High Charge Density Cations Research Articles

Cations impact radical reaction dynamics in concentrated multicomponent aqueous solutions.

Ultraviolet (UV) photolysis of nitrite ions (NO2-) in aqueous solutions produces a suite of radicals, viz., NO·, O-, ·OH, and ·NO2. The O- and NO· radicals are initially formed from the dissociation of photoexcited NO2-. The O- radical undergoes reversible proton transfer with water to generate ·OH. Both ·OH and O- oxidize the NO2- to ·NO2 radicals. The reactions of ·OH occur at solution diffusion limits, which are influenced by the nature of the dissolved cations and anions. Here, we systematically varied the alkali metal cation, spanning the range from strongly to weakly hydrating ions, and measured the production of NO·, ·OH, and ·NO2 radicals during UV photolysis of alkaline nitrite solutions using electron paramagnetic resonance spectroscopy with nitromethane spin trapping. Comparing the data for the different alkali cations revealed that the nature of the cation had a significant effect on production of all three radical species. Radical production was inhibited in solutions with high charge density cations, e.g., lithium, and promoted in solutions containing low charge density cations, e.g., cesium. Through complementary investigations with multinuclear single pulse direct excitation nuclear magnetic resonance (NMR) spectroscopy and pulsed field gradient NMR diffusometry, cation-controlled solution structures and extent of NO2- solvation were determined to alter the initial yields of ·NO and ·OH radicals as well as alter the reactivity of NO2- toward ·OH, impacting the production of ·NO2. The implications of these results for the retrieval and processing of low-water, highly alkaline solutions that comprise legacy radioactive waste are discussed.

Water Dynamics in Divalent and Monovalent Concentrated Salt Solutions

Water hydrogen bond dynamics in concentrated salt solutions are studied using polarization-selective IR pump-probe spectroscopy and 2D IR vibrational echo spectroscopy performed on the OD hydroxyl stretching mode of dilute HOD in H(2)O/salt solutions. The OD stretch is studied to eliminate vibrational excitation transfer, which interferes with the dynamical measurements. Though previous research suggested that only the anion affected dynamics in solution, here it is shown that the cation plays a role as well. From FT-IR spectra of the OD stretch, it is seen that replacing either ion of the salt pair causes a shift in absorption frequency relative to that of the OD stretch absorption in bulk pure water. This shift becomes pronounced with larger, more polarizable anions or smaller, high charge-density cations. The vibrational lifetime of the OD hydroxyl stretch in these solutions is a local property and is primarily dependent on the nature of the anion and whether the OD is hydrogen bonded to the anion or to the oxygen of another water molecule. However, the cation still has a small effect. Time dependent anisotropy measurements show that reorientation dynamics in these concentrated solutions is a highly concerted process. While the lifetime, a local probe, displays an ion-associated and a bulk-like component in concentrated solutions, the orientational relaxation does not have two subensemble dynamics, as demonstrated by the lack of a wavelength dependence. The orientational relaxation of the single ensemble is dependent on the identity of both the cation and anion. The 2D IR vibrational echo experiments measure spectral diffusion that is caused by structural evolution of the system. The vibrational echo measurements yield the frequency-frequency correlation function (FFCF). The results also show that the structural dynamics are dependent on the cation as well as the anion.

The effect of metal cations on the nature of the first electronic transition of liquid water as studied by attenuated total reflection far-ultraviolet spectroscopy

The first electronic transition (Ã←X̃) of liquid water was studied from the perspective of the hydration of cations by analyzing the attenuated total reflection far-ultraviolet (ATR-FUV) spectra of the Group I, II, and XIII metal nitrate electrolyte solutions. The Ã←X̃ transition energies of 1 M electrolyte solutions are higher (Li(+): 8.024 eV and Cs(+): 8.013 eV) than that of pure water (8.010 eV) and linearly correlate with the Gibbs energies of hydration of the cations. The increases in the Ã←X̃ transition energies are mostly attributable to the hydrogen bond formation energies of water molecules in the ground state induced by the presence of the cations. The deviation from the linear relation was observed for the high charge density cations, H(+), Li(+), and Be(2+), which reflects that the electronic energies in the excited states are also perturbed. Quantum chemical calculations show that the Ã←X̃ transition energies of the water-cation complexes depend on the hydration structures of the cations. The calculated Ã←X̃ transition energies of the water molecules hydrating high charge density cations spread more widely than those of the low charge density cations. The calculated transition energy spreads of the water-cation complexes directly correlate with the widths of the Ã←X̃ transition bands measured by ATR-FUV spectroscopy.

Charge density-dependent modifications of hydration shell waters by Hofmeister ions.

Gadolinium (Gd(3+)) vibronic sideband luminescence spectroscopy (GVSBLS) is used to probe, as a function of added Hofmeister series salts, changes in the OH stretching frequency derived from first-shell waters of aqueous Gd(3+) and of Gd(3+) coordinated to three different types of molecules: (i) a chelate (EDTA), (ii) structured peptides (mSE3/SE2) of the lanthanide-binding tags (LBTs) family with a single high-affinity binding site, and (iii) a calcium-binding protein (calmodulin) with four binding sites. The vibronic sideband (VSB) corresponding to the OH stretching mode of waters coordinated to Gd(3+), whose frequency is inversely correlated with the strength of the hydrogen bonding to neighboring waters, exhibits an increase in frequency when Gd(3+) becomes coordinated to either EDTA, calmodulin, or mSE3 peptide. In all of these cases, the addition of cation chloride or acetate salts to the solution increases the frequency of the vibronic band originating from the OH stretching mode of the coordinated waters in a cation- and concentration-dependent fashion. The cation dependence of the frequency increase scales with charge density of the cations, giving rise to an ordering consistent with the Hofmeister ordering. On the other hand, water Raman spectroscopy shows no significant change upon addition of these salts. Additionally, it is shown that the cation effect is modulated by the specific anion used. The results indicate a mechanism of action for Hofmeister series ions in which hydrogen bonding among hydration shell waters is modulated by several factors. High charge density cations sequester waters in a configuration that precludes strong hydrogen bonding to neighboring waters. Under such conditions, anion effects emerge as anions compete for hydrogen-bonding sites with the remaining free waters on the surface of the hydration shell. The magnitude of the anion effect is both cation and Gd(3+)-binding site specific.

A review on the problems of the solid state ions diffusion in cathodes for rechargeable Mg batteries

This work summarizes the results of our studies devoted to Mg ions mobility in Chevrel phases (CPs), MxMo6T8 (M – metal, T = S, Se) and presents our vision of the problem of multivalent ions’ diffusion in intercalation compounds. A simplified analysis of the main factors, which affect the activation energy barriers, as well as experimental data of Mg ions insertion into different hosts, show that low Mg ions mobility in common transition metal oxides or sulfides cannot be explained only by strong ionic interactions, but it is rather caused by a hard redistribution of the bivalent cations charge in inorganic materials. In contrast to these hosts, CPs allow for a high mobility of multivalent cations, because their unusual crystal structure with octahedral Mo6-clusters enables a fast and efficient attainment of local electro-neutrality upon insertion of cations of high charge density. Analysis of diffusion pathways based on the detailed structural determinations sheds light on important aspects of the electrochemical behavior of CPs, such as partial Mg ions trapping in the course of reversible Mg insertion and the ways to avoid it.

Acceleration of oxidative folding of bovine pancreatic ribonuclease A by anion-induced stabilization and formation of structured native-like intermediates

Phosphate anions accelerate the oxidative folding of reduced bovine pancreatic ribonuclease A with dithiothreitol at several temperatures and ionic strengths. The addition of 400 mM phosphate at pH 8.1 increased the regeneration rate of native protein 2.5-fold at 15°C, 3.5-fold at 25°C, and 20-fold at 37°C, compared to the rate in the absence of phosphate. In addition, the effects of other ions on the oxidative folding of RNase A were examined. Fluoride was found to accelerate the formation of native protein under the same oxidizing conditions. In contrast, cations of high charge density or ions with low charge density appear to have an opposite effect on the folding of RNase A. The catalysis of oxidative folding results largely from an anion-dependent stabilization and formation of tertiary structure in productive disulfide intermediates (des-species). Phosphate and fluoride also accelerate the initial equilibration of unstructured disulfide ensembles, presumably due to non-specific electrostatic and hydrogen bonding effects on the protein and solvent.

Relative role of anions and cations in the stabilization of halophilic malate dehydrogenase.

Halophilic malate dehydrogenase unfolds at low salt, and increasing the salt concentration stabilizes, first, the folded form and then, in some cases, destabilizes it. From inactivation and fluorescence measurements performed on the protein after its incubation in the presence of various salts in a large range of concentrations, the apparent effects of anions and cations were found to superimpose. A large range of ions was examined, including conditions that are in general not of physiological relevance, to explore the physical chemistry driving adaptation to extreme environments. The order of efficiency of cations and anions to maintain the folded form is, for the low-salt transition, Ca(2+) approximately Mg(2+) > Li(+) approximately NH(4)(+) approximately Na(+) > K(+) > Rb(+) > Cs(+), and SO(4)(2)(-) approximately OAc(-) approximately F(-) > Cl(-), and for the high-salt transition, NH(4)(+) approximately Na(+) approximately K(+) approximately Cs(+) > Li(+) > Mg(2+) > Ca(2+), and SO(4)(2)(-) approximately OAc(-) approximately F(-) > Cl(-) > Br(-) > I(-). If a cation or anion is very stabilizing, the effect of the salt ion of opposite charge is limited. Anions of high charge density are always the most efficient to stabilize the folded form, in accordance with the order found in the Hofmeister series, while cations of high charge density are the most efficient only at the lower salt concentrations and tend to denature the protein at higher salt concentrations. The stabilizing efficiency of cations and anions can be related in a minor way to their effect on the surface tension of the solution, but the interaction of ions with sites only present in the folded protein has also to be taken into account. Unfolding at high salt concentrations corresponds to interactions of anions of low charge density and cations of high charge density with the peptide bond, as found for nonhalophilic proteins.

Oxide solution mechanisms in silicate melts: Systematic variations in the activity coefficient of SiO 2

Phase equilibria and spectroscopic data are used to develop a simple model for the interaction of various oxide components and molten SiO 2. Network modifying oxides, M x O y produce nonbridging oxygens thereby depolymerizing the SiO 2 network. The energetics of nonbridging oxygen formation are least favorable when the field strength of the metal cation is high. This produces relatively strong M-O and Si-O-Si bridging bonds at the expense of weaker Si-O-M bonds ( De Jong et al., 1980). This relationship is manifested by an increase in positive deviations from ideality with increasing cation field strength in M x O y -SiO 2 systems; the activity coefficient of SiO 2 is inversely correlated with Si-O-M bond strength. Network forming oxides (aluminates, phosphates, titanates. zirconates, etc.) may copolymerize with the SiO 2 network. Mixing on the same quasi-lattice produces solutions which approach ideality. Deviations from ideality in such solutions can be linked to distortions in the SiO 2 network. Discrete anion formers (phosphates, titanates, chromates, zirconates) complex with metal oxides other than SiO 2 to form discrete structural units which do not copolymerize with SiO 2. The SiO 2 network is essentially shielded from the high charge density cations in such systems and unmixing is common. As a result, the relative deviations from ideality in such melts are high. It is important to recognize that oxides such as P 2O 5, TiO 2 and ZrO 2 may act as either network-formers or discrete anion formers depending upon melt composition, and are probably distributed between these two “sites” in most geologically important liquids. The latter structural role is favored in more basic compositions.

The production of ethyl acetate from ethylene and acetic acid using clay catalysts

AbstractWyoming bentonite, exchanged with cations of high charge density, is an efficient and selective catalyst for the production of ethyl acetate, in a single step, from ethylene and acetic acid. The reaction occurs in the interlamellar space of the clay where it is proposed that strong Brönsted acid sites on the clay coupled with highly polarized intercalated reactants produce a reaction which is otherwise difficult to carry out.

Investigations on the use of 4-(5-Nonyl) pyridine as an extractant for chlorocomplexes of gallium(III)

A study of the distribution of gallium(III) between hydrochloric acid solutions and 0.1M 4-(5-nonyl)-pyridine in benzene has been undertaken. Gallium can be quantitatively extracted from moderate to concentrated acid solutions and also from dilute acid solutions containing high concentrations of neutral cations of high charge density. The effects of acidity, the solvent concentration and diverse ions on the extraction have been investigated. An attempt has been made to elucidate the stoichiometry of the extracted species. The formulation of (NPy H)+ GaCl 4 − ) and (NPy−H−NPy)+ GaCl 4 − is expected at macro and trace concentrations of the metal. The extraction coefficients of several elements have been recorded under conditions optional for the extraction of gallium; and their factors for separations are estimated.

Minor and trace element partitioning between immiscible ocelli-matrix pairs from lamprophyre dikes and sills, Monteregian Hills petrographic province, Quebec

Many lamprophyre dike and sill rocks in the Monteregian Hills petrographic province of southwestern Quebec contain felsic segregations (ocelli) which have been interpreted as globules of immiscible liquid (Philpotts 1976). Ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements. The major element data, when plotted on a Greig diagram, outline a field of possible silicate-liquid immiscibility at higher alumina+alkali content than that previously mapped in iron-rich experimental systems. The trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high-charge density cations are preferentially concentrated in the matrix (mafic) material, a result which is consistent with theoretical and experimental studies.

The role of P2O5 in silicate melts

Phase equilibria data in the systems SiO2-P2O5, P2O5-MxOy, and P2O5-MxOy-SiO2 are employed in conjunction with Chromatographic and spectral data to investigate the role of P2O5 in silicate melts. Such data indicate that the behavior of P2O5 is complex. P2O5 depolymerizes pure SiO2 melts by entering the network as a four-fold coordinated cation, but polymerizes melts in which an additional metal cation other than silicon is present. The effect of this polymerization is apparent in the widening of the granite-ferrobasalt two-liquid solvus. In this complex system P2O5 acts to increase phase separation by further enrichment of the high charge density cations Ti, Fe, Mg, Mn, Ca, in the ferrobasaltic liquid. P2O5 also produces an increase in the ferrobasalt-granite REE liquid distribution coefficients. These distribution coefficients are close to 4 in P2O5-free melts, but close to 15 in P2O5-bearing melts.The dual behavior of P2O5 is explained in a model which requires complexing of phosphate anions (analogous to silicate anions) and metal cations in the melt. This interaction destroys Si-O-M-O-Si bonds polymerizing the melt. The higher concentration of Si-O-M-O-Si bond complexes in immiscible ferrobasaltic liquids relative to their conjugate immiscible granite liquids explains the partitioning of P2O5 into the ferrobasaltic liquid.

Complexation of sodium and potassium with macrocyclic polyethers; activation of the counter anions

Complexation of inorganic salts of sodium and potassium, MX (X = NCS −, I −, Br −, Cl − and PO 4 −3) with benzo-15-crown-5 (I) and dibenzo-30-crown-10 (II) in ethanolic (methanolic) and aq-ethanolic (aq-methanolic) media has been carried out using diverse organic protonated molecules, HL, in the reaction medium. Polyethers I and II with potassium form [K−-I 2] + and [K-II] + which due to their low charge density fails to stabilise the counter anion, X. Consequently, reactions X −+HL=X...HL −=HX+L −  HL L,(HL) n − take place in this order. In addition to some new complexes of the old type, viz. [K-I 2]Benz,2 BenzH and [K-II]NCS, BenzH (BenzH = benzoic acid and Benz its anion) new series of complexes (i) [K-II]L,HL (HL = 2-nitrophenol (OnpH), 2,4-dinitrophenol (DnpH), or 2,4,6-trinitrophenol (PicH)), (ii) [K-II]Pic, and (iii) [K-R]X, (HL) n (for R = I, HL = a molecule of OnpH or DnpH for X = Cl −, and BenzH for X = NCS −; and for R = II, HL = two molecules of OnpH when X = NCS −) have been isolated. With I and II, sodium forms cations [Na-I] + and [Na 2-II] +, which possess nearly double the charge density compared to analogous potassium cations, activation of only Cl − and PO 4 3− could be noticed that too towards strong proton donor, PicH; Pic − produced in situ, however, is not allowed to undergo homoconjugation by the high charge density cations. The bimetallic compound [Na 2-II]2Pic so obtained is also the first of its type.

Enthalpies of solution of some electrolytes having high-charge density cations in N,N-dimethylformamide

Enthalpies of solution were measured for GdCl3, MgC12, BaBr2, LiBr, and KI at various concentrations in anhydrous N,N-dimethylformamide at 25C, and the data extrapolated to infinite dilution to obtain standard enthalpies of solution. With the exception of GdCI3, the limiting slopes were in agreement with the Debye-Huckel limiting slopes. The two-stage enthalpy effect previously observed for MgC12 was not observed when trace amounts of hydrolyzed salt were absent.