The influence of polar solvents on ions and ion pairs in the anionic polymerization of styrene

In the rather polar organic solvents dimethoxyethane, tetrahydrofuran and 3-methyltetrahydrofuran, the behaviour of contact ion pairs, solvent-separated ion pairs and free carbanions of polystyryl sodium has been investigated by kinetic and conductivity measurements. Both the equilibrium between the two kinds of ion pairs and the dissociation of solvent-separated ion pairs to free anions are followed over a wide range of temperature. Thereby, conditions can be found under which the polymerization takes place almost exclusively via one of the two types of ion pairs.The thermodynamic parameters of the equilibria and the Arrhenius parameters of the propagation rate constants of the different kinds of propagating chain ends are reported. The equilibria between these species are strongly influenced by the solvent whereas the individual propagation rate constants are scarcely affected by the solvent. The “effective charge distance” in the solvent-separated ion pair can be estimated from the corresponding dissociation constant. The mobility of the polymer carbanion is discussed.     

Electrochemical studies of ion pairs and triplets formed by alkali metal cations with 1,2-naphthosemiquinone and 1,2-naphthoquinone dianion

The properties of ion associates formed by alkali metal cations with 1,2-naphthosemiquinone radical anions and 1,2-naphthoquinone dianions have been polarographically studied in dimethylsulfoxide solutions. It was established that these radicals and dianions coexist in equilibria with corresponding ion pairs and triple ions. The values of association constants were calculated on the basis of the slightly modified method of De Ford and Hume. Some structural aspects of ion association in the systems investigated are discussed.     

Indirect detection in reversed-phase liquid chromatography: Ion-pair retention models for cations on polystyrene polymers

Summary The distribution equilibria of cationic compounds in reversed-phase chromatographic systems (ion-pair chromatography) have been studied on the basis of their effect on a detectable mobile phase component. The solid phase was a polystyrene-divinylbenzene copolymer and the detectable component, a quaternary ammonium ion, 1-methylpyridine. The solutes were mono- and divalent amines and quaternary ammonium ions. The cations can be retained by ion-pair adsorption and ion exchange. Expressions for the ion-pair retention of the solutes and the mobile phase cation (system peak) have been developed assuming Langmuir distribution of ion pairs to a solid phase with one kind of binding site. The validity of the expressions has been tested by evaluation of ion-pair distribution constants using non-linear curve fitting techniques. Good agreement for the constants of common ion pairs was obtained from different kinds of capacity ratio expressions. Ion exchange retention can appear beside ion-pair retention, and it has been observed in the pH range 1.6–6.1. The effect depends not only on cations in the mobile phase, but also on the nature of the buffering systems.     

The role of association/complexation equilibria in the anionic polymerization of (meth)acrylates

The kinetics of the anionic polymerization of methacrylates and acrylates in THF as well as the MWD of the polymers formed depend on the concentration of active centres and of additives, such as lithium halides and lithium alkoxides. These results are discussed on basis of multiple equilibria between non-associated, associated, and complexed ion pairs which are supported by viscosity measurements. The position of these equilibria determines the rate of polymerization, whereas the dynamics of interconversion determine the polydispersity. In the absence of additives the rate of monomer addition to non-associated ion pairs competes with the rate of association. Addition of lithium chloride mainly affects the rate of interconversion between dormant and active species, whereas lithium tert-butoxide forms adducts which stabilize the active centre.     

Charge-Segregated Stacking Structure with Anisotropic Electric Conductivity in NIR-Absorbing and Emitting Positively Charged π-Electronic Systems

Squarylium-based π-electronic cation with an augmented dipole was synthesized by methylation of zwitterionic squarylium. The cation formed various ion pairs in combination with anions, and the ion pairs exhibited distinct photophysical properties in the dispersed state, ascribed to the formation of J- and H-aggregates. The ion pairs provided solid-state assemblies based on cation stacking. It is noteworthy that complete segregation of cations and anions was observed in a pseudo-polymorph of the ion pair with pentacyanocyclopentadienide as a π-electronic anion. In the crystalline state, the ion pairs exhibited photophysical properties and electric conductivity derived from cation stacking. In particular, the charge-segregated ion-pairing assembly induces an electric conductive pathway along the stacking axis. The charge-segregated mode and fascinating properties were derived from the reduced electrostatic repulsion between adjacent π-electronic cations via dipole–dipole interactions.     

Structural modifications of dicationic acetylcholinesterase reactivators studied under ion‐pairing mechanism in reversed‐phase liquid chromatography

A study focused on the chromatographic behavior of several acetylcholinesterase reactivators under ion‐pairing mechanism is reported. Among these reactivators, dicationic oximes and carbamoyl‐based pyridinium congeners were studied, which form ion pairs with alkylsulfonate anions. This mechanism was studied for some major experimental parameters, such as the chain length of the ion‐pairing agent added to the aqueous phase, its concentration, temperature, and nature of the organic modifier from mobile phase. Retention data showed one or two possibilities of forming ion pairs and the tautomerism of the studied reactivators, for different pH values of the aqueous component. Double sigmoid shapes were obtained for the studied compounds for the dependence between retention factor and pH, indicating the possibility of one or two tautomeric equilibria: at pH close to 7 these compounds are not stable as dicationic species and they participate in the retention process as nitroso forms, which are not able to form ion pairs with alkylsulfonates. The dependences of the retention factor on the organic modifier content from mobile phase were linear. Two complementary theoretical models were used to explain the functional dependences for the retention data on the experimental parameters.     

Excited-state proton-transfer dynamics of 1-methyl-6-hydroxyquinolinium embedded in a solid matrix of poly(2-hydroxyethyl methacrylate)

The excited-state intrinsic proton transfer and its geminate recombination, as well as the ground-state equilibria, of 1-methyl-6-hydroxyquinolinium embedded in a solid matrix of poly(2-hydroxyethyl methacrylate) have been studied by measuring time-resolved and steady-state fluorescence spectra along with absorption and excitation spectra. Proton transfer takes place within 3.3 ns to form ion pairs while its back-reaction occurs on the time scale of 3.7 ns. The ion pairs in the rigid alcoholic matrix go through neither diffusion to form free ions nor subsequent electronic rearrangement to form the keto species within their excited-state lifetimes.     

Electronic Structure and Structure of Ion Pairs of Anionic σ Adducts of Polynitroarenes with Acetonate Ion

The anionic Yanovskii adducts of 1,3,5-trinitro- and 1,3-dinitrobenzenes, 1,3-dinitronaphthalene, and 3,5-dinitrobenzoic acid with sodium, potassium, and tetrabutylammonium acetonates in low-polarity solvents exist mostly as contact ion pairs, while in polar solvents, as solvent-separated ion pairs and free ions. Lowering the temperature increases the fraction of solvent-separated ion pairs in low-polarity solvents and of free ions in polar solvents, by shifting the equilibria to stronger solvated ionic species. By quantum-chemical calculations of the geometry and electronic structure of the anions and of the electronic absorption spectra of the free ions and ion pairs, as well as by a spectrophotometric study of adducts with solvents of various polarity it was established that the cation on ion-pair formation coordinates with the 4-nitro group with respect to the pyramidal node.     

Reactivities of fluorenyl species as ethylene oxide polymerization initiators—IV. Kinetic studies with cryptated 9-methylfluorenylsodium

Reactivities of cryptated ion pairs and of free ions were determined by spectrophotometry for the ring-opening of ethylene oxide by 9-methylfluorenylsodium complexed by [222], in THF at +20, ?10 and ?30 C. These ionic species have the same reactivity which is much lower than that of tight ion pairs. This behaviour may be explained by the increase of delocalization of the negative charge when the interionic distance becomes larger, but coordination of ethylene oxide with the cation may also occur with tight ion pairs leading to electrophilic activation of the monomer.     

Effect of temperature on acid–base equilibria in separation techniques. A review

Studies on the theoretical principles of acid–base equilibria are reviewed and the influence of temperature on secondary chemical equilibria within the context of separation techniques, in water and also in aqueous-organic solvent mixtures, is discussed. In order to define the relationships between the retention in liquid chromatography or the migration velocity in capillary electrophoresis and temperature, the main properties of acid–base equilibria have to be taken into account for both, the analytes and the conjugate pairs chosen to control the solution pH. The focus of this review is based on liquid–liquid extraction (LLE), liquid chromatography (LC) and capillary electrophoresis (CE), with emphasis on the use of temperature as a useful variable to modify selectivity on a predictable basis. Simpli?ed models were evaluated to achieve practical optimizations involving pH and temperature (in LLE and CE) as well as solvent composition in reversed-phase LC.     

Silver Ions in Non‐canonical DNA Base Pairs: Metal‐Mediated Mismatch Stabilization of 2′‐Deoxyadenosine and 7‐Deazapurine Derivatives with 2′‐Deoxycytidine and 2′‐Deoxyguanosine

Novel silver‐mediated dA?dC, dA*?dC, and dA*?dG base pairs were formed in a natural DNA double helix environment (dA* denotes 7‐deaza‐dA, 7‐deaza‐7‐iodo‐dA, and 7‐cyclopropyl‐7‐deaza‐dA). 7‐Deazapurine nucleosides enforce silver ion binding and direct metal‐mediated base pair formation to their Watson–Crick face. New phosphoramidites were prepared from 7‐deaza‐dA, 7‐deaza‐7‐iodo‐dA, and 7‐cyclopropyl‐7‐deaza‐dA, which contain labile isobutyryl protecting groups. Solid‐phase synthesis furnished oligonucleotides that contain mismatches in near central positions. Increased thermal stabilities (higher T_m values) were observed for oligonucleotide duplexes with non‐canonical dA*?dC and dA?dC pairs in the presence of silver ions. The stability of the silver‐mediated base pairs was pH dependent. Silver ion binding was not observed for the dA?dG mismatch but took place when mismatches were formed between 7‐deazaadenine and guanine. The specific binding of silver ions was confirmed by stoichiometric UV titration experiments, which proved that one silver ion is captured by one mismatch. The stability increase of canonical DNA mismatches might have an impact on cellular DNA repair.     

Luminescence kinetics and association equilibria of lanthanide ions in aqueous solutions

Luminescence life time measurement can be used under certain conditions to determine the thermodynamic constants of complex formation between the luminescent (central ion) and ligand. The basic equations correlating the life time and the equilibria constants were derived for two cases: the time for establishing the thermodynamic equilibrium is much shorter than the life time of the excited state of the central ion; and the time for establishing the equilibrium and the life time are of the same order of magnitude.     

Thermodynamics of electrolytes. III. Activity and osmotic coefficients for 2–2 electrolytes

The peculiar behavior of 2–2 and higher valence type electrolytes is discussed in terms of various theories some of which assume, while others do not, an equilibrium between separated ions and ion pairs as distinct chemical species. It is recognized that in some case a distinct species of inner-shell ion pairs is indicated by spectroscopic or ultrasonic data. Nevertheless, there are many advantages in representing, if possible, the properties of these electrolytes by appropriate virial coefficients and without chemical association equilibria. It is shown that this is possible and is conveniently accomplished by the addition of one term to the equations of Parts I and II of this series. The coefficients of these equations are given for nine solutes. It is also noted that these equations have been successfully applied to mixed electrolytes involving one component of the 2–2-type.     

Ion association in N-methylpyrrolidone and the N-methylpyrrolidone-water mixed solvent from data on heat capacity and density of solutions

Association constants for solutions of electrolytes in N-methylpyrrolidone (MP) and the MP-water mixed solvent at 298.15 K are calculated. It is shown that, over a wide range of concentrations, concentration dependences of apparent molar heat capacities and apparent molar volumes of the electrolytes in MP and MP-water mixtures are described adequately in terms of equilibria between ions and ion pairs of a single type in the solution.     

Complexation of N-bis[2-(1,2-dicarboxyethoxy)ethyl]aspartic acid with Fe(II), Co(II), and Ni(II) in aqueous solution

In a search for environment-friendly metal chelating ligands for industrial applications, the protonation and complex formation equilibria of N-bis[2-(1,2-dicarboxyethoxy)ethyl]aspartic acid (BCA6) with Fe(II), Co(II), and Ni(II) ions in aqueous 0.1 M NaCl solution were studied at 25°C by potentiometric titration. The model for complexation and the stability constants of different complexes were determined for each metal ion using SUPERQUAD. In all cases, complex formation was dominated by stable ML^4? complexes.     

Electroconductivity study of active centers of polar vinyl monomers in the anionic polymerization

Ion equilibria in THF solutions of organolithium compounds are studied at ?30°C. The extremely low values of the dissociation constants (from 10^?11 to 10^?13M) and the formation of ion triples of the types A^?, B⁺, A^? prove that active centers exist in THF mainly as contact ion pairs with very small interionic distance (～ 2.5 Å) and tend to form higher associates. A comparison is made between the physicochemical characteristics of active centers of vinyl nitriles, ketones, and esters. A planar structure is proposed for the anion of methyl methacrylate. The results indicate that the intramolecular chelate complexation between the polar group and the counterion of the active center decreases in the order . A greater participation of ion triples in the propagation reaction is proposed.     

Pyridine induced disproportionation reaction of Co2(CO)8 in THF: Homonuclear ion pairs and dihydrogen activation

The IR spectra of the Co²⁺/Py/Co(CO)₄⁻, system in THF reveal complex associative phenomena. Three kinds of homonuclear ion pairs were distinguished by means of variable temperature IR spectroscopy. Under the conditions in which these associative equilibria exist cobalt(II) is reduced by dihydrogen to cobalt( −I) under 1 atm of syngas at room temperature.     

Carbenium ions,onium ions,and covalent species in the cationic polymerization of alkenes

The importance of exchange reactions between various types of active species present in the cationic polymerization of alkenes is addressed. Reactivities of carbocations, corresponding ion pairs, onium ions, and covalent species are discussed for model systems and in real polymerization. The influence of various parameters on the equilibria between carbenium ions, onium ions, and covalent species is described. Rate constants of the isomerization of active sites are estimated and correlated with polydispersities. General ways to improve control of cationic polymerization of alkenes are discussed.     

Complexation of iminodisuccinic acid (IDS) and 3-hydroxy-2,2′-iminodisuccinic acid (HIDS) with Cd2+, Hg2+, and Pb2+ in aqueous solution

In a search for environmentally friendly metal-chelating ligands for industrial applications, the protonation and complex formation equilibria of iminodisuccinic acid (IDS) and 3-hydroxy-2,2′-iminodisuccinic acid (HIDS) with Cd²⁺, Hg²⁺, and Pb²⁺ in aqueous 0.1?mol?L^?1 NaNO₃ solution were studied at 25°C by potentiometric titration. Models for complexation and stability constants of the different complexes were determined for each metal ion using the computer program SUPERQUAD. In all cases, complex formation was dominated by stable ML^2? complexes.     

Thermionic Emission Data on the Electronic Work Function of LnBr3 (Ln = La,Ce, Er,Ho, Lu) Crystals

A methodology based on measurements of ion-molecular equilibria using high-temperature mass spectrometry was proposed for determining the electronic work function ?_e of congruently subliming ionic crystals. Using the enthalpies of sublimation determined experimentally for the LnBr_4? ion and LnBr₃ molecule, the effective values of ?_e were for the first time obtained from the thermochemical cycles of LaBr₃, CeBr₃, ErBr₃, HoBr₃, and LuBr₃ crystals. The resulting values were (±0.20 eV) 2.54, 3.35, 3.04, 2.85, and 3.05, respectively.