Ion-pairs lifetimes

The quenching rates within the micelle were so reduced that detection of product ions proved to be possible even 100 ms after excitation. That ion pairing was significant for the expanded lifetimes was established by the observation that the ion pair lifetime was sensitive to added electrolyte, to the hydro-phobicity of the reduced product, and to the charge on the surfactant. ... 

The lifetimes of the charge separated state in degassed toluene for 1 through 4 are 5, 104, 150, and 300 ns, respectively, as determined by time-resolved transient absorption experiments [60-62], As Table 3 illustrates, this trend is preserved in the liquid crystalline environment, with the caveat that the ion pair lifetimes are increased by at least one order of magnitude. This variation allows for the study of photorefractivity as a function of the lifetime of the intramolecular charge separated state. 

Quantitation of the impact of longer ion pair lifetimes on intermolecular charge separation can be made considering the collision frequency (Z) of the ion pairs and neutral dopants in the liquid crystal. The relations Dda = kBT/6nr a and Z = 4kBTNA/3r can be utilized to obtain the relation Z = 8DtwNa. Here, Dda is the diffusion constant of the neutral dopant, Na is the number density of the dopant, a is the radius of the dopant, and q is the viscosity [64], Dda can be estimated to be equal to the self-diffusion constant of the liquid crystal, which is 3 x 10-6 cm2/sec, a 10A, and the optimal dopant concentration for 3 is 4.3 x 1017 molecules/cm3. This gives a collision frequency of 3.2 x 106 sec-1, or... 

In concentrated salt solutions, the vapor pressure is lower than that of pure water, and hence it exhibits reduced water activity. This phenomenon is explained by the fact that a considerable fraction of the water molecules are associated with the hydration of the salt ions. The binding energy of these water molecules (which forms the first and the second hydration shells) to the center ion is larger than 10 kcal/mol therefore, they are less likely to participate in the hydration of the newly formed proton. To observe successful proton dissociation, the thermodynamic stable complex must be formed within the ion-pair lifetime. The depletion of the solution from water molecules available for this reaction will lower the probability of the successful dissociation. As demonstrated in Figure 9, this function decreases with the activity of the water in the solution. 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

The validity of the above conclusions rests on the reliability of theoretical predictions on excited state barriers as low as 1-2 kcal mol . Of course, this required as accurate an experimental check as possible with reference to both the solvent viscosity effects, completely disregarded by theory, and the dielectric solvent effects. As for the photoisomerization dynamics, the needed information was derived from measurements of fluorescence lifetimes (x) and quantum yields (dielectric constant, where extensive formation of ion pairs may occur [60], the observed photophysical properties are confidently referable to the unperturbed BMPC cation. Figure 6 shows the temperature dependence of the... 

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. 

On the other hand, oxidation of a DNA base by a triplet state of the an-thraquinone (AQ5"3) generates a contact ion pair in an overall triplet state, and back electron transfer from this species to form ground states is prohibited by spin conservation rules. Consequently, the lifetime of the triplet radical ion pair is long enough to permit the bimolecular reaction of AQ- with 02 to form superoxide (02 ) and regenerate the anthraquinone. 

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

Such variation in the lifetimes of the ion pairs, which depends on the mode of activation, primarily arises from the difference in the spin multiplicities (see above). None the less, the long-lived ion-radical pair allows the in-cage proton transfer from the cation radical ArMe+ to the CA- anion radical to effectively compete with the back electron transfer,205 i.e.,... 

The lack of stereospecificity in equation (72) (as compared to equations (70) and (71)) is readily accommodated by the long lifetime of the triplet ion pair produced by the sensitized irradiation method. The latter allows the facile ring opening of DBC+ cation radical followed by isomerization of the resulting xylylene cation radical, i.e.,... 

Solvent-dependent lifetimes and ion-pairing of these intermediates can be responsible for the observed variations in the stereo- and chemo-selectivity. Assuming that bromonium ions and carbocations are formed in discrete pathways, the influence of these factors can be readily understood. On the one hand, bridged ions react stereospecifically whatever the medium the... 

O-Substituted phenylhydroxylamines also undergo rearrangement to give the 2-isomers. For example O-(arenesulphonyl) phenylhydroxylamines 47 readily form the 2-sulphonyl derivatives 48. Experiments with 180-labelled compounds led to the suggestion54 of a mechanism involving an ion pair which has only a very short lifetime. 

The regio-, stereo- and chemoselectivities have been mainly interpreted in terms of bridging of the ionic intermediate and/or ion pair dissociation. Solvent-separated ion pairs and free ions have often been considered to explain the product selectivities of these reactions. Nevertheless, the stereochemical outcomes can also be determined by the relative rates of the ion pair dissociation and of the nucleophilic trapping of the intermediate, i.e. by the lifetime of the intermediate . 

W. J. Vining, J. V. Caspar, and T. J. Meyer, Influence of environmental effects on excited state lifetimes. The effect of ion pairing on metal-to-ligand charge transfer excited states, J. Phys. Chem. 89, 1095-1099(1985). 

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