Publication
555
J. Am.
Chem. Soc., 124 (30), 8798 -8799,
2002
DOI: 10.1021/ja026804r S0002-7863(02)06804-X |
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Dynamics of Proton Transfer at Nonactivated
Carbons from Laser Flash Electron Photoinjection
Experiments
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Jean
Gamby, Philippe Hapiot and Jean-Michel
Savéant
Contribution from the Laboratoire
d'Electrochimie Moléculaire de l'Université Denis
Diderot (Paris 7), UMR CNRS 7591, 2 place Jussieu,
75251 Paris Cedex 05, France
The investigation of proton exchange dynamics at carbon atoms has been
so far limited to molecules activated by an electron-withdrawing substituent
or by the removal of one electron yielding the corresponding cation radical.
A method is proposed to overcome this limitation and extend the gathering
of data to nonactivated carbon acids, RH. It consists of using photoinjected
electrons to generate the radical R· from a rapidly or concertedly
cleaving substrate, RX. The variations of the radical "polarogram" (in
which R· is converted into R-) upon addition of an
acid are then exploited to derive the protonation rate constant of R-.
The method is demonstrated with the example of the diphenylmethyl carbanion.
The Brönsted plot thus obtained indicates that proton transfer to
this carbanion is intrinsically slow, with a barrier on the order of
1 eV. An inverted region behavior seems to appear at large driving forces. |
