Why do some reactions speed up dramatically with slight changes in driving force, while some others appear much less responsive? The interplay between kinetics and thermodynamic driving forces has been a cornerstone of physical organic chemistry. The most elegant aspect of Marcus theory lies in dissecting the reaction barrier into thermodynamic-independent and thermodynamicdependent components. 1 This dissection immediately raises a critical question: what are the "thermodynamic-independent" parameters defining the inherent nature of a family of reactions? This question forms the foundation of different attempts to describing the interplay, leading to different models. However, many previous models rely on either straightforward assumptions, or, in attempts to be more comprehensive, on extensive parameterization of reaction-specific concepts, which greatly restricts the scope and the ability to derive meaningful physical insights from these frameworks due to their more statistical nature. These include models that incorporate bond breaking and bond formation to estimate the energy of the transition state, which often require treatments very specific to one reaction class. Addressing these limitations requires a model that captures the global relationship between activation and reaction energies across the entire thermodynamic range that relies on minimal assumptions. For example, the most experimentally relevant is the Leffler equation (Figure ), which expresses the free energy of activation ∆G ‡ by a combination of the reaction's free energy ∆G and the intrinsic barrier ∆G0 ‡ , which corresponds to ∆G ‡ for ∆G = 0, in the linear approximation, ∆G ‡ = ∆G0 ‡ + α∆G. The Brønsted slope α indicates the sensitivity of the reaction rates to changes in driving force. ∆G0 ‡ and α are intrinsic properties of each family of reactions, presumably independent of thermodynamic driving force. For constant ∆G0 ‡ and α, the Leffler equation is reduced to the Bell-Evans-Polanyi (BEP) principle, describing the kinetic-thermodynamic relationship within the same family of reactions. Although the linear kineticthermodynamic relationship is usually experimentally observed, it is the approximation over a limited range of thermodynamic driving force. As the driving force becomes highly exergonic or endergonic, the linearity breaks down. Brønsted, Bell, Marcus, Jencks, and Shaik have all mentioned that the Brønsted slope must be a local property, representing the tangent to the curve describing the relationship between reaction barrier and reaction energy at a given point. In the extreme limits, the behaviour of α reveals distinct kinetic regimes. For highly exergonic reactions, α→0; conversely, for very endergonic reactions, the rate becomes directly proportional to the driving force, with α→1. BEP plots often show deviations from linearity. These outliers prompt a critical question: do they indicate curvature due to driving forces exceeding the linear range, or do they reflect reactions from distinct families with different intrinsic properties? Analysing Brønsted relationships in their exact non-linear form is essential to addressing this.