Evidence for inverted region behavior in proton transfer to carbanions

The diphenylmethane-diphenylmethyl anion acid/base couple in N,N-dimethylformamide is taken as an example for investigating the dynamics of proton transfer at carbon in a system where the acid is not activated by an electron-withdrawing group or by removal of an electron. The laser flash electron photoinjection technique is applied to the determination of the rate constant for the protonation of diphenylmethyl anion by an extended series of acids that offers a range of driving forces encompassing over 1.2 eV. The plot of the rate constant versus the pK(a) difference between diphenylmethane and the acids or of the activation free energy versus the standard free energy of the reaction exhibits clear "inverted region" behavior (by a factor of 80 in terms of rate constants). While such behaviors have been predicted and observed for outersphere electron-transfer reactions, previous evidence for proton-transfer reactions was scarce. Entropic factors, derived from an investigation of the temperature dependence of the experimental rate constants, are also discussed.