Why are proton transfers at carbon slow? Self-exchange reactions.

When 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 CH(3)H + (-)CH(3), H(2)O + OH(-), and (+)NH(4) + NH(3) self-exchange reactions. The effect of electron-withdrawing substituents and the case of cation radicals are discussed within the same framework taking the O(2)NCH(2)H + CH(2)=NO(2)(-) and (+.)H(2)NCH(2)H + (.)CH(2)NH(2) as examples. Illustrated by the CH(2)=CH-CH(2)H + (-)CH(2)-CH=CH(2) 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 O(2)N-CH(2)=CH-CH(2)H + CH(2)=CH-CH=NO(2)(-) and (+.)H(2)N-CH(2)=CH-CH(2)H + (.)CH(2)-CH=CH(2)-NH(2) couples as examples.