How Does the Intrinsic Barrier of Intermolecular Proton Transfer Depend on Molecular Parameters?

The question in the title is of fundamental importance, because the intrinsic barrier ΔG_int^≠ is the key parameter connecting thermodynamics and kinetics. With a valence bond configuration mixing model for proton self-exchange (see picture), it is shown that ΔG_int^≠ is a linear function of I_v -E(σ_NH^* ), where I_v is the ionization potential of the base and E(σ_NH^* ) the energy of the antibonding orbital of the N-H bond in the conjugated acid. With para-substituted N,N'-dimethylanilines, this manifests itself by a linear relationship between ΔG_int^≠ and the Hammett parameter σ⁺ .