| Popis: |
We have previously shown that in the case of extremely short-lived halocarbon anions RCl − (like CCl 4 − or CFCl 3 − ), one observes the decay of the solvent-separated ion pairs (R + ||Cl − ) instead of the geminate ion recombination between the solvent cation (S + ) and the fragment anion (Cl − ). If the anions live longer (e.g. CHCl 3 − ) then the expected geminate ion kinetics is seen with a primary geminate pair (S + /CHCl 3 − ) and a secondary geminate pair (S + /Cl − ), coupled by the corresponding anion fragmentation rate. Ion pairs like (R + ||Cl − ) represent a product from ion recombination. The t −0.6 kinetic treatment used so far to analyse such geminate ion mechanism now was modified to allow the product from ion recombination to be an optical absorber with e p ≠0. With this theory all the kinetic details of the production of ion pairs (CCl 3 + ||Cl − ) and their decay in the CCl 4 system can now be explained. In order to understand the mechanistic dependence on fast and slow anion fragmentation, the results are compared with additional halocarbons, covering a wider range of anion lifetimes: hexa-, penta- and tetrachloroethane. For CCl 4 the yield of ion pairs is highest and the anion lifetime shortest. The fragment radical CCl 3 is available for charge-transfer from the solvent cations for a very long time. For tetrachloroethane or chloroform the release of the fragment radical R is so late, that the ion pair (R + ||Cl − ) cannot be observed, either due to a rather low efficiency for charge transfer to R, or more likely, because the ion pairs are decaying faster, than they are produced. |
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