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The authors investigate the fundamental quantum mechanical origin of chemical activation barriers by analyzing collinear H$_2$ + H$^\bullet$ hydrogen atom transfer on its Born-Oppenheimer potential energy surface. To bypass the limitations of classical electrostatic interpretations in chemical kinetics, they extended chemical bond energy component analysis to track electronic kinetic energy changes across the reaction coordinate. Their findings establish that Pauli exclusion is a strictly necessary condition for the existence of the transition state barrier, a conclusion independently validated via absolutely-localized molecular orbital (ALMO) decomposition.
Transition state barriers are fundamentally quantum rather than electrostatic: without Pauli exclusion, collinear H$_2$ + H$^\bullet$ hydrogen atom transfer exhibits no activation barrier at all.
The transition state barrier is a foundational concept in chemical kinetics, yet its physical origin is seldom investigated. In this study we analyze the barrier to hydrogen atom transfer in collinear H$_2$+H$^{\bullet}$ on its Born$-$Oppenheimer surface. We extend upon the energy component analysis of chemical bonds to develop an electronic kinetic energy-based interpretation of the transition state barrier. This approach reveals that Pauli exclusion is necessary for the existence of the energy barrier to H$_2$+H$^{\bullet}$ atom transfer, which is consistent with a complementary analysis using absolutely-localized molecular orbitals.