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

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 V_i 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 n_m (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 n_m 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.