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

Reactions between CO2 and reduced sulfur compds. (RSC), H2S and CH3SH, were investigated using ground and excited state d. functional theory (DFT) and coupled cluster (CC) methods to explore possible RSC oxidn. mechanisms and CO2 activation mechanisms in the atm. environment. Ground electronic state calcns. 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 redn. of CO2 with activation energies of 49.64 and 47.70 kcal/mol, resp., 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) calcns. 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. [on SciFinder(R)]