Reaction-Class-Dependent Intrinsic Barriers Unify Deviant and Multimodal Bell-Evans-Polanyi Behavior in Polar Group Transfer Reactions.

The Bell-Evans-Polanyi (BEP) relationship is a foundational principle linking reaction kinetics and thermodynamics and is widely used to analyze and predict reactivity in group transfer reactions. However, substantial deviations from a single BEP correlation are frequently observed for structurally diverse reagents, limiting its general applicability. Here, by integrating experimental kinetic data with quantum-chemically derived intrinsic barriers obtained from self-exchange reactions, we demonstrate that systematic, reaction-class-dependent variations in intrinsic barriers provide a physically transparent explanation for the breakdown of single BEP correlations across electrophilic fluorination, trifluoromethylthiolation, and hydride transfer reactions. On this basis, we introduce an intrinsic barrier augmented linear free energy framework that diagnostically unifies disparate BEP regimes while remaining consistent with Marcus-type barrier decomposition. This framework captures reaction-class-specific sensitivities to intrinsic barrier and thermodynamic driving force, affording improved quantitative agreement with experiment relative to conventional BEP analysis. Beyond rationalizing outliers and multimodal BEP behavior, this framework enables physically transparent prediction of Mayr electrophilicity and nucleophilicity parameters across structurally diverse reagents. Independent experimental determination of electrophilicity parameters for previously unreported fluorine- and SCF-transfer reagents provides external validation.