Dynamics of CO₂ activation by gas-phase transition metal ions: the importance of intersystem crossing

The activation of CO₂ at isolated transition-metal centers represents a prototypical problem for understanding elementary steps relevant to single-atom catalysis. Fundamental knowledge of such systems can be acquired by investigating gas phase reactions between transition-metal ions and molecules. Because open-shell transition-metal species often possess multiple accessible spin states, their reactions with CO₂ can proceed along competing spin-changing and spin-conserving pathways. Understanding how spin-orbit coupling influences these competing pathways therefore calls for a direct comparison between scattering experiments and multi-state dynamical simulations. In this Perspective, we summarize our combined experimental and theoretical investigations of the Ta⁺, Nb⁺, and Zr⁺ + CO₂ reactions. Crossed-beam velocity map imaging provides energy- and angle-resolved differential cross sections, while trajectory surface-hopping simulations on first-principles based full-dimensional multi-spin potential energy surfaces enable a dynamical treatment of intersystem crossing and spin-conserving channels on an equal footing. In all three systems, intersystem crossing competes with the spin-conserving channel for the control of the overall reaction dynamics and kinetics. These reactions all proceed predominantly via an indirect mechanism, as evidenced by the nearly isotropic differential cross sections consistent with long-lived complex formation, and the respective product energy distributions indicate substantial energy deposition into internal modes. Despite their similar potential-energy topographies, Ta⁺ and Nb⁺ + CO₂ reactions are dominated by spin-changing pathways at all energies investigated, whereas in the Zr⁺ system the spin-conserving channel becomes competitive. This difference arises from the markedly different magnitudes of the spin-orbit coupling, which determines the efficiency of intersystem crossing and thereby the balance between the two pathways.