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 SCF3-transfer reagents
provides external validation.