Correlated Molecular Orbital Theory Study of the Al + CO 2 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 C2v -AlCO2 , among which η-AlCO2 and C2v -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.