Dynamics of proton transfer at nonactivated carbons from laser flash electron photoinjection experiments

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.