Separated Ion Pairs Research Articles

Transient Ion-Pair Separations for Electrospray Mass Spectrometry.

We report a novel ion-pair chromatography (IPC) approach for liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS), where the eluent does not contain any ion-pairing reagent (IPR). The IPR is injected on the column, much like the sample, and moves down the column. Significant amounts of a high retention factor IPR is injected, resulting in a transient but reproducible regional coating that progresses along the column. The sample is injected after a brief interval. The sample components interact with the IPR coated region during their passage; the chosen eluent gradient elutes the analytes of interest into the mass spectrometer before the IPR. Following analyte elution, the gradient is steeply raised, the IPR is washed out, and the effluent is sent to waste via a diverter valve until it is fully removed. As the nature of the analyte retention continuously changes along the column and with time, we call this transient ion-pair separation (TIPS). As the IPR never enters the MS, TIPS addresses two major drawbacks of IPC for ESI-MS: it avoids both ion suppression and ion source contamination. The potential of the generic approach for other modes of separation is discussed. An illustrative separation of two small inorganic ions, iodate and nitrate, is demonstrated on a reverse phase column by a transient prior injection of hexadecyltrimethylammonium chloride as IPR.

Ion Pair Binding in the Solid-State with Ditopic Crown Ether Uranyl Salophen Receptors.

Two ditopic uranyl salophen receptors with benzo-15-crown-5 and benzo-18-crown-6 units (R(1) and R(2), respectively) have been synthesized from commercially available starting materials. Comprehensive studies on the solid-state ion pair complexation with various alkali and ammonium halides have been conducted. From the 19 obtained solid-state structures (6 structures with R(1), 13 structures with R(2)), three general interaction motifs I-III have been observed. Interaction motif I has a separated ion pair with the cation coordinated to the crown ether unit, and the anion or oxygen containing solvent molecule coordinated to the uranyl center. The interaction motif II manifests a polymeric structure with a contact ion pair between the uranyl-coordinated anion and cation coordinated in the crown ether in the adjacent receptor. Interaction motif III consists of a more general stacked packing structure of the receptors with or without ion pairs. From the obtained solid-state structures, the complex R(2)·NaI shows an interesting formation of infinite coordination polymeric structure where the crown ether complexed sodium cations and the O═U═O units of the adjacent receptors form a nearly linear 1-D chains. In the course of this work also the first solid-state structure of uranyl salophen acetate complex was obtained (R(2)·KAcO).

Biexponential photon antibunching: recombination kinetics within the Förster-cycle in DMSO.

Time-resolved experiments with pulsed-laser excitation are the standard approach to map the dynamic evolution of excited states, but ground-state kinetics remain hidden or require pump-dump-probe schemes. Here, we exploit the so-called photon antibunching, a purely quantum-optical effect related to single molecule detection to assess the rate constants for a chemical reaction in the electronic ground state. The measurement of the second-order correlation function g((2)), i.e. the evaluation of inter-photon arrival times, is applied to the reprotonation in a Förster-cycle. We find that the antibunching of three different photoacids in the aprotic solvent DMSO significantly differs from the behavior in water. The longer decay constant of the biexponential antibunching tl is linked to the bimolecular reprotonation kinetics of the fully separated ion-pair, independent of the acidic additives. The value of the corresponding bimolecular rate constant, kp = 4 × 10(9) M(-1) s(-1), indicates diffusion-controlled reprotonation. The analysis of tl also allows for the extraction of the separation yield of proton and the conjugated base after excitation and amounts to approximately 15%. The shorter time component ts is connected to the decay of the solvent-separated ion pair. The associated time constant for geminate reprotonation is approximately 3 ± 1 ns in agreement with independent tcspc experiments. These experiments verify that the transfer of quantum-optical experiments to problems in chemistry enables mechanistic conclusions which are hardly accessible by other methods.

Sandwich Lutetacyclopentadiene with the Coordination of Lithium to the Diene Unit: Synthesis, Structure, and Transformation

The sandwich lutetacyclopentadiene 2 can be obtained in high yield by the 1:1 reaction of the 1,4-dilithio-1,3-butadiene 1 with [Cp2LuCl]. Single-crystal X-ray diffraction of 2 reveals it as a contact ion pair with the coordination of lithium to the diene unit. This coordination mode is rare in metallacyclopentadiene chemistry. Further transformation of 2 with the inorganic salt KF and crown ethers leads to the formation of sandwich lutetacyclopentadienes 3 and 5 as separated ion pairs and of 4 as a one-dimensional coordination polymer.

Molecular dynamics simulations of K+–Cl− ion pair in polar mixtures of acetone and water: Preferential solvation and structural studies

Molecular dynamics simulation studies have been performed for a series of polar mixtures of acetone and water containing a K+–Cl− ion pair. We have obtained the potentials of mean force (PMFs) for 15 mixtures spanning the whole composition range from 0% to 100% acetone. The PMFs indicate the presence of a stable contact ion pair (CIP), a solvent assisted ion pair (SAIP) and a solvent separated ion pair (SSIP) in all the compositions with acetone mole fraction (xacetone)≤0.50. However, for xacetone>0.50 and xacetone≤0.90, PMFs suggest the presence of only a stable CIP and SAIP. For xacetone>0.90, PMFs show the existence of only a stable CIP. While the stability of CIPs in mixtures increases with xacetone, the reverse trend is observed for SAIPs. The determination of thermodynamic properties suggests that entropy favours the CIPs and SAIPs in all the mixtures. The analysis of radial distribution functions using Kirkwood–Buff (KB) integrals explains the preferential solvation of the ion pair. We observed that in all the mixtures, K+–Cl− ion pair is preferentially solvated by water. Running coordination numbers of solvent molecules around the ion pair and preferential binding parameters also support the above observations. The dynamical aspects have been explored by calculating self-diffusion constants and hydrogen bond dynamics.

The synthesis and structures of rare earth 2-fluorophenyl- and 2,3,4,5-tetrafluorophenyl-N,N′-bis(aryl)formamidinate complexes

Redox transmetallation/protolysis (RTP) reactions between free rare-earth metals (La, Nd, or Yb), Hg(C6F5)2, and FFormH (FFormH=N,N′-bis(2-fluorophenyl)formamidine) in thf afforded four rare-earth complexes: trivalent [La(FForm)3(thf)2]·thf, [Nd(FForm)3(thf)x], [Yb(FForm)3(thf)] (when a slight excess of Yb was used), and divalent [Yb(FForm)2(thf)2] (when a large excess of Yb was used). With the exception of [Yb(FForm)3(thf)], the complexes did not readily crystallise from thf. However, crystallisation from either dme (La, Yb) or diglyme (Nd), produced single crystals of [La(FForm)3(dme)], [Yb(FForm)2(dme)2], and [Nd(FForm)3(diglyme)], respectively. A Nd complex of N,N′-bis(2,3,4,5-tetrafluorophenyl)formamidine (TFFormH): [Nd(TFForm)3(dme)], was also prepared by a similar RTP reaction, and was crystallised from dme. Heating the previously reported divalent TFForm complex: [Yb(TFForm)2(thf)3], in either PhMe or diglyme, separated out trace amounts of the trivalent complex [Yb(TFForm)3(thf)2] from PhMe, or the mixed oxidation state, solvent separated ion pair (SSIP) [Yb(TFForm)(diglyme)2][Yb(TFForm)4] from diglyme. The SSIP complex is the first crystallographically characterised trivalent ytterbium complex coordinating four discrete amidinate ligands.

Lithium Di- and trimethyl dimolybdenum(II) complexes with Mo-Mo quadruple bonds and bridging methyl groups.

New dimolybdenum complexes of composition [Mo2{μ-Me}2Li(S)}(μ-X)(μ-N^N)2] (3a-3c), where S = THF or Et2O and N^N represents a bidentate aminopyridinate or amidinate ligand that bridges the quadruply bonded molybdenum atoms, were prepared from the reaction of the appropriate [Mo2{μ-O2CMe}2(μ-N^N)2] precursors and LiMe. For complex 3a, X = MeCO2, while in 3b and 3c, X = Me. Solution NMR studies in C6D6 solvent support formulation of the complexes as contact ion pairs with weak agostic Mo-CH3···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of 3a and 3b. Samples of 3c enriched in (13)C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C6D6 and THF-d8 solvents. Crystallization of 3c from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li(THF)4] [Mo2(Me)2(μ-Me){μ-HC(NDipp)2}2] (4c), where Dipp stands for 2,6-iPr2C6H3. A computational analysis of the Mo2(μ-Me)2Li core of complexes 3a and 3b has been developed, which is consistent with a small but non-negligible electron-density sharing between the C and Li atoms of the mainly ionic CH3···Li interactions.

Biomimetic [2Fe-2S] clusters with extensively delocalized mixed-valence iron centers.

A complete series of biomimetic [2Fe-2S] clusters, [(L(Dep) Fe)2 (μ-S)2 ] (3, L(Dep) =CH[CMeN(2,6-Et2 C6 H3 )]2 ), [(L(Dep) Fe)2 (μ-S)2 K] (4), [(L(Dep) Fe)2 (μ-S)2 ][Bu4 N] (5, Bu=n-butyl), and [(L(Dep) Fe)2 (μ-S)2 K2 ] (6), could be synthesized and characterized. The all-ferric [2Fe-2S] cluster 3 is readily accessible through the reaction of [(L(Dep) Fe)2 (μ-H)2 ] (2) with elemental sulfur. The chemical reduction of 3 with one molar equivalent of elemental potassium affords the contact ion pair K(+) [2Fe-2S](-) (4) as a one-dimensional coordination polymer, which in turn reacts with [Bu4 N]Cl to afford the separate ion pair [Bu4 N](+) [2Fe-2S](-) (5). Further reduction of 4 with potassium furnishes the super-reduced all-ferrous [2Fe-2S] cluster 6. Remarkably, complexes 4 and 5 are [2Fe-2S] clusters with extensively delocalized Fe(2+) Fe(3+) pairs as evidenced by (57) Fe Mössbauer, X-ray absorption and emission spectroscopy (XAS, XES) and in accordance with DFT calculations.

AIMD Results for a Concentrated Solution of Tetra-ethylammonium Tetra-fluoroborate in Propylene Carbonate

Ab initio molecular dynamics (AIMD) calculations are implemented for a concentrated solution of tetra-ethylammonium tetra-fluoroborate in propylene carbonate (TEABF4/PC), which is of interest in electrochemical double-layer capacitors. AIMD calculations predict more sharply structured ion-ion center partial radial distributions than either GAFF or OPLS-aa force-fields in classic FFMD calculations. This may be due to AIMD refinement of van der Waals interactions which drive ion-pairing and phase separation of TEABF4 from PC at this elevated concentration. Quantifying the charge transfer of BF4 - and N(CH2CH3)4 + ions to neighboring solution species, the (Bader) partial charges of BF4 - and N(CH2CH3)4 + ions in PC solution differ from the formal charges by being smaller in magnitude by about 5%.

Measurement of catecholamines in rat and mini-pig plasma and urine by liquid chromatography-tandem mass spectrometry coupled with solid phase extraction.

A tandem mass spectrometry method combined with an ion-pair chromatographic separation after weak cation exchange solid phase sample extraction for epinephrine (E), norepinephrine (NE) and dopamine (DA) has been developed. Two surrogate matrixes for plasma and urine as well as stable isotope labeled internal standards were utilized for quantitation. The observed dynamic range of E, NE and DA was 0.025-100ng/ml for plasma, and 0.25-1000ng/ml for urine with a r(2) regression coefficient >0.99. Extraction recoveries were greater than 60% and the lower limit of quantitation was 25pg/ml for all three analytes in plasma. This method provided excellent sensitivity and selectivity for use with small sample volumes (≤25uL), enabling high-throughput pharmacodynamic animal model development and screening of adverse effects.

High-order Cu(II) chloro-complexes in LiCl brines: Insights from density function theory and molecular dynamics

Cu(II) complexation by chloride is relevant to the transport of copper in near-surface geologic environments, yet the existence of high-order Cu(II) chloro-complexes still remains in dispute. In this study, the structure characteristics and stabilities of [CuClx]2−xaq (x=3, 4, 5) complexes have been investigated using density functional theory (DFT) methods and molecular dynamics (MD) simulations. [CuCl3]− and [CuCl4]2− species can both be tracked, while the [CuCl5]3−aq complex cannot be recorded during MD simulations of trace Cu2+ in chloride-rich brines. DFT calculations indicate that contact ion pair (CIP) conformers of [CuCl3]− species are less stable than its solvent separated ion pair (SSIP) conformers, in which one Cl− stays in the second coordination sphere of the centered Cu2+. MD simulations also reveal that the SSIP conformer is apt to appear in the aqueous solution than its CIP conformer. It seems that the third Cl− is more likely in the second coordination shell of center Cu2+ in [CuCl3]−. Meanwhile, the characteristic peak around 385nm resolved in UV–Vis spectra experiments, which was attributed to the [CuCl3]− complex, could also be resulted from some SSIP structures of the [CuCl3]− complex. In MD simulations, the complex [CuCl4]2−aq is found more stable than [CuCl3]−aq. The surrounding water molecules around [CuClx]2−xaq (x=3, 4) enhance their stabilities in Cl− brines, especially for [CuCl4]2−aq. The hydration shell of [CuCl4]2−aq species is more intact than that of [CuCl3]2−aq, and the residence time of a water molecule in the second coordination sphere of Cu ion in the [CuCl4]2−aq complex is also obviously longer than that of [CuCl3]−aq. The [CuCl4]2−aq complex can even be recorded in the less concentrated (6.33m)Cl− solution, while the [CuCl3]−aq complex is tracked only in the 16.32mCl− brine. Meanwhile, the possibilities of [CuCl3]−aq and [CuCl4]2−aq complexes found in 16.32mCl− solutions both decrease with increasing temperature, [CuCl3]−aq cannot be recorded during our simulations at 373K and 423K, and only few [CuCl4]2−aq at 423K.

Quantum chemical study of the stereoselectivity of nucleophilic addition of 2-methylcyclohexanone to phenylacetylene

100°C, 1 h 31+G* approach. Superbasic environment was described in the context of model [3] where the super basic center is represented by the KOH · DMSO (3) complex. The solvent molecule included in the explicit form provides the description of specific solva tion effects, which appear in the formation of K+HO– solvent separated ion pair, the nonspecific effect of the other solvate environment is taken into account in terms of the polarizable continuum model (PCM) [4]. Figure 1 represents the schematic profile of vinylation reactions at the 2 and 6 positions of compound 1. The acidity of aliphatic and alicyclic ketones in dimethyl sulfoxide is rather high (pKa ≈ 23–25 [5]) and the reaction of compound 1 with base 3 leads to formation of weakly bound prereaction complexes 4 and next 2 and 6 methylcyclohexenenolate ions 5a and 5b bound to cationic center:

Ion-pair chromatography coupled to inductively coupled plasma-mass spectrometry (IPC-ICP-MS) as a method for thiomolybdate speciation in natural waters.

Molybdenum precipitates preferentially under reducing conditions; therefore, its occurrence in sediment records is used as an indicator of paleoredox conditions. Although thiomolybdates (MoO4-xSx(2-) with x = 1-4) supposedly are necessary intermediates in the process of molybdenum precipitation under anoxic conditions, there is no information about their abundance in natural environments, because of a lack of element-specific methods with sufficiently low detection limits. Here, we optimized ion-pair chromatographic separation for coupling to an inductively coupled plasma-mass spectrometry detector (IPC-ICP-MS). 2-Propanol (10%-25% gradient) replaced the previously used acetonitrile (25%-75%) as the solvent, to reduce the carbon load into the plasma. In synthetic solutions, formation of thiomolybdates was found to occur spontaneously in the presence of excess sulfide and the degree of thiolation was highest at pH 7. Excess hydroxyl led to a transformation of thiomolybdates to molybdate. Under acidic to neutral conditions, precipitation of molybdenum and hydrolysis of tetrathiomolybdate were observed. Flash-freezing was found to be suitable to stabilize tetrathiomolybdate, with <4% transformation over more than two months. High ionic strengths matrices (>2 mM) negatively affected the detection of molybdate, which eluted mainly in the dead volume, but had no negative effect on higher thiolated molybdates. Detection limits were ∼10 nM. With the newly developed IPC-ICP-MS method, thiomolybdates were found to form spontaneously in euxinic marine waters after adding a molybdate spike and occur naturally in sulfidic geothermal waters.

Ion Pair Structures in Aqueous KSCN Solution: Classical and Quantum Mechanical/Molecular Mechanical Molecular Dynamics Simulation Study#

Ion pair formation and dissociation are fundamental processes in electrolyte solutions so that understanding thermodynamic stabilities and dynamic aspects of ion pairs is of great importance. The structures of various ion pair states are here studied by carrying out classical and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations of aqueous KSCN solutions. A few different solvent potential models are considered and the resulting ion pair structures are directly compared. In particular, when KSCN is treated quantum mechanically and effective fragment potential (EFP) model for water is used, we found two stable forms of ion pairs that can be considered as contact ion pair (CIP) and solvent separated ion pair (SSIP), where the interionic distances between K+ and SCN − are found to be 4.0 and 5.5 Å, respectively. QM/EFP‐MD further indicates that the nitrogen side of SCN − is preferentially interacting with K+ in CIP. However, the corresponding CIP appears at 3.0 Å interionic distance in fully classical MD simulation at a higher KSCN concentration. Nonetheless, both QM/EFP and classical MD simulation results show that the CIP state appears to be more stable than SSIP. From the site‐site radial distribution functions (RDF) calculated from the QM/MM and classical MD trajectories, detailed ion pair structures and surrounding solvent configurations are further elucidated.

Preparation and characterization of biologically active novel copper ion-pairs of nonsteroidal anti-inflammatory drugs (NSAIDs)

The present work is concerned with preparation, separation and structure elucidation of solid ion-pairs of nonsteroidal anti-inflammatory drugs in reaction to copper(II) ion. The prepared solid ion-pairs were investigated by different analytical techniques, such as FT-IR, mass and thermogravimetric and differential thermal analyses, in addition to elemental analysis. The general formulae of the prepared ion-pairs were determined. Moreover, the formulae of prepared ion-pairs were proposed and structurally identified. The biological activities of the separated solid ion-pairs toward some kinds of insect species in flour mills in comparison with their drugs were studied, and they were found to be biologically active more than their parent drugs. The present work involved the preparation, separation and structures elucidation of Cu(II) ion-pairs of nonsteroidal anti-inflammatory drugs (NSAIDs). Their proposed structures (a–d) are given.

Thorium Triamidoamine Complexes: Synthesis of an Unusual Dinuclear Tuck-in–Tuck-over Thorium Metallacycle Featuring the Longest Known Thorium−σ-Alkyl Bond

We report the synthesis and characterization of a range of thorium(IV)–halide, −amide, and −alkyl complexes supported by a sterically demanding triamidoamine ligand. Reaction of [ThCl4(THF)3.5] with [Li3(TrenDMBS)(THF)3] (TrenDMBS = N(CH2CH2NSiMe2But)3) gave [Th(TrenDMBS)(Cl)(THF)] (1), which was converted to [Th(TrenDMBS)(I)] (2), with concomitant elimination of Me3SiCl, by treatment with Me3SiI. Treatment of 2 with [LiNEt2] afforded [Th(TrenDMBS)(NEt2)] (3), or treatment with [PhCH2K] produced the dimeric tuck-in–tuck-over alkyl complex [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-μ-CH2)}]2 (4), which features the longest reported Th−σ-alkyl bond distance of 2.875(9) Å. Reaction of 4 with [Et3NH][BPh4] to give the putative separated ion pair complex [Th(TrenDMBS)][BPh4] resulted in an intractable product mixture. In order to furnish a greater understanding of the latter reaction, the solvent-separated ion pair complex [U(TrenDMBS)(NCMe)2][BPh4] (5) was prepared from the monomeric uranium analogue of 4, [U{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-μ-CH2)}] (I), to provide a related model complex and to examine potential routes to useful TrenDMBS–actinide precursors. The nascent reactivity of 4 is suggested by the isolation of a small quantity of the oxo-insertion product [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeButCH2-μ-O)}]2 (6) when 4 is stored in diethyl ether. The complexes have been variously characterized by single-crystal X-ray diffraction, NMR spectroscopy, FTIR spectroscopy, Evans method solution magnetic moment determination, and CHN elemental analyses.

Synthesis and characterisation of halide, separated ion pair, and hydride cyclopentadienyl iron bis(diphenylphosphino)ethane derivatives.

Treatment of anhydrous FeX2 (X = Cl, Br, I) with one equivalent of bis(diphenylphosphino)ethane (dppe) in refluxing THF afforded analytically pure white (X = Cl), light green (X = Br), and yellow (X = I) [FeX2(dppe)]n (X = Cl, ; Br, ; I, ). Complexes are excellent synthons from which to prepare a range of cyclopentadienyl derivatives. Specifically, treatment of with alkali metal salts of C5H5 (Cp, series ), C5Me5 (Cp*, series ), C5H4SiMe3 (Cp', series ), C5H3(SiMe3)2 (Cp'', series ), and C5H3(Bu(t))2 (Cp(tt), series ) afforded [Fe(Cp(†))(Cl)(dppe)] , [Fe(Cp(†))(Br)(dppe)] , and [Fe(Cp(†))(I)(dppe)] (Cp(†) = Cp, Cp*, Cp', Cp'', or Cp(tt)). Dissolution of in acetonitrile, or treatment of with Me3SiI in acetonitrile (no halide exchange reactions were observed in other solvents) afforded the separated ion pair complexes [Fe(Cp(†))(NCMe)(dppe)][I] . Attempts to reduce , , and with a variety of reductants (Li-Cs, KC8, Na/Hg) were unsuccessful. Treatment of with LiAlH4 gave the hydride derivatives [Fe(Cp(†))(H)(dppe)] . This report provides a systematic account of reliable methods of preparing these complexes which may find utility in molecular wire and metal-metal bond chemistries. The complexes reported herein have been characterised by X-ray diffraction, NMR, IR, UV/Vis, and Mössbauer spectroscopies, cyclic voltammetry, density functional theory calculations, and elemental analyses, which have enabled us to elucidate the electronic structure of the complexes and probe the variation of iron redox properties as a function of varying the cyclopentadienyl or halide ligand.

Charge separation in supramolecular ferrocene(s)-zinc porphyrin-fullerene triads: A femtosecond transient absorption study

Mechanistic aspects of photoinduced charge separation in supramolecular triads, constructed using covalently linked zinc porphyrin-ferrocene(s) dyads — self-assembled via axial coordination to either pyridine or phenylimidazole appended fulleropyrrolidine ( Fc x- ZnP : PyC 60 or Fc x- ZnP : ImC 60; x = 1 or 2), has been investigated using femtosecond pump-probe transient spectroscopy. Upon photoexcitation of ZnP , charge separation from ferrocene to 1 ZnP * to yield the initial Fc +- ZnP •-: C 60 radical ion-pair or charge separation from 1 ZnP * to C 60 to yield the initial Fc - ZnP •+: C 60•- radical ion-pair, depending upon the ferrocene-zinc porphyrin intermolecular distance, was observed. These radical ion-pairs resulted in the formation of ultimate distantly separated Fc +- ZnP : C 60•- radical ion-pairs either via an electron migration (former case) or hole shift (latter case) process. Kinetics of charge separation as a function of spacer connecting the ferrocene and porphyrin, and spacer between the porphyrin and fullerene is reported. In agreement with our earlier study (J. Phys. Chem. B 2004; 108: 11333–11343), the Fc +- ZnP : C 60•- radical ion-pair persisted beyond the monitoring time window of our instrument, suggesting charge stabilization in these supramolecular triads.

A Study of the Influence of Lithium Salt Anions on Oxygen Reduction Reactions in Li-Air Batteries

The influence of lithium salts on O2 reduction reactions (ORR) in 1, 2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) has been investigated. Microelectrode studies in a series of tetrabutylammonium salt (TBA salt)/DME-based electrolytes showed that O2 solubility and diffusion coefficient are not significantly affected by the electrolyte anion. The ORR voltammograms on microelectrodes in these electrolytes exhibited steady-state limiting current behavior. In contrast, peak-shaped voltammograms were observed in Li+-conducting electrolytes suggesting a reduction of the effective electrode area by passivating ORR products as well as migration-diffusion control of the reactants at the microelectrode. FT-IR spectra have revealed that Li+ ions are solvated to form solvent separated ion pairs of the type Li+(DME)nPF6− and Li+(TEGDME)PF6− in LiPF6-based electrolytes. On the other hand, the contact ion pairs (DME)mLi+(CF3SO3−) and(TEGDME)Li+(CF3SO3−) appear to form in LiSO3CF3-containing electrolytes. In the LiSO3CF3–based electrolytes the initial ORR product, superoxide (O2−), is stabilized in solution by forming [(DME)m-1(O2−)]Li+(CF3SO3−) and [(TEGDME)(O2−)]Li+(CF3SO3−) complexes. These soluble superoxide complexes are able to diffuse away from the electrode surface reaction sites to the bulk electrolyte in the electrode pores where they decompose to form Li2O2. This explains the higher capacity obtained in Li/O2 cells utilizing LiCF3SO3/TEGDME electrolytes.

Multisite ion model in concentrated solutions of divalent cations (MgCl2 and CaCl2): osmotic pressure calculations.

Accurate force field parameters for ions are essential for meaningful simulation studies of proteins and nucleic acids. Currently accepted models of ions, especially for divalent ions, do not necessarily reproduce the right physiological behavior of Ca2+ and Mg2+ ions. Saxena and Sept (J. Chem. Theor. Comput.2013, 9, 3538–3542) described a model, called the multisite-ion model, where instead of treating the ions as an isolated sphere, the charge was split into multiple sites with partial charge. This model provided accurate inner shell coordination of the ion with biomolecules and predicted better free energies for proteins and nucleic acids. Here, we expand and refine the multisite model to describe the behavior of divalent ions in concentrated MgCl2 and CaCl2 electrolyte solutions, eliminating the unusual ion–ion pairing and clustering of ions which occurred in the original model. We calibrate and improve the parameters of the multisite model by matching the osmotic pressure of concentrated solutions of MgCl2 to the experimental values and then use these parameters to test the behavior of CaCl2 solutions. We find that the concentrated solutions of both divalent ions exhibit the experimentally observed behavior with correct osmotic pressure, the presence of solvent separated ion pairs instead of direct ion pairs, and no aggregation of ions. The improved multisite model for (Mg2+ and Ca2+) can be used in classical simulations of biomolecules at physiologically relevant salt concentrations.