H–C Bond Elongation Induced by Dipolar Rh Adatoms and Vacancies: Site Polarity-Driven Many-Body Cooperativity

Using the Rh-CH4 prototype, we demonstrate that atomic undercoordination shortens the ligand-catalyst (Rh–Rhλ–) bond and polarizes the valence electrons of Rhλ–, creating a dipole to raise the valence density-of-states (DOS). The polarity (λ) sums the spatial charge distribution over the protruding part of the dipole or the excess valence DOS. These additional electrons establish a local electric field that activates the reactant H–C bond through cooperative relaxation of the dipolar-substituted Rhλ-:Hδ+–Cδ− hydrogen bond, where “:” denotes the electron-rich pole. The resultant Rhλ-:Hδ+ attraction and Rhλ-:↔Cδ- repulsion lengthen the reactant H–C bond and thus reduce the activation energy. An R2 value of 1.0 for five scaling curves confirms the precision and robust foundation of the framework, underscoring the dominance of local electronic polarization and bond relaxation without requiring electron exchange during the precursor stage of the chemical reaction. Findings shall improve our understanding of processes such as nanocatalysis, nanotoxicity, or OH– oxidation of living organisms with undercoordination-induced dipolar formation and the site-specific dipolar-reactant many-body cooperativity.