The cytochrome bd oxygen reductase catalyzes the four-electron reduction of dioxygen to two water molecules. The structure of this enzyme reveals three heme molecules in the active site, which differs from that of heme-copper cytochrome c oxidase. The quantum chemical cluster approach was used to uncover the reaction mechanism of this intriguing metalloenzyme. The calculations suggested that a proton-coupled electron transfer reduction occurs first to generate a ferrous heme b 595. This is followed by the dioxygen binding at the heme d center coupled with an outer-sphere electron transfer from the ferrous heme b 595 to the dioxygen moiety, affording a ferric ion superoxide intermediate. A second proton-coupled electron transfer produces a heme d ferric hydroperoxide, which undergoes efficient O–O bond cleavage facilitated by an outer-sphere electron transfer from the ferrous heme b 595 to the O–O σ* orbital and an inner-sphere proton transfer from the heme d hydroxyl group to the leaving hydroxide. The synergistic benefits of the two types of hemes rationalize the highly efficient oxygen reduction repertoire for the multi-heme-dependent cytochrome bd oxygen reductase family.

Density functional theory (DFT) calculations were conducted to investigate the cobalt porphyrin‐catalyzed electro‐reduction of CO 2 to CO in an aqueous solution. The results suggest that Co II −porphyrin (Co II −L) undertakes a ligand‐based reduction to generate the active species Co II −L⋅ − , where the Co II center antiferromagnetically interacts with the ligand radical anion. Co II −L⋅ − then performs a nucleophilic attack on CO 2 , followed by protonation and a reduction to give Co II −L−COOH. An intermolecular proton transfer leads to the heterolytic cleavage of the C−O bond, producing intermediate Co II −L−CO. Subsequently, CO is released from Co II −L−CO, and Co II −L is regenerated to catalyze the next cycle. The rate‐determining step of this CO 2 RR is the nucleophilic attack on CO 2 by Co II −L⋅ − , with a total barrier of 20.7 kcal mol −1 . The competing hydrogen evolution reaction is associated with a higher total barrier. A computational investigation regarding the substituent effects of the catalyst indicates that the CoPor−R3 complex is likely to display the highest activity and selectivity as a molecular catalyst.

In this contribution, the mechanism of carbonyl sulfide (COS) absorption by N‐methyldiethanolamine (MDEA) aqueous solution was explored via theoretical computations. Detailed reaction mechanisms were analyzed using density functional theory (DFT) calculations at the B3LYP‐D3 level of theory. In total, four different pathways for COS absorption by MDEA have been considered. The most favorable pathway for the removal of COS is a three‐step mechanism including the hydrolysis, proton transfer, and dissociation of CO2, and hydrolysis is the rate‐determining step. The mechanisms of the COS absorption by different amines were investigated, and the calculated results suggest that the total energy barrier for the COS absorption by MDEA is comparable to that by monoethanolamine (MEA), diethanolamine (DEA), and diisopropylamine (DIPA), indicating the COS absorption by all the four amines are feasible, while MDEA gives a better performance in terms of thermodynamics.