Ion-pair relaxation

Detailed kinetic exploration of the relaxation of the initially formed radical ion pair would probably be of most utility in addressing the question of the degree of charge separation in exciplex or in radical ion pairs. Rentzepis and coworkers have recently found that in strongly interacting donor-acceptor pairs, for example, indene-chloranil, the initially formed ion pair relaxes within a few picoseconds to an equilibrated solvated complex (67). Time-resolved resonance Raman spectroscopy has also been used recently as a kinetic monitor for radical ion reactivity (68). 

The Argand diagram of the 2.2 M solution of Et NCl shows three relaxation processes typical for aqueous electrolyte solutions (1) ion-pair relaxation (r l = tip), (2) low frequency relaxation (rj, as 8 ps) of water, (3) high frequency relaxation a 1 ps) of water, in contrast to that of the 2 M solution of Bu NBr where the relaxation process (2) splits up into two processes. Figure 7 shows the concentration dependence of the... 

The analysis of the observed concentration dependence of relaxation times Tjp(c) of the ion-pair relaxation region, reveals two superimposed modes, a diffusional rotation mode of the ion-pair dipoles and a kinetic mode due to the re-establishment of the undisturbed equilibrium = Cd2+, M +)... 

BARTHEL - The existence of superimposed orientational and kinetic modes follows from the concentration dependences of dispersion amplitude, jp(o>c) - Ip(oo,c) and ion-pair relaxation time, jp(c). Models of the ion pair used in connexion with Eqs. (22) and (28) to calculate particle densities make assumptions on the shape and dipole moment of the ion-pair dipoles. For... 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

Chemical models of electrolytes take into account local structures of the solution due to the interactions of ions and solvent molecules. The underlying information stems from spectroscopic, kinetic, and electrochemical experiments, as well as from dielectric relaxation spectroscopy. The postulated structures include ion pairs, higher ion aggregates, and solvated and selectively solvated ions. 

The equivalent conductivity of an electrolyte solution decreases with increasing concentration due to interionic attractions described mainly by the electrophoretic and relaxation field effects 2-35>. This decrease is more pronounced if in addition the electrolyte is associated. Association of ionic salts by ion-pairing is commonly observed in solvents of low or moderate dielectric constant. The immediate goals in the analysis of conductance data are the. determination of the limiting equivalent conductance at infinite dilution, A0, and the evaluation of the association constant, KA, if ion-pairing occurs. 

Blatz and Mohler38 have performed 2D NOE NMR experiments on the protonated f-butylamine Schiff base of all-fraws-retinal using different counterions, each carrying at least one nonexchangeable proton. The study has indicated that a proton on the counterion molecule is spatially close, in aprotic solvents, to the protons of the chromophore near the positively charged nitrogen. It has also shown that the ion-pair formation is relaxed in either the presence of excess carboxylic acid (the counterion) or when using methanol as a solvent. 

Rate constant for reorganization of the ion-pair intermediate, which is assumed to be equal to the dielectric relaxation of water. 

In this connection, attention should be paid to an unusual NMR technique called nuclear magnetic relaxation dispersion (NMRD). In contrast with NMR spectroscopy, the NMRD signal arises from the nuclei of the abundant solvent molecules and not from the dissolved substances. The relaxation properties of the solvent molecules are profoundly modified if the solvent contains paramagnetic particles (see a review by Desreux 2005). A solvent molecule sails in the vicinity of an ion-radical and finds itself in the local magnetic field of this paramagnetic particle. Then, induced magnetism of the solvent molecule dissipates in the solvent bulk. This kind of relaxation seems to be registered by NMR. NMRD is applicable to studies on ion-radical solvation/desolvation, ion-pair dynamics, kinetics of ion-radical accumulation/consumption, and so on. 

The quaternary ammonium compounds paraquat and diquat are widely used non-selective contact herbicides, which are extremely toxic to humans. Fee et al. [112] established an HPLC-MS-MS procedure for the determination of these herbicides in whole blood and urine using ethyl paraquat as internal standard. After extraction with Sep-Pak C18 cartridges, analytes were separated using ion pair chromatography with heptafluorobutyric acid in 20 mM ammonium acetate and acetonitrile gradient elution. Detection was carried out in ESI MS-MS SRM mode. Using similar separation and detection conditions, paraquat, diquat, difenzoquat, and a number of structurally related quaternary nitrogen muscle relaxants (see Section 20.2.1.3) were determined in whole blood by Ariffin and Anderson [113]. 

Recently we discovered [11] that photolysis of the diphenylallyl carbanion with white light causes isomerisation of the trans,trans conformer to the cis,trans (scheme 1). When the source of the illumination is removed, conformational relaxation proceeds at a rate which is markedly sensitive to the nature of the ion pairing, being much slower for loose than for tight ion pairs. The results of an extension of this work will be presented in this paper together with an outline of the possible relevance of this photochemical phenomenon to the stereochemistry of the polymerisation of dienes. 

Conductance relaxation Is also shown to be critically dependent upon aggregation equilibria affecting non-conducting (ion-pairs) as well as Ionic species. The relaxation behavior In the presence of quadrupoles (ion-pair dimers) and triple Ions Is thoroughly analyzed. The experimental results show the potential of the field modulation techniques as a method for the Investigation of ionization processes, independent of conductance measurements. 

At increasing salt concentrations the conductance of all solutions investigated shows the emergence of other equilibria involving the ions and ion-pairs triple ions, quadrupoles. .. become stable species. Althought one expects from chemical relaxation theory the appearance of more relaxation processes, experimentally only one relaxation process, whatever the concentration of salt, is observed. This odd behavior is intimately related to the nature of equlllbriun perturbation and observation of the response in the field modulation method. 

A linear concentration dependence of the reciprocal relaxation time upon total ion-pair concentration would therefore point to a main recombination process between a simple ion and a triple ion. 

Considering these different limiting forms of the recombination term an Important tentative conclusion emerges the concentration dependence of the reciprocal relaxation time is a direct measure of the main ionic recombination process and yields therefore information on the ionic species present in solution. A linear dependence on total ion-pair concentration would therefore indicate unilateral triple ion formation or, if both kinds of triple ions are present as indicated by conductance, a sufficient difference in their stability. At this point it should be noted that the usual method of Fuoss and Draus... 

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