Electrochemistry and Homogeneous Self-Exchange Kinetics of the Aqueous 12-Tungstoaluminate(5−/6−) Couple

The Influence of Spectator Cations on Solvent Reorganization Energy Is a Short-Range Effect

In this manuscript, we use classical molecular dynamics simulation to explore the origin of specific cation effects on the rates of bulk-phase aqueous electron transfer (ET) reactions. We consider 0.6 M solutions of Cl^- and a series of different cations: Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. We evaluate the collective electrostatic fluctuations that drive Marcus-like ET and find that they are essentially unaffected by changes in the cationic species. This finding implies that the structure making/breaking properties of various cations do not exert a significant influence on bulk-phase ET reactions. We evaluate the role of ion pairing in these specific cation effects and find, unsurprisingly, that model redox anions that are more highly charged tend to pair more effectively with spectator cations than their monovalent counterparts. We demonstrate that this ion pairing significantly affects local electrostatic fluctuations for the anionic redox species and thus conclude that ion pairing is one of the likely sources of rate-dependent cation effects in aqueous ET reactions.

Ion-pairing in polyoxometalate chemistry: impact of fully hydrated alkali metal cations on properties of the keggin [PW12O40]3- anion

The counterions of polyoxometalates (POMs) impact properties and applications of this growing class of inorganic clusters. Here, we used density functional theory (DFT) to elucidate the impact of fully hydrated alkali metal cations on the geometry, electronic structure, and chemical properties of the polyoxotungstate anion [PW₁₂O₄₀]^3-. The calculations show that the HOMO of the free anion [PW₁₂O₄₀]^3- is a linear combination of the 2p AOs of the bridging oxygens, and the first few LUMOs are the 5d orbitals of the tungsten atoms. The S₀→ S₁ electron excitation, near 3 eV, is associated with the O(2p) → W(5d) transition. Anion/cation complexation leads to formation of [PW₁₂O₄₀]^3-[M⁺(H₂O)₁₆]₃ ion-pair complexes, where with the increase of atomic number of M, the M⁺(H₂O)₁₆ cluster releases several water molecules and interacts strongly with the polyoxometalate anion. For M = Li, Na and K, [PW₁₂O₄₀]^3-[M⁺(H₂O)₁₆]₃ is characterized as a "hydrated" ion-pair complex. However, for M = Rb and Cs, it is a "contact" ion-pair complex, where the strong anion-cation interaction makes it a better electron acceptor than the "hydrated" ion-pair complexes. Remarkably, the electronic excitations in the visible part of the absorption spectrum of these complexes are predominantly solvent-to-POM charge transfer transitions (i.e. intermolecular CT). The ratio of the number of intermolecular charge transfer transitions to the number of O(2p)-to-W(5d) valence (i.e. intramolecular) transitions increases with the increasing atomic number of the alkali metals.

Experimental measurements of U24Py nanocluster behavior in aqueous solution

Uranyl peroxide nanoclusters may impact the mobility and partitioning of uranium at contaminated sites and could be used in the isolation of uranium during the reprocessing of nuclear waste. Their behavior in aqueous systems must be better understood to predict the environmental fate of uranyl peroxide nanoclusters and for their use in engineered systems. The aqueous stability of only one uranyl peroxide nanocluster, U60 (K16Li44[UO2(O2)OH]60), has been studied to date [Flynn, S. L., Szymanowski, J. E. S., Gao, Y., Liu, T., Burns, P. C., Fein, J. B.: Experimental measurements of U60 nanocluster stability in aqueous solution. Geochemica et Cosmochimica Acta 156, 94–105 (2015)]. In this study, we measured the aqueous stability of a second uranyl peroxide nanocluster, U24Py (Na30[(UO2)24(O2)24(HP2O7)6(H2P2O7)6]), in batch systems as a function of time, pH, and nanocluster concentration, and then compared the aqueous behavior of U24Py to U60 to determine whether the size and morphology differences result in differences in their aqueous behaviors. Systems containing U24Py nanoclusters took over 30 days to achieve steady-state concentrations of monomeric U, Na, and P, illustrating slower reaction kinetics than parallel U60 systems. Furthermore, U24Py exhibited lower stability in solution than U60, with an average of 72% of the total mass in each nanocluster suspension being associated with the U24Py nanocluster, whereas 97% was associated with the U60 nanocluster in parallel experiments [Flynn, S. L., Szymanowski, J. E. S., Gao, Y., Liu, T., Burns, P. C., Fein, J. B.: Experimental measurements of U60 nanocluster stability in aqueous solution. Geochemica et Cosmochimica Acta 156, 94–105 (2015)]. The measurements from the batch experiments were used to calculate ion activity product (IAP) values for the reaction between the U24Py nanocluster and its constituent monomeric aqueous species. The IAP values, calculated assuming the activity of the U24Py nanocluster is equal to its concentration in solution, exhibit a significantly lower nanocluster concentration dependence than those IAP values calculated assuming an activity of 1 for the nanocluster. The inclusion of a deprotonation reaction for U24Py minimizes the pH dependence of the calculated IAP values. The modeling results suggest that the U24Py nanocluster experiences sequential deprotonation. Taken together, the results indicate that the aqueous behavior of the U24Py nanocluster, like that of U60, is best described as that of an aqueous complex.

Recent advances in application of (27)Al NMR spectroscopy to materials science

Valuable information about the local environment of the aluminum nucleus can be obtained through (27)Al Nuclear Magnetic Resonance (NMR) parameters like the isotropic chemical shift, scalar and quadrupolar coupling constants, and relaxation rate. With nearly 250 scientific articles per year dealing with (27)Al NMR spectroscopy, this analytical tool has become popular because of the recent progress that has made the acquisition and interpretation of the NMR data much easier. The application of (27)Al NMR techniques to various classes of compounds, either in solution or solid-state, has been shown to be extremely informative concerning local structure and chemistry of aluminum in its various environments. The development of experimental methodologies combined with theoretical approaches and modeling has contributed to major advances in spectroscopic characterization especially in materials sciences where long-range periodicity and classical local NMR probes are lacking. In this review we will present an overview of results obtained by (27)Al NMR as well as the most relevant methodological developments over the last 25years, concerning particularly on progress in the application of liquid- and solid-state (27)Al NMR to the study of aluminum-based materials such as aluminum polyoxoanions, zeolites, aluminophosphates, and metal-organic-frameworks.

Computational characterization of the internal bonding and solvation structure for [Nb10O28]aq6-

The chemistry of polyoxometalate ions is richly explored, but mostly via synthesis and experimental studies. Less emphasis has been placed on developing a robust computational understanding of their aqueous dynamics. In this work, we utilize both a previously created force-field model and ab initio molecular dynamics to explore the solvation structure of [Nb10O28]aq(6-). We examine characteristic behaviors of both cluster-water interactions and intra-cluster bond strengths. We show that cluster-water interactions are dictated by electrostatic interactions, which in turn are dictated by the shape of the cluster; ultimately reflecting a quantitatification of the steric shielding to the cluster's oxygen sites. We also show that bond strengths within the cluster do not correlate with oxygen reactivities, lending credence to previous suggestions that oxygen exchange for decaniobate clusters occurs via intermediates that form by significant structural reorganizations.

Kinetics of reduction of aqueous hexaammineruthenium(III) ion at Pt and Au microelectrodes: electrolyte, temperature, and pressure effects

Rate constants kel obtained by impedance spectroscopy for the reduction of Ru(NH3)6(3+) at polycrystalline Pt and Au ultramicroelectrodes depend strongly on the identity and concentration of the anion present in the order CF3SO3(-) < Cl(-) < ClO4(-), but not on the cation of the supporting electrolyte (Na(+), K(+), H(+)). For Cl(-) as the sole anion present, kel is directly proportional to the total [Cl(-)], such that kel would be zero if Cl(-) were hypothetically absent, indicating that Cl(-) is directly involved in mediation of the Ru(NH3)6(3+/2+) electron transfer. For CF3SO3(-) as the sole counterion, the dependence of kel on the total [CF3SO3(-)] is not linear, possibly because blocking of the available electrode surface becomes dominant at high triflate concentrations. Volumes of activation ΔVel(⧧) for reduction of Ru(NH3)6(3+) at an electrode in presence of Cl(-) or CF3SO3(-) are much more negative than predictions based on theory (Swaddle, T. W. Chem. Rev.2005, 105, 2573) that has been successful with other electron transfer reactions but which does not take into account the involvement of the anions in the activation process. The strongly negative ΔVel(⧧) values probably reflect solvation increases peculiar to activation processes of Ru(III/II) am(m)ine complexes, possibly together with promotion of desorption of surface-blocking Cl(-) or CF3SO3(-) from electrodes by applied pressure. Frumkin corrections for Ru(NH3)6(3+) within the diffuse double layer would make ΔVel(⧧) even more negative than is observed, although the corrections would be small. The strongly negative ΔVel(⧧) values are inconsistent with reduction of Ru(NH3)6(3+) in direct contact with the metallic electrode surface, which would entail substantial dehydration of both the electrode and Ru(NH3)6(3+). Reduction of Ru(NH3)6(3+) can be regarded as taking place in hard contact with adsorbed water at the outer Helmholtz plane.

Evidence for concerted pathways in ion-pairing coupled electron transfers

Ion-pairing with electro-inactive metal ions may change drastically the thermodynamic and kinetic reactivity of electron transfer in chemical and biochemical processes. Besides the classical stepwise pathways (electron-transfer first, followed by ion-pairing or vice versa), ion-pairing may also occur concertedly with electron transfer. The latter pathway avoids high-energy intermediates but a key issue is that of the kinetic price to pay to benefit from this thermodynamic advantage. A model is proposed leading to activation/driving force relationships characterizing such concerted associative electron transfers for intermolecular and intramolecular homogeneous reactions and for electrochemical reactions. Contrary to previous assertions, the driving force of the reaction (defined as the opposite of the reaction standard free energy), as well as the intrinsic barrier, does not depend on the concentration of the ion-pairing agent, which simply plays the role of one of the reactants. Besides solvent and intramolecular reorganization, the energy of the bond being formed is the main component of the intrinsic barrier. Application of these considerations to reactions reported in recent literature illustrates how concerted ion-pairing electron-transfer reactions can be diagnosed and how competition between stepwise and concerted pathways can be analyzed. It provided the first experimental evidence of the viability of concerted ion-pairing electron-transfer reactions.

Electron transfer reactions of peroxydisulfate and fluoroxysulfate reactions with the cyanide complexes M(CN)n4- (M=Fe(II), Ru(II), Os(II), Mo(IV), and W(IV))

The stoichiometry and the kinetics of oxidation of the cyanide complexes M(CN)n4- (M = Fe(II), Ru(II), Os(II), Mo(IV), and W(IV)) by the peroxydisulfate ion, S2O8(2-), and by the much more strongly oxidizing fluoroxysulfate ion, SO4F-, were studied in aqueous solutions containing Li+. Reactions of S2O8(2-) with M(CN)n4- are known to be strongly catalyzed by Li+ and other alkali metal ions, and this applies also to the corresponding reactions of SO4F-. The primary reactions of S2O8(2-) and SO4F- have both been found to be one-electron processes in which the equally strong O-O and O-F bonds are broken. The primary reaction of S2O8(2-) consists of a single step yielding M(CN)n3-, SO4-, and SO42-, whereas the primary reaction of SO4F- comprises two parallel one-electron steps, one leading to M(CN)n3-, SO4-, and F- and the other yielding M(CN)n-1(2-), CN-, SO4- and F-. The relationship between the rate constants and the standard free energies of reaction for the Li+-catalyzed reactions of SO4F- and S2O8(2-) with M(CN)n(4-), and for the uncatalyzed reactions of S2O8(2-) with bipyridyl and phenanthroline complexes MLn2+ (M = Fe(II), Ru(II), and Os(II)) studied previously, suggests that the intrinsic barrier for all three sets of reactions is similar, i.e., unaffected by the Li+ catalysis, and that the electron transfer and the breakage of the O-O and O-F bonds are concerted processes.