Computational Studies of CO₂ Activation via Photochemical Reactions with Reduced Sulfur Compounds

Abstract

Reactions between CO₂ and reduced sulfur compounds (RSC), H₂S and CH₃SH, were investigated using ground and excited state density functional theory (DFT) and coupled cluster (CC) methods to explore possible RSC oxidation mechanisms and CO₂ activation mechanisms in the atmospheric environment. Ground electronic state calculations at the CR-CC­(2,3)/6-311+G­(2df,2p)//CAM-B3LYP/6-311+G­(2df,2p) level show proton transfer as a limiting step in the reduction of CO₂ with activation energies of 49.64 and 47.70 kcal/mol, respectively, for H₂S and CH₃SH. On the first excited state surface, CR-EOMCC­(2,3)/6-311+G­(2df,2p)//CAM-B3LYP/6-311+G­(2df,2p) calculations reveal that energies of <250 nm are needed to form H₂S–CO₂ and CH₃SH–CO₂ complexes allowing facile hydrogen atom transfer. Once excited, all reaction intermediates and transition states are downhill energetically showing either C–H or C–S bond formation in the excited state whereas only C–S bond formation was found in the ground state. Environmental implications of these data are discussed with a focus on tropospheric reactions between CO₂ and RSC, as well as potential for carbon sequestration using photocatalysis