Publication 583

J. Am. Chem. Soc., 126 (45), 14787-14795, 2004 DOI: 10.1021/ja046467h S0002-7863(04)06467-4
Why Are Proton Transfers at Carbon Slow? Self-Exchange Reactions

Contribution from the Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université, CNRS No. 7591, Université de Paris 7, Denis Diderot, 2 place Jussieu, 75251 Paris Cedex 05, France

hen the quantum character of proton transfer is taken into account, the intrinsic slowness of self-exchange proton transfer at carbon appears as a result of its nonadiabatic character as opposed to the adiabatic character of proton transfer at oxygen and nitrogen. This difference is caused by the lesser polarity of C-H bonds as compared to that of O-H and N-H bonds. Besides solvent and heavy-atom intramolecular reorganizations, the kinetics of the reaction are consequently governed at the level of a pre-exponential term by proton tunneling through the barrier. These contrasting behaviors are illustrated by an analysis of the CH₃H + ^-CH₃, H₂O + OH^-, and ⁺NH₄ + NH₃ self-exchange reactions. The effect of electron-withdrawing substituents and the case of cation radicals are discussed within the same framework taking the O₂NCH₂H + CH₂=NO₂^- and ⁺H₂NCH₂H + CH₂NH₂ as examples. Illustrated by the CH₂=CH-CH₂H + ^-CH₂-CH=CH₂ couple, it is shown that the "imbalanced character of the transition state" is related to heavy-atom intramolecular reorganization. Combination of these various effects is finally analyzed, taking the O₂N-CH₂=CH-CH₂H + CH₂=CH-CH=NO₂^- and ⁺H₂N-CH₂=CH-CH₂H + CH₂-CH=CH₂-NH₂ couples as examples.