Ion-pairing processes

The energy transferred by the photon or electron to the molecule may be lost by the latter in a number of different ways. If this energy is lower than the ionization threshold, the usual pathways are (1) reradiation or (2) dissociation into neutral fragments or charged fragments (from ion-pair processes) in other words ... 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

Two genuine electrostatic non-stoichiometric theories were developed one by Stahlberg and co-workers and the other by Cantwell and co-workers. They considered the processes involved in IPC as based solely on the formation of the electrical double layer and disregarded the ion-pairing process in the bulk eluent. 

Chapter 2) details the peculiarities of the ion-pairing process of hydro-phobic ions and it is clear that interactions other than electrostatic are essential to quantitatively describe the duplex formation that is not even conceivable in... 

In another methodology, a similar ion-pairing process is involved [10]. However, in this case the sample preparation is described for fresh material. The (fresh) cactns material is pnlped and extracted with methanol/ammonia (as in the previous example), or in aqneons bnffer at pH 4.0. An ocatdecyl silyl (ODS) stationary phase and a mobile phase of 5.0 mM of aqueous octylamine o-phosphate... 

The data collected in Table 2 show that the three electrodes based on the Foe /Foe, Coc /Coc and BCr /BCr systems are equivalent. When the experiments are carried out in different solvents, including those which exhibit low dielectric permittivity, one should take into account the possibility of ion-pair formation by cations of the reference system Foe, Coc or BCr with anions of the background electrolyte. Therefore, when the behavior of one system is analyzed in several solvents the potentials should be corrected for the ion-pairing process, using (if known) the stability constant of such species or making a theoretical estimate. 

Such data, both in single and mixed solvents, should be carefully corrected for the double layer influence. The ion-pairing processes of reactant ions in the solvents should be studied more widely and carefully, because they may significantly influence both thermodynamic and kinetic parameters of electrode reactions. 

A detailed study was carried out on (benzophenonylmethyl)-tri- -butylammonium triphenylbutylborate (BTAB). Nano- and picosecond laser photolysis demonstrated electron transfer from the borate counteranion to the excited triplet state of the benzophenone moiety. This leads to formation of a benzophenone moiety and a boranyl radical that dissociates rapidly to form butyl radicals. In the nonpolar solvent benzene the short lifetime of the triplet state (300 ps) suggests an intra-ion-pair process. The addition of 1 % MeCN caused an increase in the triplet lifetime to 1.2 ns, suggesting formation of a solvent-separated ion pair. For a lO" m solution in neat MeCN triplet decay is a function of tetrabutylammonium triphenyl- -butylborate concentration. 

The propagation reaction in the 1,1-diphenylhexyllithium initiated polymerization (toluene, —30°C) shows simple first order kinetics in monomer and initiator over a limited range of concentrations [174], The precipitant-soluble (and presumably inactive) polymer chains always form, in this case, a constant fraction of the added initiator. These represent about 80% of the potential polymer chains, fep values evaluated on the basis that the active chain concentration is always 20% of that of the added initiator are shown in Fig. 19. The results correspond to an activation energy of 5.0 kcal mole" together with a pre-exponential factor of 10 . Extrapolation to 25°C leads to a value of fep of 20 1 mole" sec". The values are reasonable for an ion-pair process, but may represent minimum values since it is not sure that all high polymer chains are active. Figure 19 also includes a point at —60°C determined from data... 

The last seven years witnessed the development of separator columns with high efficiencies, which resulted in a significant reduction of analysis time. In addition, separation methods based on the ion-pair process were introduced as an alternative to ion-exchange chromatography, since they allow the separation and determination of both anions and cations. 

Ion Pairing Upon dissolution, the metal ion can coordinate with a variety of counterions in the aqueous layer. This ion-pairing process can be rationalized by means of the Lewis acid-base concept. When the metal ion (the Lewis acid, that is, a species that accepts an electron pair) and the counterion (the Lewis base, that is, a species that donates an electron pair) interact, the electrons involved form a covalent bond. Lewis acids and bases are more likely to coordinate if their electrons possess similar properties. So-called soft acids or bases have valence... 

The tetra-cationic species [M-TRP]" + interacts strongly with the tetra-anionic [M -TPPS]" molecules by electrostatic and r-interactions. This ion-pairing process is directed by molecular recognition as can be inferred from the structural and electric charge complementarity of those species (Figure A face-to-face dimer is initially formed when they are... 

These theories are based on the interaction of the solute ion with the charged surface layer established by the adsorbed counterion and by adsorbed competing ions. The nonstochiometric models apply the Poisson-Boltzmann equation to estimate retention from an electrostatic point of view. The electrical double-layer model applied uses different approaches such as liquid partition , surface adsorption, diffuse layer ion-exchange , and sru face adsorption doublelayer models. It is not possible to draw conclusions about the ion pair process from chromatographic retention data, but each model and theory may find use in describing experimental results under the particular conditions studied. 

Iron(lll) and other metals can form insoluble complexes with phosphate anion. It is likely that surface metal ions and/or colloidal iron are combining with one or more phosphate groups on the nucleic add fragments. If this happens, the ion-pair process chromatographic separation can be interrapted, causing the peaks to broaden. In extreme cases, it is possible that this metal contamination is so severe that nucleic acid fragments are completely prevented from eluting from the system and no peaks are detected. 

Electron impact (E<50 eV) on NF3 (p = 5x10" Torr) gaveNF" in addition to F"and Fgjfor E = 50 eV, the relative abundances were F" F2 NF =1000 5.0 0.7. The ionization efficiency curve of NF" exhibited two appearance potentials indicating two processes for NF" formation The dissociative capture process NF3 + e" NF + 2F forming NF" with little or no kinetic energy occurred at zero electron energy, the ion-pair process NF3 + e" NF" +F + F + e" presumably corresponded to the appearance potential at 23.0 eV [27]. 

The ionisation phenomena in aqueous solutions can hardly be described by the above mentioned theories due to the complexities of the pressure-dependence of the solvent-structure. However in less structured media, e.g. low polar aprotic solvents, a quantitative description of pressure effects on equilibria and rates of ion-pairing processes should be feasible. More specifically, with the solvent considered as a continuum the permittivity and viscosity of which increase with pressure, we may predict an enhancement of dissociation of the ion-pair. Due to the higher viscosity the mobility of the ions is decreased and this means, from the viewpoint of dynamics, a decrease in recombination rate of the free ions with increasing pressure. 

Electrolyte solutions in low polar media are characterized by a very complex conductance behaviour mainly due to the long range of Coulombic interaction. However, at low electrolyte concentration, a simple ion-pair dissociation into free ions is the predominant process. This situation is experimentally come out by the reciprocal relation between equivalent conductance and the square root of electrolyte concentration. In relaxation experiments this simple behaviour is likewise characterized by a linear dependence of the reciprocal relaxation time of the ionisation equilibrium on the square root of total concentration. It is therefore relatively easy to delimit the experimental conditions within which the simple ion- pairing process is present and not obscured by the progressive emergence of higher ionic aggregates, e.g, triple ions, quadrupoles. .. 

Virtually all of these reactions depend on solvation of the ion pair to make the reactions exothermic. As such, Gj reactions are properly a subclass of Gs processes (see Section 6.15 and Ref Ic). We have maintained the distinction here to emphasize the relationship between radical and ion pair processes. 

A S°, contributions of the ion pairing process are subtracted from the overall entropy change, A. S° ... 

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