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.

H2S‐Mediated Thermal and Photochemical Methane Activation

“…The CASPT2 estimate of the vertical excitation energy in the CH 4 +H 2 S complex of 143.02 kcal mol −1 (200 nm) is in excellent agreement with values from TD‐DFT calculations and experimental data from the isolated H 2 S molecule 21. The geometry optimization on the S1 surface lowers the S1 energy by approximately 40 kcal mol −1 and places the system very close in energy to the conical intersection, less than 3 kcal mol −1 above the minimum.…”

“…The initial excitation of the S0 optimized CH 4 +H 2 S complex required 139.47 kcal mol −1 or 205 nm, a much smaller energy than that required for excitation of CH 4 alone. The optimized excited‐state complex lies ∼30 kcal mol −1 lower with an asymmetry in the SH bond length of 1.34 and 1.94 Å, which indicates neutral biradical formation,21 with the CH 4 geometry remaining unperturbed and the SH⋅⋅⋅C interatomic distance decreasing to 2.82 Å in the S1 state, from 2.95 Å in the S0 state. This result is fully consistent with that predicted from the vertical excitation data, for which the SH bond is weakened as there is increased interaction between the two molecular moieties.…”

“…Conceptually, this would describe a feasible state‐of‐the‐art method of gas‐phase methane activation, by the presence of a localized spin in the vicinity, although it is difficult to obtain because gas‐phase metal clusters cannot be used on a large scale. On the other hand, H 2 S has been shown to produce a neutral biradical in the gas phase by room‐temperature photolysis, via sulfur p z lone‐pair excitation into the symmetric SH σ* molecular orbital 21. The excitation energy [∼205 nm calculated using time‐dependent DFT (TD‐DFT) and CAM‐B3LYP/6‐311+G(2df,2p)] is smaller than the electronic transition of methane (∼140 nm), which results in molecular photodissociation into methyl and hydrogen radicals as the first step 22.…”

“…As a first step, we explored the feasibility of Reaction (7) in the gas phase by employing quantum chemical calculations. We divided our approach between thermal and photolytic (radical) methods 21. In particular, we considered that CH 4 photodissociation proceeds by light excitation at 140 nm,22 which is a considerable amount of energy.…”

“…Instead, we investigated the H 2 S as a component already present in the reaction mixture, considering it as a source of reactive intermediates. This is based on a recently investigated phenomenon of neutral biradical formation from H 2 S, triggered by ∼200 nm light irradiation 21. In this work, we attempted to combine conventional high‐temperature gas transformations with photochemically induced radical‐stimulated reactions to identify possible pathways of CH 3 SH formation from SQNG without any prior separation or purification.…”