Ion pair interconversion

For many chemical reactions with high sharp barriers, the required time dependent friction on the reactive coordinate can be usefully approximated as the tcf of the force with the reacting solute fixed at the transition state. That is to say, no motion of the reactive solute is permitted in the evaluation of (2.3). This restriction has its rationale in the physical idea [1,2] that recrossing trajectories which influence the rate and the transmission coefficient occur on a quite short time scale. The results of many MD simulations for a very wide variety of different reaction types [3-12] show that this condition is satisfied it can be valid even where it is most suspect, i.e., for low barrier reactions of the ion pair interconversion class [6],... 

Ion pair interconversions are of considerable interest in connection with the kinetics of inorganic ionic association and conductivity. Here we are concerned with the kinetics of the interconversion process between the CIP and the SShIP configurations (see Figure 22),... 

Table 8 TST Rate Constants (in ps ) and Constant of Equilibrium for the High Temperature Ion Pair Interconversion (Tr = 1-05)...

Table 9 TST Rate Constants (in ps" ). Constant ofEquilibrium, and Transmission Coefficient for the Na -Cl Ion Pair Interconversion in Ambient Aqueous Solution at Infinite Dilution...

JCS(P1)2673 89H1121]. As proven by crossover experiments, interconversion of the tautomers occurs intermolecularly, an ion pair 37c being the intermediate in the dissociation-recombination mechanism of the rearrangement. 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

Reactions with Radicai ions of Like Charge. In the final section, we briefly mention reactions between radical ions of like charge. One of the longstanding problems of radical ion chemistry involves the actual stmcture of ketyls. Following extensive conductivity and magnetic susceptibility studies in the 1930s detailed ESR and optical studies have demonstrated the existence and interconversion of at least four distinct species a free ketyl anion a monomer ion pair a... 

The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l -4). For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7.) Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8). 

Other probes of the initial distribution have been tried. A few studies of the time-dependent ion-pair recombination probability have been made recently with picosecond pulse radiolysis equipment. A magnetic field alters the rate of interconversion between a triplet and singlet ion-pair. If this rate is fast enough to compete with the recombination rate of ions, the yield of recombined ion-pairs is markedly affected by the magnetic field. 

The simultaneous presence of two types of ion pairs was revealed in the above discussed investigation by the two sets of hyperfine lines appearing in the ESR spectrum. This was possible because interconversion... 

Although the ESR studies of ion pairs give the most detailed information about such systems and provide valuable data about the thermodynamics, as well as about kinetics of the interconversion, other methods of investigations are also important. An interesting spectrophotometric technique was developed in our laboratory by Smid and Hogen-Esch (21). Their work led to the understanding of various factors which affect the ability of ethereal solvents to interact with alkali ions of fluorenyl salts. 

In the past, NMR spectroscopy has not been a useful technique to investigate structure and dynamic behavior of ion-pairs. Results of such studies indicated that interconversion of the different ion-paired species present was fast enough to average the spectra for the different species present41. However, recently NOE experiments, pioneered by Pochapsky and coworkers42, have been successfully applied to map some ion-paired salts. Further, it has been found that at sufficiently low temperatures, interconversion and reorganization processes within and between ion-paired salts become slow enough to reveal NMR spectra of the ion-pairs not perturbed by dynamic effects. 

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