A Semi‐Empirical Model of the Energy Barrier of Proton Transfer Reactions

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Abstract
The energy barrier in proton transfer reactions is described by a Johnston‐type equation (1) (n = order of bond to be broken). The barrier model is discussed in terms of free energies. The Vi values are free energies of ionic cleavage in aqueous solution of the XH and YH bonds; they are computed from eqns. (4c) and (4d). The values of p1 and p2 affect curvature (absence or presence of maximum) and symmetry of the barrier. It is postulated that pi is a typical constant of the reacting bond and can be transferred from one transition state to another.With the aid of eqn. (1) and its first derivative, values of pi and nm (bond order at maximum of barrier) can be based on quantities determined experimentally, Δ≠ and ΔG. For OH bonds, pi ≈ 1.0. For CH bonds pi is larger than 1.0 and depends on the structure of the carbanionic moiety (influence of resonance and inductive effects).As there cannot be a maximum if p1 = p2 = 1.0, the suggested model of the barrier leads to a better understanding why proton transfer must be ‘fast’ in some reactions and ‘slow’ in others. The computed values of nm may be utilized to gain some insight into the nature of the transition states; they supply a basis for the discussion of primary hydrogen isotope effects.

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