Charge transfer to the solvent

This type of charge reduction by charge transfer to the solvent molecule occurs in general when SI are polar solvent molecules of aprotic character such as dimethyl-sulfoxide, dimethyl formamide, and acetonitrile. Protic solvents such as water lead to charge reduction which involves an intracluster proton transfer reaction ... 

Franck and Scheibe (8) were the first to interpret the absorption spectra of halide ions in solution as due to a charge transfer to the solvent (CTTS). The solvation configuration around the ion cannot change during the transition. Therefore the energy required for the transition at the band maximum ... 

Solvation of the halide ions in various solvents has been studied by measuring the chemical shifts for Cl , Br , and 1 in water and several other polar solvents [23]. The chemical shifts are strongly solvent dependent and correlate to the UV absorption band energy corresponding to charge transfer to the solvent. 

Solutions of ferrocene (Fn) in CCI4 show a new band in the electronic spectrum due to charge transfer to the solvent 73> [see section B],... 

Figure 6. Dynamics ofprimary electron-transfer processes triggered hy the femtosecond UV excitation of an aqueous sodium chloride solution ([H20]/[NaCl] = 55). The different steps of an electron photodetachment from the halide ion (Cl ) involve charge transfer to the solvent state (1,2), transient electron-atom couplings (4, 5), and the nonequilibrium state of excess electrons (3). The final steps of the multiple electron photodetachment trajectories (6, 7) are also reported. These data are obtained from time-resolved UV-IR femtosecond spectroscopic data published in references 85 and 86.

To obtain a molecular understanding of the nature of coordinate r, one can probe the charge transfer to the solvent itself (Barthel et al., 2001). 

Anion Solvent Ref. Charge transfer to the ion Cage geometry1) Pox- Pohj Interaction1) energy -AE... 

Similar interface to that used for ESI. In APCI, a corona discharge is used to ionize the analyte in the atmospheric pressure region. Ions are formed by charge transfer from the solvent as the solution passes through a heated nebulizer into the APCI source... 

Born pointed out that a mole of gaseous ions of radius r and charge ze would be similarly stabilized on transfer to the solvent, the work difference being —AG°olv. Strictly speaking, the foregoing argument is not really applicable to ions, especially if z = 1, since one cannot charge up an ion with increments less than e, but the result is still valid ... 

Symmetry requirements for concerted reductive elimination of dialkyls have been considered (314), and for trialkylgold(III) species reductive elimination from a trigonal, three-coordinate intermediate was found to be symmetry forbidden. Solvent participation or the involvement of T-shaped species, however, was suggested as possible. Charge transfer to the high-oxidation state gold(III) center and reductive elimination from such a charge transfer state was proposed as an alternative reaction pathway. 

Talamoni and coworkers17 found that in methanol, allylamine has no influence on the yield of 0-Ps. Other amines, such as w-propylamine and triethylamine, increase the yield of the o-Ps. The authors ascribe this difference to the higher ionization potential of allylamine, making positive charge transfer from the solvent ions (the holes) ineffective. The ionization potential of allylamine (9.6 V) is considerably higher than that of other amines (w-propylamine 8.8 eV, triethylamine 7.8 eV, aniline 7.7 eV). They found a correlation between the ionization potentials and the enhancement factors. In water, allylamine also enhances the formation of 0-Ps, due to the much higher ionization potential of water, 12.6 eV (while the value for methanol is only 10.8 eV). 

Change in solvent polarity has been shown to alfect the relative contribution of exciton resonance and charge transfer to the stabilisation of excited complexes (Eunice et al., 1979). It was found, for example, that the quenching of the fluorescence of anthracene by amines and phosphines in nonpolar solvents showed a better correlation between log and the singlet energy of the quencher than with the oxidation potential of the quencher. The reverse is true when polar solvents are used, showing, as had been postulated in earlier work (Davidson and Lambeth, 1969), that charge transfer is important in such solvents. 

Electrospray ionization is similar in effect to the thermospray technique and is useful for similar applications. The difference resides in the use of a high electric field to nebulize the sample solution (or sample and eluant), creating droplets with excess electric charge. As the droplet solvent evaporates during traverse of a desolvation chamber, charge transfers to the analyte molecules and these are released as gaseous ions. A further refinement in this technique is the use of electronic lenses to direct ions more efficiently into the mass spectrometer. Because the analyte is not subject to heating, there is also less possibility for thermal decomposition of complex lipid components. 

Despite improvements in experimental techniques, the fundamental processes in radiation-induced cationic polymerizations remain largely hypothetical. Pulse-radiolysis studies - on styrene solutions have led to the conclusion that charge transfer from the solvent produces a styrene cation-radical which then dimerizes to form both associated dimer cation-radicals and bonded dimer cation-radicals. These initial steps are thought to be sev al orders of magnitude faster than the subsequrat prop tion reactions. The presence of trace impurities can dictate the course of polymerization, and rate studies provide circumstantial evidence for the theory that nucleophiles can neutralize the cations in these systems and allow free-radical polymerization to occur alone. 

In this section, the results of a computational study 48 will be used to illustrate the effects of the solvent—and the significant complexity of these effects—in quantum charge transfer processes. The particular example... 

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