CO₂ activation by gaseous zirconium cations: competition between spin-changing and spin-conserving pathways

Area of Science:

• Chemical Physics
• Physical Chemistry
• Reaction Dynamics

Background:

• Multi-state reactivity explains reactions in open-shell systems via intersystem crossing when spin-conserving pathways are inaccessible.
• Understanding molecular-level multi-state reactivity requires well-defined model systems for detailed experimental and theoretical analysis.
• The oxygen atom transfer from CO2 to transition metal cations in the gas phase serves as a key prototype reaction.

Purpose of the Study:

• To investigate the reaction dynamics and kinetics of the Zr+ + CO2 system.
• To elucidate the role of multi-state reactivity and intersystem crossing in this oxygen atom transfer reaction.
• To provide a detailed molecular-level understanding of the competition between different spin channels.

Main Methods:

• Joint experimental and theoretical study employing energy and angle-resolved differential cross sections.
• Trajectory simulations on full-dimensional coupled potential energy surfaces using machine-learned first-principles data.
• Analysis using non-adiabatic transition-state theory to explain observed energy dependencies.

Main Results:

• Experimental data indicates dominant indirect atomistic dynamics, consistent with similar reactions involving other transition metal cations.
• Trajectory simulations reveal a competition between an exothermic intersystem crossing pathway to doublet ZrO+ and an endothermic spin-conserving pathway to quartet ZrO+.
• Product ion velocity distributions confirm a switch in the dominant reaction channel as it becomes energetically accessible.
• Integral cross sections and thermal rate coefficients exhibit three distinct energy dependence regimes (E^-0.5, E^-1, and positive dependence) explained by submerged crossing points.

Conclusions:

• The study highlights the critical importance of electron spin states in governing the reaction dynamics and kinetics of Zr+ + CO2.
• A delicate balance between contributions from different spin states to the interaction potential dictates the overall reaction outcome.
• The findings offer insights into multi-state reactivity, particularly in systems involving transition metal cations and small molecules.