Pauli exclusion as a necessary condition for the existence of the H${}_2$ + H${}^{\bullet }$ atom transfer barrier

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.