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Publication 575
J. Phys.
Chem. A, 107 (38), 7445 -7453, 2003
DOI: 10.1021/jp035108q S1089-5639(03)05108-9 |
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Investigating the Dynamics of Carbanion Protonation by Means
of Laser Flash Electron Photoinjection from an Electrode
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Jean
Gamby , Philippe Hapiot and Jean-Michel Savéant
Laboratoire d'Electrochimie Moléculaire,
Université de Paris 7 - Denis Diderot, Case Courrier 7107,
2 place Jussieu, 75251 Paris Cedex 05, France, and Laboratoire
d'Electrochimie, Synthèse et Electrosynthèse Organiques
- UMR 6510, Université de Rennes 1, Campus de Beaulieu
- Bat. 10C, 35042 Rennes Cedex, France
The investigation of protonation/deprotonation at carbon is traditionally
limited to molecules where acidity has been boosted by introduction of
an electron-withdrawing group or by removal of an electron. These restrictions
can be removed by application of the laser flash electron photoinjection
technique. A thin layer of radicals is initially formed upon reduction
of an appropriate substrate by the photoinjected electrons. The time-resolved
current-potential responses for the reduction of the radicals thus generated
are sensitive to the rate of the protonation of the ensuing carbanion
by purposely added acids. The second-order rate constant may then be
extracted from the half-wave potential versus time data with satisfactory
accuracy in a wide range of values that extends up to the diffusion limit.
The method is demonstrated with the example of diphenylmethyl and benzyl
carbanions. Several observations may be derived from these first illustrating
experiments. There is a large kinetic isotope effect. Proton transfer
is intrinsically slow, showing that this property is not the result of
the presence of an electron-withdrawing group. The intrinsic barrier
is larger in the benzyl case than in the diphenylmethyl case. Unusual
temperature effects (negative activation enthalpy) are observed at least
in some cases, calling for systematic investigation in future studies. |
