Influence of anions on intervalence charge transfer (IVCT) in mixed-valence dinuclear complexes

Spectroelectrochemical studies of the intervalence charge transfer (IVCT) characteristics of both diastereoisomeric forms of the dinuclear complex [{Ru(bpy)2}2(mu-dpi-)]n+ [bpy=2,2'-bipyridine; dpi-=4,5-di(2-pyridyl)imidazolate] showed that the degree of inter-metal electronic coupling (or valence delocalization) is dependent on stereochemical identity. Increasing the relative concentration of the strongly associating anion toluene-4-sulfonate in acetonitrile/[(n-C4H9)4N]{B(C6F5)4} solution differentially decreased the level of delocalization for the two diastereoisomers. In a comparative investigation of electrochemical and spectroelectrochemical techniques of the anion-induced electron localization in [{Ru(bpy)2}2(mu-dpo)]5+ [dpo=3,4-di(2-pyridyl)-1,2,5-oxadiazole], differences were observed between the two methods in the order and extent of effects induced by a number of inorganic anions (PF6-, BF4-, ClO4-). It was determined that the measure of coupling derived from electrochemical methods was less reliable than that obtained from spectral methods. Comparative electrochemical studies were undertaken on [{M(bpy)2}2(mu-BL)]n+ {M=Ru, Os; BL=dpo, dpi-), which revealed substantial differences in DeltaEox (the separation between the redox potentials for the MII-MII/MIII-MII and MII-MIII/MIII-MIII couples) for the two metal centers and therefore the comproportionation constant Kc, dependent on the neutral or anionic nature of the bridging ligand.     

The effect of the water content of acetonitrile on the electrochemistry of ferrocenyl thiol self-assembled monolayers

A novel kind of ethylene-ferrocenyl dodecyl thiol bearing an electron-withdrawing pyridium group (FcCH=CH-Py(CH2)10SH) was synthesized and mixed with HOOCC(10)SH on an Au electrode to form mixed self-assembled monolayers (SAMs) in ethanol solution. The influence of the water content of acetonitrile on the electrochemistry of the SAMs was studied by cyclic voltammetry and ac voltammetry measurement. The results showed that the current decreased and the oxidation potential of the ferrocene group shifted negatively with increasing water content of acetonitrile.     

Intramolecular charge delocalization in the singlet excited state of blocked pyrylium ions

The dual fluorescence emission of the pyrylium ion 3 and of the partly blocked 4 has been studied extensively under various conditions. The short-wavelength emitting species N^* of 3 is short-lived (≤200 ps at room temperature) while the long-wavelength emitting species A^* is long-lived (>3 ns, except in acetic acid). This long-wavelength fluorescence undergoes an important solvatochromic shift and the difference Δ between the absorption and fluorescence maxima versus Lippert’s solvent polarity function Δf is linear. Increasing the viscosity of the medium, or decreasing the temperature, decreases the long-wavelength emission quantum yield while that of the short-wavelength fluorescence and its lifetime (from <100 ps to >4 ns) both increase, indicating that A^* is formed from N^*. Introducing an ortho methyl group on the paraanisyl substituent (compound 4) blocks its rotation and reduces the fluorescence I_A./I_N. ratio, but it does not suppress completely the long-wavelength emission. This favors a ground state configuration where the phenyl substituent would be orthogonal to the xanthylium moiety. A strong interaction of 3 and 4 with aliphatic nitriles is characterized from the quenching of the fluorescence emission (with rate constants of ca. 2×10⁸ M⁻¹ s⁻¹). A static quenching process also occurs indicating a ground state interaction with the solvent. In pure aliphatic nitriles, this interaction is the main deactivation pathway of the singlet excited state, and practically no fluorescence nor triplet formation can be observed.     

Effect of low-molecular-weight organic anions on surface charge of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes. In the present paper, the effect of the anions of four low-molecular-weight organic acids on the surface charge of three variable charge soils was investigated. The results showed that the presence of organic anions led to an increase in negative charge and a decrease in positive charge. Positive charges decreased to a larger extent than negative charges. The effect of different anions on surface charge followed the order citrate > malate > oxalate > acetate. For hyper-rhodic ferrasol and rhodic ferrasol, the change of positive charge decreased with the increase in pH, while that of negative charge increased with the increase in pH. Among different soils the extent of change in surface charge was related to their iron oxides content. When free iron oxides were removed from the soil, the effect of organic anions on surface charge decreased sharply. These findings may be of practical significance for variable charge soils low in nutrient-retaining capacity by increasing the retaining capacity for cations such as potassium and calcium considerably but decreasing that for anions such as nitrate remarkably.     

Stable dimeric aromatic cation-radicals. Structural and spectral characterization of through-space charge delocalization

The spontaneous assembly of aromatic cation-radicals (D(+)(*)()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)(*)() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)(*)() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)(*)()).     

Influence of water content on failure of phenolic composites in fire

This paper evaluates the structural performance of flame resistant phenolic matrix composites exposed to fire. Experimental fire tests were performed on a glass-phenolic composite under combined static loading and one-sided radiant heating. The reduction to the tension and compression failure strengths of the phenolic composite was measured in these tests for heat flux conditions ranging from 10 kW/m² (∼225 °C) to 75 kW/m² (∼700 °C). It was discovered that the failure strengths of the phenolic composite decreased rapidly in the event of fire, particularly under compressive loading when failure occurred more rapidly than under tensile loading. The phenolic composite, despite having high flame resistance, loses strength more rapidly and fails sooner than a more flammable vinyl ester composite. The study shows that greater flammability resistance does not necessarily result in better structural performance in fire. The poor structural performance of the phenolic composite was due to explosive delamination damage and cracking caused by vaporisation of water in the matrix phase. It is shown that removing water from phenolic composites by natural or artificial ageing reduces the incidence of delamination cracking and thereby improves the materials' structural performance in fire. It is concluded that phenolic composites do not provide good structural performance in fire, even though they have low flame and smoke properties. However, reducing the water content in the matrix phase below about 10% can greatly improve the structural performance of phenolic composites during fire.     

Effect of ortho substitution on the charge localization of dinitrobenzene radical anions

Optical and electron paramagnetic resonance spectroscopies were used to study the radical anions of several m-dinitrobenzenes and p-dinitrobenzenes with substituents on ortho positions relative to the nitro groups. 1,4-Dinitrobenzene, 1,4-dimethyl-2,5-dinitrobenzene, and 2,5-dinitrobenzene-1,4-diamine radical anions are delocalized (class III) mixed valence species, but in the dinitrodurene radical anion the nitro groups are forced out of the ring plane due to the steric hindrance, which results in localization of the charge. The radical anions m-dinitrobenzene, 2,6-dinitrotoluene, and dinitromesitylene are all localized (class II) mixed valence species, as is common for m-dinitrobenzenes, and the rate of intramolecular electron transfer reaction strongly decreases with the number of methyl substituents. The same mechanism of rotation of the nitro groups out of the ring plane due to steric hindrance caused by neighboring methyl groups is also responsible for slowing the reaction. However, 2,6-dinitroaniline radical anion and 2,6-dinitrophenoxide radical dianion are charge-delocalized because the strong electron releasing amino and oxido groups increase the conjugation between the two charge-bearing units.     

Investigation of charge localization and charge delocalization in model molecules by multiphoton ionization photoelectron spectroscopy and DFT calculations

In this work we focus on the question to which degree a surplus charge is localized or delocalized in extended molecular systems. Molecules consisting of a flexible tail and the benzene chromophore, such as n-propylbenzene, 2-phenylethyl alcohol and 2-phenylethylamine, are used as model molecules. Their S0-S1 resonance enhanced multiphoton ionization (MPI) spectra containing origin transitions of different conformers appear at similar wavelengths. This shows, that in the neutral the electronic excitation is localized at the benzene chromophore. Geometry differences between the neutral and the cation can be qualitatively derived from intensities of vibrational transitions or the onset behavior in MPI high-resolution photoelectron (MPI-PE) spectra. We identify two possible reasons for structural changes: Charge-dipole interaction and charge delocalization. Whereas both effects can be active for the folded gauche conformers, the charge-dipole interaction is expected to be small for the extended anti conformers and geometry changes are attributed to charge delocalization. Density functional calculations of structures and energies qualitatively confirm the experimental results for all molecules and their conformers. They predict charge delocalization into the end group of below 20% for n-propylbenzene and 2-phenylethyl alcohol. In the case of 2-phenylethylamine the charge is equally shared by the near-isoenergetic charge sites of the benzene chromophore and the amine group.     

Quantifying the role of water in protein-carbohydrate interactions

Water-mediated interactions play a key role in carbohydrate-lectin binding, where the interactions involve a conserved water that is separated from the bulk solvent and present a bridge between the side chains of the protein and the carbohydrate ligand. To apply quantum mechanical methods to examine the role of conserved waters, we present an analysis in which the relevant carbohydrate atoms are modeled by methanol, and in which the protein is replaced by a limited number of amino acid side chains. Clusters containing a conserved water and a representative amino acid fragment were also examined to determine the influence of amino acid side chains on interaction energies. To quantify the differential binding energies of methanol versus water, quantum mechanical calculations were performed at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31+G(d) level in which either a methanol molecule was bound to the conserved water (liganded state) or in which a water molecule replaces the methanol (unliganded state). Not surprisingly, the binding of a water to clusters containing charged amino acid side chains was more favorable by 1.55 to 7.23 kcal/mol than that for the binding of a water to the corresponding pure water clusters. In contrast, the binding energy of water to clusters containing polar-uncharged amino acid side chains ranged from 4.35 kcal/mol less favorable to 4.72 kcal/mol more favorable than for binding to the analogous pure water clusters. The overall trend for the binding of methanol versus water, in any of the clusters, favored methanol by an average value of 1.05 kcal/mol. To extend these studies to a complex between a protein (Concanavalin A) and its carbohydrate ligand, a cluster was examined that contained the side chains of three key amino acids, namely asparagine, aspartate, and arginine, as well as a key water molecule, arranged as in the X-ray diffraction structure of Con A. Again, using methanol as a model for the endogenous carbohydrate ligand, energies of -5.94 kcal/mol and -5.70 kcal/mol were obtained for the binding of methanol and water, respectively, to the Con A-water cluster. The extent to which cooperativity enhanced the binding energies has been quantified in terms of nonadditive three-body contributions. In general, the binding of water or methanol to neutral dimers formed cooperative clusters; in contrast, the cooperativity in charged clusters depended on the overall geometry as well as the charge.     

Influence of aluminum on the lipid composition of pond snails present in media with different acidities

It has been established that the toxic action of aluminum on the hydrobiontLymnaea stagnalis L. increases in an acid medium. The dual action of acidification and a toxicant lowers the survival rate. The lipid and phospholipid compositions of pond snails of control and experimental groups at various concentrations of toxicant and various pH values of the water have been investigated.Institute of Water Problems, Northern Karelian Scientific Center of the Russian Academy of Sciences, Petrozavodsk. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 784–789, November-December, 1995. Original article submitted December 13, 1994.     

Interfacial acidities, charge densities, potentials, and energies of carboxylic acid-functionalized silica/water interfaces determined by second harmonic generation

Second-harmonic studies were carried out to determine the interfacial acidity, the potential, and the interfacial energy density of an acid-functionalized silica/water interface between pH 2 and 12. The interfacial potential changes over 3 orders of magnitude, from 10-2 mV to several tens of millivolts, and the interfacial energy density changes by 7 orders of magnitude, from less than 10-7 mJ/m2 to several millijoules per square meter. The methodology presented in this study provides quantitative thermodynamic information necessary for understanding and predicting how solvated species interact with functionalized organic adlayers at liquid/solid interfaces over a wide pH range.     

Physical organic chemistry of transition metal carbene complexes. 23. Kinetic and thermodynamic acidities of cationic benzothienyl- and selenylcarbene complexes of rhenium in aqueous acetonitrile.

The pK(a) values of a cationic selenyl- (5H(+)) and a benzothienylcarbene complex (6H(+)) and rate constants for the reversible deprotonation of these complexes by water, carboxylate ions, primary aliphatic amines, secondary alicyclic amines (5H(+) only), and OH(-) (5H(+) only) were determined in 50% MeCN-50% water (v/v) at 25 degrees C. In comparison with neutral Fischer-type carbene complexes such as 1H, the cationic complexes 5H(+) and 6H(+) are much more acidic, and the intrinsic barriers to proton transfer are substantially higher. This paper discusses a variety of factors that contribute to these differences, with the most important ones being that 5H(+) and 6H(+) are cationic, which makes the C(5)H(5)(NO)(PPh(3))Re moiety a stronger pi-acceptor than the (CO)(5)M moieties, coupled with the fact that the deprotonated forms of 5H(+) and 6H(+ )are aromatic molecules.     

New approach to the analysis of low levels of anions in water.

Optimized electromigrative sample introduction improves detection limits for anions in capillary electrophoresis. Reproducible results are achieved for micromolar and nanomolar levels of concentration. The new method offers shorter runtimes, improved resolution and greater simplicity in comparison with ion chromatography. The technique was applied to water samples from the power industry. Trace levels of anions are monitored routinely in water for steam generation in conventional and nuclear power plants. Reproducible and accurate results are presented for pure water samples containing typical concentrations of anions as well as for more specific types of samples, such as water compositions in the primary and secondary circuits of a nuclear power plant.     

Influence of water content on the self-assembly of metal-organic frameworks based on pyridine-3,5-dicarboxylate

A 0D discrete molecule [Co(3,5-pdc)(H2O)5].2H2O (1) was obtained in quantitative yield from the reaction of CoCl2.6H2O and pyridine-3,5-dicarboxylate (3,5-pdc) in pure water solvent at ambient temperature. While a 1D zigzag chain species, [{Co(3,5-pdc)(H2O)4}.H2O]n (2), was produced in a water-rich environment, a 2D layer compound, [Co(3,5-pdc)(H2O)2]n (3), with a 6(3) topology was generated under a water-reduced condition and a 2D sheet structure, [{Cu(3,5-pdc)(py)2}.H2O.EtOH]n (4), was formed under a water-poor condition. Compounds 1, 2, and 4 were characterized by single-crystal X-ray diffraction analysis. The 1D zigzag chain 2 shows a recoverable collapsing property. Compound 4 adopts a 2D sheet structure with a 4.8(2) topology, observed for the first time for the 3,5-pdc-related metal-organic frameworks. Water content was found to be an important factor in determining the topologies of the products in the self-assembly of divalent metal ions (Co2+, Cu2+) and pyridine-3,5-dicarboxylate under mild conditions.     

The association of aromatic anions in water

Molar conductances of dilute solutions of sodium 2-naphthalenesulfonate at 25°C and of sodium 2-anthraquinonesulfonate have been measured at 25 and 37°C. These data are interpreted to show that the anthraquinonesulfonate anion is dimerized in solution. Dimerization constants and the enthalpy and entropy of dimerization are calculated. Spectrophotometric absorbancies have been measured at 25, 30, 35 and 40°C for a series of aqueous solutions containing both sodium 4-dimethylaminobenzenesulfonate and sodium 3-nitrobenzenesulfonate. Equilibrium constants and enthalpy and entropy changes for formation of the 1 1 complex between these two ions have been calculated from these data. The formation of these complexes between like-charged ions, and of dimers of other aromatic solutes in water is discussed in terms of a two-state model of hydrophobic hydration.     

Effect of low-molecular-weight organic anions on electrokinetic properties of variable charge soils

It is known that some inorganic anions can be adsorbed by variable-charge soils specifically, resulting in the lowering of the zeta potential of the clay particle. Reasoning similarly, organic anions should also have such an effect. In this article, the effect of the anions of five low-molecular-weight (LMW) organic acids existing widely in soils on the zeta potentials of two variable-charge soils was examined. The results showed that the presence of organic anions led to a decrease in zeta potential. The effect of different anions on zeta potential followed the order oxalate>citrate>malate>maleate>acetate. The effect increased with the increase in anion concentration and decreased with the increase in pH. The extent of the effect on different soils was apparently related to their iron oxide content. The presence of organic anions also led to a decrease in the isoelectric point (IEP) of the soil. The IEPs of two soils in organic anion systems followed the order acetate>maleate>malate>citrate. No IEP was detected for the oxalate system.     

Ultrafast anisotropy dynamics of water molecules dissolved in acetonitrile

Infrared pump-probe experiments are performed on isolated H(2)O molecules diluted in acetonitrile in the spectral region of the OH stretching vibration. The large separation between water molecules excludes intermolecular interactions, while acetonitrile as a solvent provides substantial hydrogen bonding. Intramolecular coupling between symmetric and asymmetric modes results in the anisotropy decay to the frequency-dependent values of approximately 0-0.2 with a 0.2 ps time constant. The experimental data are consistent with a theoretical model that includes intramolecular coupling, anharmonicity, and environmental fluctuations. Our results demonstrate that intramolecular processes are essential for the H(2)O stretching mode relaxation and therefore can compete with the intermolecular energy transfer in bulk water.