Formation Of Contact Ion Pairs Research Articles

Contact ion pair formation and ether oxygen coordination in the polymer electrolytes M[N(CF 3SO 2) 2] 2PEO n for M = Mg, Ca, Sr and Ba

The polymer electrolytes M[N(CF 3SO 2) 2] 2PEO n for M = Mg, Ca, Sr and Ba have been investigated using infra-red spectroscopy, differential scanning calorimetry and impedance spectroscopy. The effects of varying concentration ( n = 6–40) and temperature (25–95°C) on the contact ion pair formation and cation coordination have been studied. Contact ion pairs are only found for the most concentrated samples ( n < 9). Ion pairs occur with two types of structures: one type is found for samples containing Mg 2+ and the other for samples with Ca 2+, Sr 2+ and Ba 2+. The conductivities of the samples are discussed in terms of radii and coordination numbers of the cations.

Representation of polygalacturonic acid by subunits in the analysis of its potentiometric titration curves

The potentiometric titration curves of aqueous solutions of polygalacturonic acid were determined in the presence of either tetraalkylammonium or alkaline ions in order to explain the effects of the size and the nature of the counterion on the degree of dissociation. In the evaluation of potentiometric curves, the approach proposed by Strauss was used and successive ionization constants of appropriately chosen oligomeric subunits of the polymer molecule were determined. The partition functions and Gibbs free energies of ionization per mole of polygalacturonic acid at the ambient chemical potential of hydrogen ions, and the Gibbs free energies per mole of polyacid related to the hydrogen ion in its standard state, were calculated and are given as a function of hydrogen ion activity or of the degree of ionization. From the above functions the differences in the counterion specificities were observed and are discussed on the basis of the formation of contact ion pairs between an alkaline ion and the carboxylate group. In addition, the values of the entropy and enthalpy changes per mole of repeating unit (hydrogen ion at standard state potential) accompanying the stepwise dissociation reactions were calculated on the basis of the temperature dependence of the stepwise dissociation constants via the Gibbs—Helmholtz relation, and the thermodynamic data were correlated with the structural properties of the investigated polymer solute.

Hetero‐ ansa ‐Bridged Main Group Metallocenes — a Structural and NMR Study

1,2-Bis(dimethylamino)-1,2-di-9-fluorenyldiborane(4) (1) is doubly metallated with group 1 and 2 metals to yield metal 1,2-bis(dimethylamino)-1,2-di-9-fluorenyldiborate(4) 2-Li to 2-Ba. The conformations of these compounds in solution are investigated by NMR spectroscopy, and the solid-state structures of 2-Li · 5 THF, 2-K · 4 THF, 2-K · 2 DME, and 2-Ca · 2 THF are determined by X-ray diffraction. All four structures can be described as main group metallocenes. The potassium derivatives form contact ion pairs, leading to polymeric chain-like structures. In contrast, 2-Li · 5 THF exhibits the rare combination of a contact ion pair with a solventseparated ion pair, giving the monomeric structure [Li(THF)4][(fl2B2(NMe2)2)Li · THF]. 2-Ca · 2 THF, which is also monomeric, adapts a severely distorted geometry.

Conductometric Study of Ion Association between Tris(oxalato)chromate(III) and Alkali Metal Ions at Temperatures from 0 to 50 °C

Abstract Conductivities for aqueous solutions of alkali metal salts of tris(oxalato)chromate(III) ([Cr(ox)3]3−) were measured at various temperatures from 0 to 50 °C. Ion-association constants (KA) between the complex ion and alkali metal ions increased in the order Li+ &amp;lt;&amp;lt; Na+ &amp;lt; K+ &amp;lt; Rb+ &amp;lt; Cs+ and had minimum values (KA(min)) at characteristic temperatures (tmin) in the observed temperature range except for the lithium ion. The appearance of minima of KA suggested the formation of contact ion pairs besides that of solvent-separated ones. The contact ion-pair formation was considered to be assisted by weak hydration of alkali metal ions and enhanced with decreasing their hydration strength. The lithium ion was presumed to form only solvent-separated ion pairs on the basis of extremely small ion-association constants. Temperature dependence of log KAfor each ion association could be represented by the equation: log KA = p(t − tmin)2 + log KA(min). A common relationship between p and tmin was found out for the ion-association systems of [Cr(ox)3]3−, [Co(NH3)6]3+, and [Co(en)3]3+ (en = ethylenediamine), and p was expressed by an empirical quadratic equation in tmin. The standard enthalpy changes of ion association (ΔHas°) at any temperature for these complex systems could be reproduced within ±0.2 kJ mol−1 by use of these equations with the experimental tmin. Hydration behavior of [Cr(ox)3]3− was also discussed based on limiting molar conductivities and their temperature dependence.

Raman and infrared spectroscopic investigation of contact ion pair formation in aqueous cadmium sulfate solutions

Raman and IR data for aqueous CdSO4 and (NH4)2SO4 solutions have been recorded over broad concentration and temperature ranges. Whereas the v1-SO 4 2− band profile is symmetrical in (NH4)2SO4 solutions, in CdSO4 solutions a shoulder appears on the high frequency side which increases in intensity with increasing concentration and temperature. The molar scattering coefficient of the v1-SO 4 2− band is the same for all forms of sulfate in (NH4)2SO4 and CdSO4 solutions and is independent of temperature up to 99°C. The high frequency shoulder is attributed to the formation of a contact ion pair [Cd2+OSO 3 2− ] (1∶1 associate). Also the v3-SO 4 2− antisymmetric stretching mode shows a splitting in the CdSO4 solution. Further spectroscopic evidence for contact ion pair formation is provided by IR spectroscopy. No higher associates or anionic complexes are required to interpret the spectroscopic data. The degree of association has been measured as a function of concentration and temperature. The thermodynamic association constant, KA=0.15±0.05 kg-mol−1 at 25°C is estimated from the Raman data by an extrapolation procedure by taking account of the activity coefficients. Values are reported for the activity coefficient of the ion pair. From the Raman temperature dependence studies, the enthalpy of formation for the contact ion pair is estimated to be 10±1 kJ-mol−1.

Mechanism of Contact Ion Pair Formation in Photolyzed Mixtures of N,N,N',N'-Tetramethylbenzidine with Halogen-Containing Compounds in Nonpolar Solvent

Time variation of microwave dielectric absorption has been examined for photolyzed solutions of N,N,N',N'- tetramethylbenzidine (TMB) admixed with a halogen-containing compound AX (AX=CCl 4 , C 2 H 5 I, C 2 H 5 Br, C 6 H 5 I, and C 6 H 5 Br) in benzene. In each solution the dielectric absorption signals show rapid growth after photoirradiation, indicating formation of a contact ion pair (CIP) TMB + X - (X denotes a halogen atom). The amplitudes of the signals are dependent on the AX concentration and are large for CCl - but relatively small in other molecules when compared at the same concentration

Mechanisms of competing solvolytic elimination and substitution reactions. The role of ion-pair intermediates in aqueous solvents

Solvolysis of 9-(2-bromo-2-propyl)fluorene 1-Br in mixtures of water with methanol or acetonitrile at 25 °C provides the elimination product 9-isopropenylfluorene 2, and the substitution products 9-(2-hydroxy-2-propyl)fluorene 1-OH and 9-(2-methoxy-2-propyl)fluorene 1-OMe. The Grünwald–Winstein parameter was measured in methanol–water mixtures as mobs= 0.70, which is composed of the parameter for the elimination reaction, mE= 0.65, and the substitution reaction, ms= 0.83. The parameters for the corresponding chloride 1-Cl were measured as mobs= 0.82, mE= 0.76 and ms= 0.97. The kinetic deuterium isotope effects for the reactions of the hexadeuteriated analogue 9-(2-bromo[2H6]-2-propyl)fluorene ([2H6]-1-Br) were measured as (kEH+ksH)/(kED6+ksD6)= 2.3 ± 0.1 for the disappearance of the substrate in 70 vol% methanol in water, and kEH/kED6= 3.1 ± 0.1 and ksH/ksD6= 1.4 ± 0.1 for the elimination and substitution, respectively. In pure acetonitrile the alkene 2 is the sole product and the isotope effect was found to be kEH/kED6= 2.3 ± 0.1. These results strongly indicate a branched mechanism involving rate-limiting formation of a common contact ion pair which either undergoes nucleophilic attack by the solvent or is dehydronated. The intermediate shows a very small discrimination between different nucleophiles. Thiocyanate ion and azide ion, which are assumed to react with diffusion-controlled rates with the ion pair, are only a few times more reactive than a water molecule in 70 vol% methanol in water, kSCN/kHOH= 3 and kN3/kHOH= 4, ratios of second-order rate constants. Methanol was found to be slightly less reactive than water, kMeOH/kHOH= 0.7.

Contact ion pairs formed from photolyzed solutions of N,N,N',N'-tetramethylbenzidine in carbon tetrachloride

The time variation of microwave dielectric absorption has been examined for photolyzed solutions of N,N,N',N'- tetramethylbenzidine (TMB) in CCl 4 . The dielectric absorption signals show a rapid growth followed by a second-order decay approaching a flat level higher than the baseline. These are attributed to the formation of contact ion pairs (TMB + Cl - ) by reaction of the excited state of TMB with CCl 4 and a subsequent dimerization process. The dipole moment of the contact ion pair and the rate constant for its dimerization are determined to be 12.4O -1/2 D and 1.7×10 9 O -1 M -1 s -1 , respectively, where O is the quantum yield for the ion-pair formation with a value larger than 0.26 (possibly ≥0.5)

Specific helix stabilization of poly(L‐glutamic acid) in mixed counterion systems

AbstractThe coil–helix transitions of poly (L‐glutamic acid) in aqueous alcohol solutions have been investigated for mixed counterion systems. It has been found that coexistence of two kinds of counterion species, i.e., two alkali metal counterions, alkali and alkaline earth metal, and two alkaline earth metals, specifically stabilizes or destabilizes the helix conformation depending upon the combination of the counterion species. The most striking enhancement of the helix content was observed for the combination of Li+ and K+ counterions. It has been suggested that the helix stabilization is attributed to the reduction of the free energy in the contact ion pair formation between the polymer charges and the counterions in the mixed counterion systems. © 1993 John Wiley &amp; Sons, Inc.

Cation–calixarene interactions: tetraalkylammonium cation binding by calixarene anions

Syntheses of a number of tetraalkylammonium derivatives of various calix[n]arenes (n= 4, 6, 8) have been conducted by reacting the tetraalkylammonium hydroxides with the parent calixarenes. 1 H NMR studies in acetone and dimethylsulfoxide (DMSO) solvents have shown that cation inclusion by the anionic calixarenes occurs in some cases for only the tetramethyl- and tetraethyl-ammonium cations, indicating that different forms of ion association must occur in solution. The nature of cation–anion interactions in the solid tetraethylammonium salts of the monoanions of p-tert-butylcalix[4]arene (L2) and its dihomooxa analogue (L3) has been explored by room temperature single-crystal X-ray studies of the two salts and the parent ligand L3, all recrystallised variously solvated from acetonitrile. In both solid salts, columnar arrays of alternating cations and anions are found: in the dihomooxa ligand (L3) salt, the cations are captured within the ligand cup, forming contact ion pairs, the column comprising stacks of these. By contrast, the columnar array found in the parent ligand (L2) salt consists of discrete moieties, with the anion cavity occupied by acetonitrile. The free (neutral) ligand L3, as its acetonitrile monosolvate, contains the acetonitrile within the ligand cavity also. The structure determination of the unsubstituted calix[4]arene ligand, L1, is recorded as its ⅓ acetonitrile solvate, with location of all phenolic hydrogen atoms bridging their associated oxygen atoms; the acetonitrile in this case simply occupies a lattice void, as in the isomorphous acetone solvate.

Solvent-induced and salt-induced coil-helix transition of poly( l-lysine) salts in water/alcohol mixtures

Coil-helix transition has been studied for poly( l-lysine) salts, with counterion species Cl − (PLLHCl), Br −(PLLHBr) and NO − 3(PLLHNO 3), in aqueous alcohol mixtures (water/MeOH, EtOH, 2PrOH, tBuOH) as a function of the alcohol composition and/or salt concentration. The dependence of the helix content on the solvent species and composition is interpreted in terms of the three solvent properties i.e. dielectric constant, Gutmann-Mayer acceptor number and water activity, which have been found to be critical for contact ion-pair formation in the study of poly( l-glutamic acid). The salt-induced transition is also analysed by taking account of the contact ion-pair formation. Comparisons are made with the results on the poly( l-glutamic acid) systems.

A direct nitrogen-15 NMR study of neodymium(III)-nitrate complex formation in aqueous solvent mixtures

A study of contact ion-pair formation between the neodymium (III) and nitrate ions in aqueous solvent mixtures has been carried out by a direct, low temperature, nitrogen-15 (15N) nuclear magnetic resonance (NMR) technique. At low temperatures, −90 to −120°C ligand exchange is slow enough to permit the observation of15N NMR signals for uncomplexed nitrate ion, and this anion in the primary solvation shell of Nd(III). In aqueous mixtures with inert acetone and Freon-12, resonance signals for Nd(NO3)2+, Nd(NO3) 2 1+ , and two higher complexes are observed. Signal areas indicate these additional species are possibly a combination of the tetra-, penta-, and hexanitrato complexes, but not the trinitrato. In water-methanol, a medium of higher dielectric constant, complexation is much less and signals only for the mono-and dinitrato complexes are observed. The effect of solvent on complexation is demonstrated more clearly by a series of measurements in water-methanol-acetone mixtures.

Inhibition of Clostridium perfringens phospholipase C by ammonium and sulfonium dications.

Polyamines, alkyldiammonium and alkyldisulfonium salts, inhibit the Clostridium perfringens phospholipase C-catalyzed hydrolysis of 1-S-phosphocholine-2-O-hexadecanoyl-1-mercapto-2-ethanol (1) at pH 7.5, 37 degrees C, mu = 1.0 with KCl. Simple saturation kinetics are observed as both 1 and [Ca2+] are varied and simple linear inhibition is observed. The data are consistent with a non-competitive mechanism that involves binding of the inhibitors to free enzyme, E.[Ca2+] and E.[Ca2+].[S]; the inhibition constants for decamethylenebis(trimethylammonium) and decamethylenebis(dimethylsulfonium) bromides are 90 and 0.28 mM, respectively. It is suggested that the enhanced inhibition by the alkyldisulfonium salts results from more favorable equilibrium constants for contact ion-pair formation or from the formation of tetracoordinate sulfuranes.

A study of ion pairing in acetonitrile solutions containing magnesium perchlorate using attenuated total reflection FTIR spectroscopy

A study of ion pairing for Mg(ClO 4 ) 2 dissolved in acetonitrile has been carried out using attenuated total reflection FTIR spectroscopy. The present results indicate that the coordination number for ligands around the Mg 2+ ion is approximately four, and not six as previously reported. Ion pairing is relatively strong with 70% of the Mg 2+ ions forming contact ion pairs with one perchlorate ion. The spectral characteristics of acetonitrile associated with Mg 2+ have been clearly distinguished from those of the free molecule

Spectrochemistry of solutions, part 23. Changes of enthalpy and entropy in the formation of contact ion pairs: A vibrational spectroscopic appraisal using thiocyanate and azide solutions

The vibrational spectra of solutions have been analyzed to assess both qualitatively and quantitatively the changes in enthalpy and entropy for ion pair formation in solutions of LiNCS, Mg(NCS)2, and LiN3 in liquid ammonia, dimethylformamide, dimethylsulphoxide and acetonitrile. Contrary to predictions both the ΔH ass and ΔS ass terms are all positive in the cases examined, indicating that the driving force in the ion association process derives from solvent-solute restructuring, and not the energy of the interaction between the cation and anion. This characteristic of contact ion pair formation is likely to be found to be applicable over a wide range of solvents. The following specific values of the thermodynamic parameters at 298 K have been obtained: LiNCS/DMF, ΔG=−1.3 (1) kJ mol−1, ΔH ass =+1.8 (5) kJ mol−, ΔS ass =+10 (2) J mol−1 K−1; LiNCS/DMSO, ΔG=+0.9 (2) kJ mol−1, ΔH ass =+0.3 (3) kJ mol−1; Mg(NCS)2/DMF, ΔG ass =−4.0 (3) kJ mol−1, ΔH ass =+15 (4) kJ mol−1, ΔS=+64 (17) kJ mol−1; LiN3/DMSO, ΔG ass =−2.5 (3) kJ mol−1, ΔH ass =+4.9 (9) kJ mol−1, ΔS ass =∼+25 (10) J K−1 mol−1.

Counterion and solvation effects on the conformations of the radical anions of 2,5-dicyanovinyl substituted furan and thiophene. An EPR and ENDOR study

The radical anions of two tetranitrile compounds, i.e., 2,2′-(furan-2,5-diyldimethylidene)-bispropanedinitrile (1) and 2,2′-(thiophene-2,5-diyldimethylidene)bispropanedinitrile (2), have been investigated in a 1,2-dimethoxyethane/N,N,N′,N′,N″,N″-hexamethylphosphoric triamide mixture, 1,2-dimethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran as the solvents and with Li+, Na+, K+, Cs+ as the counterions and in N,N-dimethylformamide with Zn2+ as counterion. A remarkable dependence of the molecular symmetry upon ion-pair formation is found for both molecules. The symmetry of both radical anions increases to C2ν under conditions favouring contact ion-pair formation.

Counterion-specific helix formation of poly( l-glutamic acid) in a crosslinked membrane

A poly( l-glutamic acid) (PLGA) membrane was prepared by crosslinking with poly(ether diisocyanate). The counterion specificity in helix stabilization was Li + ⩾ Na + > K + ⩾ Cs + when the membrane was immersed in aqueous alcohols and Na + > Li + ⩾ K + ⩾ Cs + in aqueous dioxane. These counterion specificities were interpreted as a result of contact ion-pair formation between the polymer charge and the bound counterion. The coil-helix transitions of PLGA in membrane/solvent mixture systems were compared with those in solution systems.

Studies on the mechanisms of gelation of kappa-carrageenan and agarose

It has been established that hydrogen bonds control both gelation and helix formation completely in the case of agarose and partially in the case of kappa-carrageenan, the major role belonging in the latter case to the interactions of a polysaccharide with metal ions. Na+ and K+ ions form contact ion pairs with sulphate groups of kappa-carrageenan. It is supposed that an increase in the number of contact ion pairs together with association of macromolecules having unordered conformation, a decrease in the second virial coefficient, and a decrease in the refraction index increment (i.e., an increase in the solvation degree of dissolved particles) is a necessary condition for forming the kappa-carrageenan gel network. A sufficient condition of kappa-carrageenan gelation is the intermolecular coordination binding of ions such as K+ ions, promoting gelation. The coil-to-helix transition of the polysaccharide is controlled by shielding the charge of kappa-carrageenan-sulphate groups. Hydrophobic interactions proved to be unessential for gelation of either agarose or kappa-carrageenan.

On the effects of association in the statistical theory of ionic systems. Analytic solution of the PY-MSA version of the Wertheim theory

The statistical mechanical approach to a fluid of dimerizing hard spheres proposed recently by Wertheim is extended to a two-component mixture of oppositely charged hard spheres. The pair potential is represented as a sum of the short-range hard-sphere potential, an intermediate-range square-well potential between the oppositely charged spheres, and a long-range Coulomb potential. The square-well potential, which is responsible for association, results from a single auxiliary site. If the site is located in such a way that only dimers are formed, the model presented here can be viewed as a model of chemical association in a fluid consisting of molecular ions. The model with the site located in the centre of hard sphere can be used to describe the formation of contact ion pairs in ionic systems. An analytical solution of the Ornstein-Zernike-like integral equation is obtained within a mixed Percus-Yevick and mean spherical approximation closure. The fraction of unbonded ions and the radial distribution functions are presented, and are studied as a function of the system density, the inverse reduced temperature, and the depth of the square well.

ESCA studies of phase-transfer catalysts in solution: ion pairing and surface activity

Phase-transfer catalysts in solution have been studied by means of electron spectroscopy. Different anion-cation distributions at the surface were found depending on the anion identity. Thus, tetrabutylammonium perchlorate and tributyl-3-iodopropylammonium iodide show strong evidence of the formation of contact ion pairs at the surface. Conversely, the tetrabutylammonium nitrate and chloride show a more diffuse character of the anion distribution with respect to the surface. The observed differences in surface structure between the salts correlate with the variation in transfer coefficients from aqueous to organic phase