Cation-anion pair formation, influence

Si spin lattice relaxation times. Direct evidence of pairing between alkali metal cations and silicate anions has recently been presented by McCormick et al. (14) using alkali metal NMR spectroscopy (14). The formation of cation-anion pairs was found to influence both the chemical shift and spin-relaxation time of the cation. Additional manifestations of ion pairing were observed in studies of the silicon spin relaxation and exchange between silicate anions (L5). This work revealed that cation-anion contacts affect the silicon T2 values, an effect also seen in Figure 1. 

In solvent-separated ion pairs, the solvation shells of the cation and the anion touch each other in solvent-bridged ion pairs, the ions share solvent molecules. In contact ion pairs, the cation and the anion are bound directly to each other and are surrounded by a common solvation shell. In penetrated ion pairs, an empty space between edge groups in one ion of a salt is occupied to a certain degree by a counterion. The two latter types of ion pair may have quite a different electronic distribution than the corresponding naked ions. The following examples show the influence of ion-pair formation. 

The reactivity of an anion as an electron donor will of course be related to its state in the solution. Ion properties are strongly solvent dependent and may differ greatly according to the strength of their interaction with the surrounding molecules. This is also true in the case of excited state anions as shown by their solvent dependent photophysics. In weakly polar solvents interionic interactions may dominate resulting in ion pair formations and a strong influence of the counter-cation on the anion properties will result. In polar solvents, ions are solvated and their mutual influence is considerably diminished. 

It would be difficult to find more comprehensive or more detailed studies on the physical chemistry of seawater than those done at the University of Miami (Millero, 2001). Several programs were developed for calculation of activity coefficients and speciation of both major ions and trace elements in seawater. The activity coefficient models have been influenced strongly by the Pitzer method but are best described as hybrid because of the need to use ion-pair formation constants (Millero and Schreiber, 1982). The current model is based on Quick Basic computes activity coefficients for 12 major cations and anions, 7 neutral solutes, and more than 36 minor or trace ions. At 25 °C the ionic strength range is 0-6 m. For major components, the temperature range has been extended to 0-50 °C, and in many cases the temperature dependence is reasonably estimated to 75 °C. Details of the model and the parameters and their sources can be found in Millero and Roy (1997) and Millero and Pierrot (1998). Comparison of some individual-ion activity coefficients and some speciation for seawater computed with the Miami model is shown in Section 5.02.8.6 on model reliability. 

Besides alkali halides, alkali and alkaline earth azides have been most thoroughly inveistigated for radiation coloration. By irradiation of freshly precipitated potassium azide at 196°C with radiation of A = 2537 A, Tompkins and Young19 obtained bands due to the presence of F-centres and V-centres. Ageing was found to have marked influence on these bands. The proposed mechanism of ageing involves the formation of anion and cation vacancy pairs... 

Dai and Carr published a detailed study of the role of ion pairing of anionic additives on the separation of cationic drugs on the reversed-phase column [15]. Two major retention mechanisms were examined by which anionic additives can influence the retention of cations (i) ion-pair formation in the mobile phase with subsequent retention of the neutral ion pair, and (ii) pre-sorption of anionic addi-... 

The ammonium catalyst can also influence the reaction path and higher yields of the desired product may result, as the side reactions are eliminated. In some cases, the structure of the quaternary ammonium cation may control the product ratio with potentially tautomeric systems as, for example, with the alkylation of 2-naph-thol under basic conditions. The use of tetramethylammonium bromide leads to predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding of the hard quaternary ammonium cation with the hard oxy anion, whereas with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the binding between the cation and the oxygen centre is weaker [11], Similar effects have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The stereochemistry of the Darzen s reaction and of the base-initiated formation of cyclopropanes under two-phase conditions is influenced by the presence or absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium salts are capable of influencing the enantioselectivity of several nucleophilic reactions (Chapter 12). 

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

The data discussed in Sections 8.5 and 8.6 make it clear that in the low-dielectric media typically employed for polymerisation reactions, the counteranions in metallocene ion pair catalysts are closely associated with the cationic complex as either inner-sphere or outer-sphere ligands. If anions are coordinated in the transition state, they must be expected to exert a significant influence on the stereochemistry of alkene polymerisation, even though the formation of syndiotactic and isotactic 1-alkenes have been readily explained by considering only the cationic metallo-cenium species and their ligand structure [21, 23, 122, 132, 133]. 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

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