Anion ion pairs

In order to characterize the alkali metal salts of the 11-vertex nido-dicarbaborates [C2B9H11]2- in solution, a series of salts were synthesized and studied by NMR spectroscopy and quantum mechanical calculations. Comparison of the theoretical nB, 13C, and H NMR chemical shifts of the isomeric dianions 7,8-, 7,9-, and [2,9-C2B9Hn]Z, and the ion pairs [ap-M-CzBgHn]- and MHP-MC2B9HH] with the observed data indicate that the alkali metal salts M2[C2B9Hn] exist in solution as anionic ion pairs [ap-M-C2B9Hn]- with the metal in apical position (ap) coordinated at the five-membered open face of the cluster.14... 

Table 5.4 Relative reactivities (normalized to iodide) for a series of nucleophiles under phase transfer, homogeneous dipolar aprotic, and homogeneous protic conditions, and the hydration number of the quaternary onium-anion ion pair [43]...

Assay of adrenaline by chromatography with an anionic ion-pairing reagent... 

The catalysts must supply the system with lipophilic cations in order to form, with required anions, ion pairs able to enter nonpolar media. The most typical catalysts are tetraalkyl ammonium (TAA) salts R4N+X, particularly those having at least 16 carbon atoms in the four R groups. Similar lipophilic catalysts are tetraalkylphosphonium and -arsonium or trialkylsulfonium salts, which are less available and usually less stable. They are therefore of negligible practical use. There are a few reports on the use of trialkylamines as catalysts in some two-phase reactions. Usually these amines are qua-ternized by a reactant actually these reactions are catalyzed by TAA salts. More complicated is the generation of dihalocarbenes with trialkylamines. The amines form, with the carbene, an ammonium ylide, which acts as a base in the organic phase. 

In the Er complex [248] there are cation-anion ion-pairs. In the anionic unit, three Cp2Er units are connected by hydrogen and chlorine bridges in a triangular array with one /i3-H having Er-Hav 2.18 A, two (/r2-H), Er-Hav of 2.34 A. 

Popa, T.V. et al. Capillary zone electrophoresis of a-heUcal diastereomeric peptide pairs with anionic ion-pairing reagents. J. Chromatogr. A. 2004,1043, 113-122. 

Shibue, M., Mant, C.T., and Hodges, R.S. Effect of anionic ion-pairing reagent concentration (1-60 mM) on reversed-phase hquid chromatography elution behaviour of peptides. J. Chromatogr. A. 2005,1080, 58-67. 

Anionic polymerizations are generally much faster than free-radical reactions although the A p values are of the same order of magnitude for addition reactions of radicals and solvated anionic ion pairs (free macroanions react much faster). The concentration of radicals in free-radical polymerizations is usually about 10 -10 M while that of propagating ion pairs is 10 -10 M. As a result, anionic polymerizations are lO -lO times as fast as free-radical reactions at the same temperature. 

A number of so-called double ion-pair methods have been described for the analysis of hydrophobic amines in which the mobile phase contains both a quaternary ammonium ion and an alkyl sulfate or sulfonate. At first glance, this combination of mobile phase additives is counterintuitive because one would expect the effect of the anionic and cationic additives to cancel. However, the combination of cationic masking agents to reduce peak tailing and an anionic ion-pairing agent to enhance retention is sometimes necessary for the reversed-phase separation of hydro-phobic amines. 

In order to study the mutual solubilities of hydrophobic but also hygroscopic imida-zolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ILs in combination with the anions bis-(trifhioromethylsulfonyl)imide, hexafluorophosphate, and tricy-anomethane with water, UV spectroscopic measurements were carried out at temperatures between 288.15 and 318.15 K. Continuum model calculations were used to support these measurements of the Gibbs energy, enthalpy, and entropy. It was found that the hydrophobic tendency increases from imidazolium to pyridinium to pyrrolidinium to piperidinium and with increasing alkyl chain length within the same cation-varying anion ion pair [157],... 

The CFS hollow fiber suppressor (see Section 3.4.3) that was developed for cation exchange chromatography can also be applied to cation analysis via ion-pair chromatography. It features good solvent stability and sufficient membrane transport properties for the anionic ion-pair reagent. This suppressor is regenerated with tetramethylam-monium hydroxide using a concentration of c = 0.04 mol/L. 

MPIC Anions Ion-pair formation neutral NH4OH TMAOH c> TPAOH d> TBAOHe) In addition to the anions listed under HPIC anionic surfactants, metal-cyano complexes, aromatic carboxylic acids... 

Under some circumstances ion-ion interactions can be more important than ion-dipole interactions. This is especially true when the valence of the ion is greater than one, and the electrolyte concentration is high. Then, the formation of ion pairs and higher aggregates is possible. Two types of ion pairs have been recognized, namely, contact ion pairs in which the cation and anion are in physical contact, and solvent-separated ion pairs in which one or two solvent molecules are situated between the cation and anion. Ion pairing must be considered in developing a complete picture of an electrolyte solution. 

Effective equilibrium constants in 0.1 M CaCl2 were measured by Cavanaugh (14) or estimated by the Radian equilibrium program (26). As given in Table IV, these constants include the effects of ionic strength and ion pairing on the activity of the anions. Ion pairs, such as CaSO, are not treated as separate species, but are accounted for as an effect on the activity of simple ions, such as SO3. 

---