Bridging Electron and Nuclear Motions in Chemical Reactions through Electrostatic Forces from Reactive Orbitals

This study presents a physics-based framework for understanding chemical reactions, highlighting the critical role of the occupied reactive orbital (ORO), the most stabilized occupied orbital during a reaction, in guiding atomic nuclei via electrostatic forces. These forces, termed reactive-orbital-based electrostatic forces (ROEFs), arise from the negative gradient of orbital energy, creating a direct connection between orbital energy variations and nuclear motion. Through the analysis of 48 representative reactions, we identify two predominant types of ROEF behavior: reactions that sustain reaction-direction ROEFs either from the early stages or just before the transition state. These forces carve grooves along the intrinsic reaction coordinates on the potential energy surface (PES), shaping the reaction pathway. This clarifies which types of electron transfer contribute to lowering the reaction barrier. Remarkably, variations in OROs align closely with the curly arrow diagrams widely used in organic chemistry, bridging the intuitive representation of electron transfer with the rigorous PES-based theoretical framework. This integration facilitates a unified discussion of electron transfer and the electrostatic forces driving nuclear motion. By unifying electronic and nuclear motion theories, this study provides a cohesive framework for understanding the driving forces behind chemical transformations, offering profound insights into the electronic basis of reaction mechanisms.