As ∆Er becomes more negative, an earlier TS and lower energy barrier is expected, in line with Hammond's postulate -as depicted by the vertical movement of the product parabola relative to the reactant (Figure ). Truncating eq 1 at first order and introducing a proportionality constant, 𝛼, recovers the Bell-Evans-Polanyi (BEP) principle (eq 2), in which the activation barrier (∆E ‡ ) is assumed to vary linearly with the reaction driving force (∆Er) between two reactions.