Formation Of Solvent-separated Ion Pairs Research Articles

Electrical Conductance And Theoretical Study Of Glutamic Acid In Different Solvents at 310.16k

The electrical conductivities of glutamic acid in the water, methanol, and ethanol are measured at 310.16 K and the conductivity parameters: Association constant (KA), equivalent conductance at infinite dilution (Λo) and the distance parameter (R). (Λo, KA, and R ) are calculated. Values of Λo are found to be in the order: water > methanol > ethanol , but the order is reversed for the amount of KA and R. This indicates the increase of ion-solvent interaction and formation of solvent separated ion-pair in the above order. The main interest is to find an accurate yet efficient solvation model for semiempirical quantum-mechanical and Density Function Theory (DFT) methods applicable to amino acids (glutamic acid) in the context of computer-aided conductivity studying. It was reparametrization of the Conductor like Screening Model (COSMO) and Conductor-like Polarizable Continuum Model (CPCM) CPCM solvent model for Parameterization method three (PM3), Hartree- Fock (HF) & DFT calculation in Gaussian interface, version 16.0 which used to calculate many descriptors of the glutamic molecule and study the relationship between these descriptors and the association constant (KA). Molecular volume (MV), Connolly parameters, and the entropy were showed the same result corresponding with the experimental parameters..

Electrolyte Dependency on Ca2+ Insertion and Extraction Properties of V2O5

The influence of the electrolyte structure on the Ca2+ ion insertion and extraction properties of V2O5 was studied by changing the electrolyte concentration or solvent. The electrolyte structure was analyzed by Raman spectroscopy. The most significant changes were found in the molar ratio of the contact ion pair (CIP) in the total ionic species. Among the various electrolytes, 0.3 M Ca(TFSI)2 dissolved in ethylene carbonate and dimethyl carbonate (0.3 M EC:DMC) and 0.5 M Ca(TFSI)2 dissolved in triglyme (0.5 M G3) have relatively small molar ratios of CIPs. The electrochemical performance was strongly related to the molar ratio of the CIPs. A high coulombic efficiency and high capacity were observed when using 0.3 M EC–DMC. Moreover, 0.5 M G3 showed the highest capacity despite its low coulombic efficiency. This could be related to the formation of solvent-separated ion pairs (SSIPs) due to the low polarity of G3 and its solvation form that encapsulates Ca2+. SSIPs had a reductively unstable character as that of the CIPs. Surface analysis revealed that the thinner the surface film produced, the lower the CIP content. This was deemed responsible for the rate performance enhancements, given the potential electrochemical instability of the Ca-containing CIPs.

Impact of Solvation Energetics on the Oxygen Reduction Reaction in Alkali-Metal/Oxygen Batteries

Alkali-metal/Oxygen batteries are a potential high-energy alternative to Li-ion batteries1. In these systems, the critical performance bottleneck is the oxygen reduction reaction (ORR) which in turn depends on the non-aqueous electrolyte that mediates ion and oxygen transport2. Herein, we show that a multifold increase of the practical discharge power of these batteries can be achieved by tuning the tripartite interactions between the salt cation, anion and the solvent in the electrolyte3. A single descriptor of the solvation energetics, the solvent reorganization energy (λ), is shown to capture the influence of electrolyte composition on both reactant transport (DO2 ) and reaction kinetics (k) at the electrode and found to be broadly applicable to alkali-metal/oxygen (i.e. Li-, Na- and K-O2) batteries. Increasing cation size (from Li+ to K+) doubled k/λ, indicating an increased sensitivity of k to the choice of anion, while variations in DO2 were minimal over this cation size range. These multifarious effects were captured in the Electrochemical Thiele Modulus incorporating Marcus-Hush kinetics and guidelines are presented for the choice of electrolyte solvent and salt when designing systems for high discharge rates. In a model symmetric K-O2 cell4, both the formation of solvent-separated ion-pairs and the existence of unsolvated anions reduced overpotential losses by 200mV compared to electrolytes with partial anion solvation, successfully demonstrating the rational, ab-initio design of electrolytes for high-power alkali-metal/Oxygen batteries.

Tuning anion solvation energetics enhances potassium–oxygen battery performance

The oxygen reduction reaction (ORR) is a critical reaction in secondary batteries based on alkali metal chemistries. The nonaqueous electrolyte mediates ion and oxygen transport and determines the heterogeneous charge transfer rates by controlling the nature and degree of solvation. This study shows that the solvent reorganization energy (λ) correlates well with the oxygen diffusion coefficient [Formula: see text] and with the ORR rate constant [Formula: see text] in nonaqueous Li-, Na-, and K-O2 cells, thereby elucidating the impact of variations in the solvation shell on the ORR. Increasing cation size (from Li+ to K+) doubled [Formula: see text], indicating an increased sensitivity of k to the choice of anion, while variations in [Formula: see text]were minimal over this cation size range. At the level of a symmetric K-O2 cell, both the formation of solvent-separated ion pairs [K+-(DMSO)n-ClO4- + (DMSO)m-ClO4-] and the anions being unsolvated (in case of PF6-) lowered ORR activation barriers with a 200-mV lower overpotential for the PF6- and ClO4- electrolytes compared with OTf- and TFSI- electrolytes with partial anion solvation [predominantly K+-(DMSO)n-OTf-]. Balancing transport and kinetic requirements, KPF6 in DMSO is identified as a promising electrolyte for K-O2 batteries.

Ion pairing in protic ionic liquids probed by far-infrared spectroscopy: effects of solvent polarity and temperature.

The cation-anion and cation-solvent interactions in solutions of the protic ionic liquid (PIL) [Et3NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent-separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion-pair formation in trialkylammonium-containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids.

Cooperative hydration of carboxylate groups with alkali cations

We study the orientational dynamics of water molecules in solutions of formate salts using femtosecond mid-infrared spectroscopy. We observe that combining the formate ion with small cations like Na(+) or Li(+) leads to a cooperative effect on the water dynamics. This observation points at the formation of solvent-separated ion pairs.

Mechanism of Anionic Dearomatizing Reactions of Diphenylphosphinamide Derivatives: A Theoretical and Experimental Study

The mechanism of the anionic dearomatisation of phosphinamide derivatives has been investigated both theoretically and experimentally. The potential-energy surface of model reactions was studied at the Becke3LYP/6-31+G* level of theory, and according to this study, a pre-reactive complex is formed between the alkyllithium and the phosphinamide. This complex evolves preferentially through NC(alpha)-metalation of the phosphinamide. The intramolecular nucleophilic addition of the carbanion to the ortho position of the aromatic ring leads to the dearomatised products, in a reaction that has been shown to be under thermodynamic control. Coordinating co-solvents, such as hexamethyl phosphoramide (HMPA) or N,N'-dimethyl-N,N'-propylene urea (DMPU), appear to influence the reaction by favouring the formation of solvent-separated ion pairs. The cyclisation reaction of allylphosphinamide derivatives was also studied. It was found that both the alpha- and gamma-attack of the allyl anion can take place, however the formation of the seven-membered ring products derived from the gamma-attack are clearly favoured.

Formation of Solvent-Separated Ion Pairs in Calix[4]arene Possessing Two Ester Groups: Molecular Tweezers

Binding constants (Ka) of calix[4]arene 1,3-alternate conformer 2 bearing two ester groups for alkali picrates have been determined in tetrahydrofuran. It has been established that (i) 2 forms the 1:1 complexes with alkali metal cations, (ii) the M+-2 complexes are not considerably solvent-separated because of the “weak encapsulation effect” for two ester groups of 2, however the Ka values for 2 are generally as large as those for 1 having four ester groups. Interestingly, the largest Ka was attained for K+ and 2 complexes. This is the first comparison for the recognition capacity between two ester groups and four ester groups in calix[4]arene ester complexes.

Intermolecular Interactions in Lactone-Based Electrolytes

The viscosities (η), conductivities (σ), and enthalpies of dissolution of (LiTF) and (LiTFSI) are measured in γ-butyrolactone (BL), γ-valerolactone (VL), and mixtures of ethylene carbonate (EC) with tetrahydrofuran (THF) and diglyme (DG): EC:THF (20:80, in moles) and EC: DG (45:55, in moles). From the variations of η with the salt concentration, the B and D coefficients in the extended Jones-Dole equation are determined at different temperatures. The concentration dependence of the conductivity is analyzed on the basis of a model involving (i) a chemical equilibrium between free ions and ion pairs, (ii) the use of the Walden product to correct molar conductivities for viscosity variations, and (iii) the calculation of ion activity coefficients by the cube root law derived from the quasi-lattice model. According to this model, the limiting molar conductivities and ion pair dissociation constants are inferred. The solvation enthalpies of the salts are deduced from the heat of dissolution of LiTF and LiTFSI in BL, VL, and the EC:DG mixture. The formation of solvent-separated ion pairs in lactones and contact ion pairs in the EC mixture is deduced from the analysis of the experimental data as the result of a competition between ion-ion and ion-solvent interactions. The dissociation coefficient of ion pairs determined by Raman spectroscopy agrees well with those deduced from conductivity measurements. From the variation of the area of the band at 676 cm−1 with salt concentration, a coordination number of 4 molecules is found for the solvation of the by BL molecules. © 2002 The Electrochemical Society. All rights reserved.

Hydroxamic Acids as Weak Base Indicators: Protonation in Strong Acid Media

The protonation equilibria of N-phenylbenzohydroxamic, benzohydroxamic, salicylhydroxamic, and N-p-tolylcinnamohydroxamic acids have been studied at 25 degrees C in concentrated sulfuric, hydrochloric, and perchloric acid media; the UV-vis spectral measurements were analyzed using the Hammett equation and the Bunnett-Olsen and excess acidity methods. The medium effects observed in the UV spectral curves were corrected with the Cox-Yates and vector analysis methods. The H(A) acidity function based on benzamides provided the best results. The range of variation of the solvation coefficient m is similar to that of amides, this indicating similar solvation requirements for amides and hydroxamic acids. For the same substrate, the observed variations of pK(BH)(+) with the mineral acid used was justified by formation of solvent-separated ion pairs; for the same mineral acid, the observed changes in pK(BH)(+) can be explained by the solvation of BH(+). The change of the pK(BH)(+) values was in reasonably good agreement with the sequence of the catalytic efficiency of the mineral acids used, HCl > H(2)SO(4) > HClO(4).

Regioselectivity of Addition of Organolithium Reagents to Enones: The Role of HMPA.

Regioselectivity of Addition of Organolithium Reagents to Enones: The Role of HMPA.

Photo-induced electron transfer in polychromophoric systems

Abstract

Solvent Effect on Hydrogen Bonding Interaction between 5,6,7,8-Tetrahydro-2-naphthol in the Ground and Excited States and Triethylamine

Abstract In the ground state 5,6,7,8-tetrahydro-2-naphthol (ThOH) forms with triethylamine (TEA) simple hydrogen bond, hydrogen-bonded ion pair or solvent-separated ion pair complex depending on the polarity of solvents. Fluorescence emission of ThOH is quenched by TEA in all solvents. The quenching rate constant kq is large in weakly polar solvents than in highly polar solvents and there is no evidence of formation of solvent-separated ion pair in latter solvents. It is suggested that restricted electron delocalization in excited ThOH molecule lowers the tendency of deprotonation of the OH group. The likely pathway of energy depletion is through intermediate singlet contact CT exciplex of the type A−D+ between ThOH and TEA.

Formation of Solvent-Separated Ion Pairs in Calixarene Ester–Alkali Picrate Complexes

Abstract Association constants (Ka) of calix[n]arene esters (1n: n=4, 6, 8) for alkali picrates have been determined in tetrahydrofuran and compared with the 1n-induced bathochromic shifts of the absorption maxima of alkali picrates. It has been established that (1) 1n’s mostly form the 1 : 1 complexes with alkali metal cations, (2) the Ka values for 1n are generally smaller than those for crown ethers, and (3) the certain M+–1n complexes are considerably solvent-separated because of the “encapsulation effect” of the calixarene cavity. This is the first example for the systematic investigation on the relation between Ka and the geometry of the calixarene ester complexes.

Photoreactions of fluorenone with electron-rich alkenes: radical ion pair formation in polar and non-polar solvents

Flash photolysis studies showed that triplet fluorenone reacts with electron-rich olefins like tetramethyl-1,3-dioxole and tetraethoxyethene to give radical ions and radical ion pairs. The fraction of contact ion pairs and solvent-separated ion pairs was determined in various solvents. Ion pair formation was detected even in non-polar solvents like benzene and cyclohexane, although the formation of solvent-separated ion pairs is endothermic according to Weller and Mataga. The application of the special salt effect confirms our observations of an equilibrium between contact ion pairs and solvent-separated ion pairs. The same technique was used to show that excited fluorenone reacts with triethylamine to give both solvent-separated ion pairs and contact ion pairs in polar solvents or only contact ion pairs in non-polar solvents.

Conductivities of 1:1 electrolytes in mixed aprotic solvents—II. dimethoxyethane mixtures with propylene carbonate and 4-butyrolactone

The conductivities of five 1:1 electrolytes have been measured in mixtures of dimethoxyethane with propylene carbonate and 4-butyrolactone. As in previous studies involving mixed ethereal solvents, a maximum in the molar conductances is found for a specific solvent mixture. From consideration of single ion mobilities, Stokes' radii, and the Bjerrum treatment of ion association, we conclude that while small cations are highly coordinated by dimethoxyethane which promotes the formation of solvent-separated ion pairs, a significant number of contact ion pairs exist. Conductivity data for tetraalkylammonium chlorides suggest that these salts form both ion pairs and higher (neutral) aggregates such as a quadrupoles.

Pulse radiolysis of solutions of trans-stilbene. Radical-anions and ion-pairs in tetrahydrofuran

On pulse radiolysis of solutions of trans-stilbene (t-St) in THF the radical-anion of t-St is formed by reaction of e–s with t-St [reaction (5)] with k5=(1.16 ± 0.03)× 1011 dm3 mol–1 s–1. The transient absorption spectrum observed with λmax at 500 and 720 nm is attributed to the unassociated St˙–. The subsequent decay of the radical-anion is accounted for by reaction with the counter-cation of THF formed on radiolysis and with radiolytically generated radicals; rate constants for these processes are estimated. Addition of sodium tetrahydridoaluminate (NAH) results in the radical-anion being associated with Na+ as a contact ion-pair and a shift of λmax to 490 nm. In the presence of the lithium salt the absorption spectrum of the radical-anion reverts to 500 nm and this is interpreted in terms of formation of solvent-separated ion-pairs. On pulse radiolysis of solutions containing NAH the main reaction forming St˙– is that of (Na+, e–s) ion-pairs with t-St [reaction (12)] with k12=(1.29 ± 0.04)× 1010 dm3 mol–1 s–1. In addition there is a delayed formation of St˙– over a period of microseconds and a possible mechanism for this is considered. The presence of tetrahydridoaluminate salts also greatly enhances the stability of St˙– and at high doses per pulse little decay was observed over 700 µs. Reasons for the enhanced stability of St˙– are considered. The variation of G(St˙–) with [NAH] was studied and was found to attain a plateau value of 2.0 at the higher concentrations. Pulse radiolysis of NAH/THF solutions containing crown ether yielded an absorption spectrum closely resembling that of e–s, but kinetic studies on the solutions containing trans-stilbene indicated that the species is an ion-pair between e–s and Na+ in which the sodium is complexed with the crown ether, i.e.(Na+C, e–s). This species reacts with trans-stilbene with a rate constant of (3.86 ± 0.07)× 1010 dm3 mol–1 s–1.

Conductometric study of ion—ion and ion-solvent interaction—1. Conductances of lithium acetate in water, methanol and 50, 60 and 70% (W/W) dioxane—water mixtures at 25°C

Conductances of lithium acetate in water, methanol and 50, 60 and 70% (w/w) dioxane—water mixtures were measured. The data was analysed using the Fuoss-Hsia equation in the form of Fernandez—Prini coefficients to find the ion-pair formation constants ( K A ) and molar conductances at infinite dilution. The log K A vs 1/ D plot is a straight line passing through the origin in agreement with the Dension—Ramsey theory. The Walden products, solvation numbers and solvodynamic radii were also calculated. The results indicate the formation of solvent separated ion-pairs (SSIP).

Spectroscopic studies of ionic solvation. XIX. Fluorine-19 and sodium-23 NMR studies of ionic interactions in nonaqueous solutions of hexafluorophosphate salts

Fluorine-19 and sodium-23 NMR measurements were carried out on sodium hexafluorophosphate solutions in a number of solvents. In solvents of medium polarity and donicity (e.g., propylene carbonate, acetone, acetonitrile) the 19 F chemical shift moved upfield with increasing concentration of the salt. This behavior is indicative of anion-cation interactions which may be of long-range type, i.e., formation of solvent-separated ion pairs; the possibility of contact ion pair formation, however, cannot be excluded. In solvents of low polarity and donicity (acetic acid, tetrahydrofuran), the salt is essentially completely associated in the 0.1–1.0M concentration range. On the other hand, in solvating solvents with high dielectric constants, such as dimethyl-formamide, dimethylsulfoxide, and formamide, there is very little ionic association in the same concentration range. The above conclusions are supported by 23 Na chemical shift measurements. Potassium hexafluorophosphate solutions do not show any concentration dependence of the 19 F chemical shifts, while for tetra-n-butylammonium solutions the 19 F resonance moves downfield with increasing concentration of the salt.

Raman, infrared, and proton magnetic resonance investigation of solutions of zinc nitrate in anhydrous methanol

Raman, infrared, and proton magnetic resonance spectra of solutions of zinc nitrate in anhydrous methanol are reported. It is concluded that above −30 °C the zinc and nitrate ions can form contact ion-pairs, but that below −30 °C association is limited to the formation of solvent-separated ion-pairs. The apparent solvation number of zinc, as determined by area measurements on the proton magnetic resonance spectra, decreases with increasing solute concentration, and an interpretation of this effect is given.