Spin current in the early stage of radical reactions and its mechanisms.

This article focuses on the formulation and application of a perturbation theory of electronic spin flux—the atomic-scale flow of spin density—in radical chemical reactions, specifically analyzing the early-stage spin dynamics in the reaction of methyl radical (CH3·) with oxygen (O2) under three distinct total spin configurations. The authors derive a general perturbative expression showing that the driving perturbation of electronic fluxes is the time derivative of the electron-nucleus interaction, and they expand this expression in molecular orbitals to identify key flux components. Ab initio nonadiabatic calculations reveal that in the reaction with triplet O2 (total spin S_tot = 1/2), spin flux flows from O2 to CH3·, reducing spin polarization, whereas in the singlet O2 reaction (S_tot = 1/2), the spin flux direction reverses, flowing from CH3· to O2; no significant intermolecular spin flux occurs in the non-reactive quartet configuration (S_tot = 3/2). The study establishes that spin flux arises from superpositions of electronic configurations with distinct local spin structures and plays a critical role in rearranging local spin densities associated with chemical bonds, providing a quantum-mechanical observable framework for understanding electron dynamics beyond traditional static potential energy surface approaches.