Correlated Molecular Orbital Theory Study of the Al + CO₂ Reaction

Density functional theory (DFT) and correlated molecular orbital electronic structure calculations were used to study the Al + CO₂ → AlO + CO reaction on the electronic ground-state potential-energy surface (PES). Geometries were optimized using DFT (M11/jun-cc-pV(Q+d)Z) and more accurate energies were obtained using the composite Weizmann-1 theory with Brueckner doubles (W1BD). The results comprise the most complete, most systematic characterization of the Al + CO₂ reaction surface to date and are based on consistent application of high-level methods for all stationary points identified. The pathways from Al + CO₂ to AlO + CO on the electronic ground-state PES all involve formation of one or more stable AlCO₂ complexes denoted η-AlCO₂, trans-AlCO₂, and C_2v-AlCO₂, among which η-AlCO₂ and C_2v-AlCO₂ are the least and most stable, respectively. We report a new minimum-energy pathway for the overall reaction, namely formation of η-AlCO₂ from reactants and dissociation of that same complex to products via a bond-insertion reaction that passes through a fourth (weakly metastable) AlCO₂ complex denoted cis-OAlCO. Natural Bond Orbital analysis was applied to study trends in charge distribution and the degree of charge transfer in key structures along the minimum-energy pathway. The process of aluminum insertion into CO₂ is discussed in the context of analogous processes for boron and first-row transition metals.