Ion pairs formation

The Gibbs energy, G, of interaction between two point charges Qi and Qj, separated over a distance r j, is given by Coulomb s equation  

Charged gronps are almost always located at the protein s snrface. If charges are present in the interior of the molecnle, they occur as ion pairs. Upon nnfolding, such ion pairs are disrnpted and the ionic gronps become hydrated. This is schematically depicted in the following lignre. 

The change in the Gibbs energy of Conlomb interaction due to the disruption of one ion pair in such an unfolding process eqnals  

Taking values of r j and e for the N- and the D-state as given earlier, per mol of ion pair = 26kJ. (Note that because (r, D is mainly 

In solvent-separated ion pairs, the primary solvation shells of the cation and the anion actually remain intact. In solvent-shared ion pairs, a single solvent molecule exists in the space between the 

1 Detention of Unpaired Electron in a Framework of One Specific Molecular Fragment 

one-electron transfer causes cis - trans isomerization. It is quite effective with free migration of an unpaired electron over the molecular framework. It is less effective when the unpaired electron is confined within the limits of a coordination complex. Such fixation of the unpaired electron hinders the rotation around the C=C bond. In a similar manner, a ball tied to a 

The isomerization is more effective when the (nitroanion-radical + alkali counterion) ion pair does not exist. The following needs to be noted It is assumed that intimate ion pairs possess larger electron affinity and are characterized by larger contribution to the free energy of electron transfer than free ions (Grigoriev et al. 2001). 

The potassium salt of the 2,2 -dipyridyl acetylene anion-radical represents another important example. In this case, the spin and charge are localized in the framework of N-C-C=C-C-N fragment. The atomic charge on each nitrogen atom is -0.447, that is, close to unity in total. The energy of this ion pair is minimal when the potassium counterion is located midway between the two rather close nitrogen lone pairs. Such a structure is consistent with the fact that the ESR spectrum of this species is almost insensitive to temperature. It means that the counterion does not hop between two remote sites of the anion-radical (Choua et al. 1999). 

The equilibrium constant X, 2 for formation of the ion-pair (sometimes referred to, less specifically, as Kq ) can be derived from the Fuoss—Eigen equation [59, 60] 

Hiickel activity coefficient, and b is the Bjerrum ratio of potential and kinetic energies, defined as 

Only in the case of Co(III) and Cr(IIl) is it feasible to investigate the outer-sphere properties directly by O resonance. In other cases, Leffler 

Sometimes a dissociation constant is quoted. This expresses the equihbrium in terms of the dissociation of the ion pair  

Most early work was given in terms of the dissociation constant, mainly because of the analogy with acid-base equilibria where acid behaviour was considered in terms of the dissociation of the weak acid, HA(aq), into ions, H30 (aq) and A (aq). However, this analogy must not be taken too far. In the weak acid situation, dissociation of a molecular species, HA(aq), occurs, but in the ion pair situation dissociation is from a species which is not a molecule rather it is a species which is held together by simple coulombic electrostatic interactions. It is vital that this distinction is made clear. 

In modern work the association constant is usually quoted, where  

In the example quoted above, the species present are the free ions, Na (aq) and SO Caq) and the ion pair NaSO (aq), and the Na+(aq) is distributed between free Na (aq) and ion pair, NaS04 (aq). The stoichiometric or total concentration of Na (aq) is given by  

As with acid-base equilibria, a further equation describing electrical neutrality can be written  

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It is important to evaluate the surface distortion associated with the assymetric field at the surface, a difficult task often simplified by assuming that distortion is limited to the direction normal to the plane [64, 6S]. Benson and co-workers [6S] calculated displacements for the first five planes in the (100) face of sodium chloride and found the distortion correction to of about 100 ergs/cm or about half of itself The displacements show a tendency toward ion pair formation, suggesting that lateral displacements to produce ion doublets should be considered [66] however, other calculations yielded much smaller displacements [67]. 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

Ion-pair formation. An ionization process in which a positive fragment ion and a negative fragment ion are the only products. 

This alkyl migration is believed to proceed via ion-pair formation. These and many other simple 0-alkyIated cyclic hydroxamic acids are thermally stablebelow 180°. 

Detailed kinetic studies of the substitution reactions of anions with heterocyclic compounds to include, for example, the effects of solvent, added salts, and ion pair formation have not been made as yet. 

At pH 7, [13]aneN3 or [12]-[15]aneN4 accommodate only two nitrogen-bound protons and these dipositive ammonium cations are apparently unable to provide sufficient electrostatic attraction to polycarboxylate anions for ion-pair formation. In contrast, the macrocyclic spermines, pentaamines and hexaamines accommodate more than three nitrogen-bound protons at pH 7 and for these ligands 1 1 associations... 

Steadman, J., and Syage, J. A. (1991). Time-resolved studies of phenol proton transfer in clusters. 3. solvent structure and ion-pair formation. J. Phys. Chem. 95 10326-10331. 

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

UV spectra are, however, very useful for the determination of acid-base and ion pair formation equilibria, and for photochemical investigations (e. g., determination of quantum yield in photolytic dediazoniation, Tsunoda and Yamaoka, 1966 fluorescence and phosphorescence at low temperature, Sukigahara and Kikuchi, 1967a). 

The proton is not the only entity that can dissociate from a substrate or bond to it. We can enumerate other interactions, such as metal-ligand complexation, ion-pair formation, charge-transfer complex formation, etc. For the sake of brevity, we treat all of these as... 

This reaction scheme agrees with the experimental rate equation, with k = k 2ftk 2 i/k-126 and k = k 21/k-126- The rate constant at a high [SO42-] would plateau at the value of Jk 26. On the other hand, one can consider ion-pair formation ... 

An alternative view that also favors an intermediate mechanism is that of Schleyer and co-workers, who believe that the key to the problem is varying degrees of nucleophilic solvent assistance to ion-pair formation. They have proposed an Sn2 (intermediate) mechanism. ... 

A brief discussion of the systematics of solvent effects on the p/, pr, and values of Tables II and III is presented in the discussion section. However, it is worthy of note here that sets 7, 37, 38, 39, 40, and 41, which involve nonhydroxylic solvents, are fitted with comparable precision to that for reaction series in aqueous or mixed aqueous organic solvents. The present analysis does not support the previous assignment (7b) of ion-pair formation of benzoic acids... 

Figure 1. Ion-Pair Formation, ArCO H + (C HsKHI C = NH, Benzene 25°... 

Polyphosphazenes sulfonates XIX with the anion covalently attached to the polymer are a new class of cation conductors that have been synthesized by Shriver [625]. They were obtained by reaction of Na0C2H4S03Na with an excess of polydichlorophosphazene in the presence of 15-crown-5, followed by the reaction of the partially substituted product with the sodium salt of poly(ethylene glycol methyl ether). The conductivity at 80 °C of the polymer with x=1.8, m=7.22 is 1.7x10 S cm This low conductivity can be attributed to an extensive ion pair formation between the sodium and sulfonate ions. 

Direct measurement of adsorptive stripping voltaimnetric peaks using HMDE 0.60 V and accumulation potential of -0.40V Dilution in phosphate buffer and water, analyzed in Vis region Ion pair formation with octadecyltrimethylammonium bromide at pH 5.6, extraction of ion pair into n-butanol Sample solution mixed with 1 M HCl, ethanol and purification on Sephadex DEAE 25 gel, gel beads are filtered off, packed into 1 nun cell and absorbance measured... 

All these methods give similar results but their sensitivities and resolutions are different. For example, UV-Vis spectrophotometry gives good results if a single colorant or mixture of colorants (with different absorption spectra) were previously separated by SPE, ion pair formation, and a good previous extraction. Due to their added-value capability, HPLC and CE became the ideal techniques for the analysis of multicomponent mixtures of natural and synthetic colorants found in drinks. To make correct evaluations in complex dye mixtures, a chemometric multicomponent analysis (PLS, nonlinear regression) is necessary to discriminate colorant contributions from other food constituents (sugars, phenolics, etc.). 

Lau, O.W. et al., Spectrophotometric determination of single synthetic food colour in soft drinks using ion-pair formation and extraction, Int. J. Food Sci. TechnoL, 30, 6, 793, 1995. 

The high dielectric constant of water normally militates against the formation of ion-pairs for simple salts because a high dielectric constant reduces the strength of the electrostatic forces. The phenomenon is more readily observed in solvents of low dielectric constant for a typical mono-monovalent salt, ion-pair formation takes place only when the dielectric constant is less than 41 (Fuoss Kraus, 1933). 

Ion-pair formation lowers the concentrations of free ions in solution, and hence the conductivity of the solution. It must be pointed out that ion-pair formation is not equivalent to the formation of undissociated molecules or complexes from the ions. In contrast to such species, ions in an ion pair are linked only by electrostatic and not by chemical forces. During ion-pair formation a common solvation sheath is set up, but between the ions thin solvation interlayers are preserved. The ion pair will break up during strong collisions with other particles (i.e., not in all collisions). Therefore, ion pairs have a finite lifetime, which is longer than the mean time between individual collisions. 

For ion-pair formation the electrostatic attraction energy = N iZ j(Q°yi4neQer (per mole of ion pairs) should be larger than the ion pair s mean thermal energy (i.e., at least 2RT). This condition yields for the critical distance of ion pair formation in aqueous solutions at 25°C ... 

Ion-pair formation (or the formation of triplets, etc.) is a very simple kind of interaction between ions of opposite charge. As the electrolyte concentration increases and the mean distance between ions decreases, electrostatic forces are no longer the only interaction forces. Aggregates within which the ions are held together by chemical forces have certain special features (i.e., shorter interatomic distances and a higher degree of desolvation than found in ion pairs) and can form a common solvation sheath instead of the individual sheaths. These aggregates are seen distinctly in spectra, and in a number of cases their concentrations can be measured spectroscopically. 

Gutfelt et al. (1997) have evaluated various ME formulations as reaction media for synthesis of decyl sulphonate from decylbromide and sodium sulphite. The reaction rate was fast both in water-in-oil and in bicontinuous ME based on non-ionic surfactants. A comparison was made with this reaction being conducted in a two-phase. system with quats as phase-transfer catalyst but was found to be much less efficient. However, when two other nucleophiles, NaCN and NaNOj, were used the PTC method was almost as efficient as the ME media. It seems that in the case of decyl sulphonate there is a strong ion pair formation between the product and the PTC. The rate in the ME media could be further increased by addition of a small amount of a cationic surfactant. 

Cools, A. A., Janssen, L. H. Influence of sodium ion-pair formation on transport... 

Recently, the abilities of primary to tertiary alkylammonium ions with Me, Et, and -Bu groups to transport water to NB have been studied [48]. As the result of careful consideration of the ion-pair formation, it has been shown that the hydration numbers (w ) of the ammonium ions in NB, being little affected by the alkyl chain length, are simply dependent on the class of the ammonium ion % = 1.64, 1.04, 0.66, respectively, for the primary, secondary, and tertiary ammonium ions. 

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. 

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