the distribution of the nanoparticles in the PEM was achieved via the use of multilayer heterostructure containing PDDA/PSS bilayer blocks assembled at different salt concentrations. Because excess charges and counterions are always present in any PEM, this approach can be applied to fabricate a wide variety of composite thin Films based on electrostatic self-assembly.

Counterion exchange at the surface of polyelectrolyte multilayer film for wettability modulation

Counterions present at the surface of polyelectrolyte multilayers (PEMs) were utilized for modulation of surface wettability via ion exchange. The PEM film was dipped in aqueous solutions of different anions, respectively, and the water contact angle of the surface varied from about 10 degrees to 120 degrees, depending on the hydration characteristics of the anion. The ion exchange mechanism was verified by X-ray photoelectron spectroscopy. The process was rapid and reversible. Ionic strength of the polyelectrolyte solution used for preparing the PEMs was found to be crucial to the surface wetting properties and the reversibility and kinetics of the process, and the effects were correlated to the surface density of the excess charge and counterion. This work provides a general, facile and rapid approach of surface property modulation.

CHRONOAMPEROMETRIC STUDIES ON MIGRATION AND DIFFUSION OF COUNTERION IN POLY(3-METHYLTHIOPHENE)

Small amplitude potential step experiments were carried out to study the counterion transfer process in oxidized poly(3-methylthiophene) (PMT) film. The results demonstrate that anion transfer process in PMT film is migration rather than diffusion. A porous metal electrode model-single hole model, which takes into account both the ionic resistance of the film and the uncompensated solution resistance, was found suitable to describe the potential step experiments. According to this model, the ionic resistivity of oxidized PMT film was calculated to be 5.0 x 10(4) OMEGA.cm, and, in turn, the diffusion coefficent of ClO4- ion in PMT film 3.7 x 10(-9) cm2/s.

Lanthanide(III) nitrobenzenesulfonates as new nitration catalysts: The role of the metal and of the counterion in the catalytic efficiency

Lanthanide(III) complexes of p-nitrobenzenesulfonic acid, Ln(p-NBSA)(3), m-nitrobenzenesulfonic acid, Ln(m-NBSA)(3), and 2,4-nitrobenzenesulfonic acid, Ln(2,4-NBSA)(3), were prepared, characterized and examined as catalyst for the nitration of benzene, toluene, xylenes, naphthalene, bromobenzene and chlorobenzene. The initial screening of the catalysts showed that lanthanum(III) complexes were more effective than the corresponding ytterbium(III) complexes, and that catalysts containing the bulky 2,4-NBSA ligand were less effective than the catalyst containing p-NBSA (nosylate) or m-NBSA ligands. Examination of a series of Ln(p-NBSA)(3) and Ln(m-NBSA)(3) catalysts revealed that there is a clear correlation between the ionic radii of the lanthanide(III) ions and the yields of nitration, with the lighter lanthanides being more effective. The X-ray single crystal structure of Yb(m-NBSA)(3).6H(2)O shows that two m-NBSA ligands are directly bound to the metal centre while the third ligand is not located in the first coordination sphere, but it is hydrogen bonded to one of the water molecules which is coordinated to ytterbium(III). NMR studies suggest that this structure is preserved under the conditions used in the nitration reaction. The structure of Yb(m-NBSA)(3) is markedly different from the structure of the well-known ytterbium(III) triflate catalyst. The coordination of the nitrobenzenesulfonate counterion to the lanthanide(III) ion suggests that steric effects might play an important role in determining the efficiency of these novel nitration catalysts. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).

Substituent and counterion effects on the formation of p-dimer dications of end-capped heptathienoacenes

We have investigated the impact of the functionalization and the chemical nature of counterions on the p-dimer dications formation in two end-capped heptathienoacenes. Radical cations of an a-substituted heptathienoacene with triisopropylsilyl groups do not p-dimerize, while those of an a,b-substituted heptathienoacene with four n-decyl side chains show a high propensity toward p-dimerization, increased by PF6 ÿ counterions.

Radical cations of end-capped tetrathienoacenes and their p-dimerization controlled by the nature of a-substituents and counterion concentration

Radical cations of a soluble rigid tetrathienoacene are capable of forming stable p-dimer dications at ambient temperature when the short backbone becomes extended with conjugated thiophene-2-yl substituents in the a-positions. On the other hand, simple attachment of methyl groups on the a-carbon of the external thiophen-2-yl rings proved sufficient to inhibit the dimerization. Stable radical cationswere also exclusively formed for tetrathienoacene derivatives end-capped with bulky TIPS and phenyl substituents.

The role of counterion on the thermotropic phase behavior of DODAB and DODAC vesicles

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Counterion effects on the structure and solubilization properties of anionic detergents. Role of hydrophobic cations in stabilizing micelles and attenuating their solvent properties

A series of alkyl sulfate detergents has been investigated in the presence of the cations Na +, methylviologen(2+) (MV 2+), 4-(cyanomethyl)pyridinium(1+) (CMP +), and tetramethylammonium (TMA +). The binding of these ions to the aqueous micellar assemblies has been measured through studies of luminescence quenching with the extramicellar probe, RuL 34-, where L = 4,4′-dicarboxy-2,2′-bipyridine. A general comparison of the alkyl sulfate aggregates with the nonquenching cations Na + and TMA + shows that the latter ion reduces the critical micelle concentration but at the same time depresses the ability of the detergent assemblies to bind or solubilize the hydrophobic quencher cations MV 2+ or CMP +. The reduced binding ability of the TMA + aggregates compared to that of the corresponding Na + soaps shows up largely in the form of a reduced favorable ΔS° for the solubilization in the case of the former. The results are in accord with a picture of the TMA + micelle as being more stable and more disordered than the corresponding assembly with Na + as the counterion. © 1989 American Chemical Society.

Counterion condensation for rigid linear polyelectrolytes

Diese Doktorarbeit studiert steife, lineare Polyelektrolyteim Rahmen eines Zellenmodells. Im Mittelpunkt steht dabeidas Phänomen der Gegenionenkondensation an der Oberflächeeines geladenen Makroions. Seine Abhängigkeit vonParametern wie Dichte, Bjerrum-Länge, Valenz undIonenstärke wird untersucht, und seine Auswirkungen aufwichtige Observablen wie Ionenverteilungen und osmotischerDruck werden diskutiert. Von theoretischer Seite werdendiese Probleme mit Hilfe der nichtlinearen undlinearisierten Poisson-Boltzmann Gleichung sowieallgemeineren Dichtefunktionaltheorien behandelt.Molekulardynamik-Simulationen ergänzen die theoretischenErgebnisse und grenzen den Bereich ihrer Gültigkeit ab. Ausgehend von der Poisson-Boltzmann Theorie wird einneuartiges Kriterium fuer Gegenionenkondensationvorgeschlagen, welches mit der Manning-Theorie verträglichist. Ein neuer Korrekturterm fuer die freie Energie inPoisson-Boltzmann Näherung wird hergeleitet, ausgehend vomModell eines einkomponentigen Plasmas. Die entsprechendenFunktionale der freien Energie werden mittels einerneuartigen Monte-Carlo Methode minimiert. Diedurchgeführten Computersimulationen untersucheninsbesondere die qualitativ neuen Phänomene, welche beihoher Ionenstärke auftreten, wie etwa Ladungsumkehr, einnegativer osmotischer Druck oder ein nicht-monotoneszeta-Potential. In all diesen Fällen wird die Bedeutungmultivalenter Ionen offensichtlich. In den Simulationen werden elektrostatische Wechselwirkungenmittels Particle-Mesh-Ewald Algorithmen berechnet. DerenAufbau wird in einem einheitlichen mathematischen Rahmenanalysiert. Speziell fuer die P3M Methode wird erstmalseine analytische Fehlerabschätzung hergeleitet.

Dressed polyions, counterion condensation, and adsorption excess in polyelectrolyte solutions.

The phenomenon of Manning-Oosawa counterion condensation is given an explicit statistical mechanical and qualitative basis via a dressed polyelectrolyte formalism in connection with the topology of the electrostatic free-energy surface and is derived explicitly in terms of the adsorption excess of ions about the polyion via the nonlinear Poisson-Boltzmann equation. The approach is closely analogous to the theory of ion binding in micelles. Our results not only elucidate a Poisson-Boltzmann analysis, which shows that a fraction of the counterions lie within a finite volume around the polyion even if the volume of the system tends towards infinity, but also provide a direct link between Manning's theta-the number of condensed counterions for each polyion site-and a statistical thermodynamic quantity, namely, the adsorption excess per monomer.