Concerted or stepwise mechanism? New insight into the water-mediated neutral hydrolysis of carbonyl sulfide

An ETS-NOCV-based computational strategies for the characterization of concerted transition states involving CO2

Due to the presence of both a slightly acidic carbon and a slightly basic oxygen, carbon dioxide is often involved in concerted transition states (TSs) with two (or more) different molecular events interlaced in the same step. The possibility of isolating and quantitatively evaluating each molecular event would be important to characterize and understand the reaction mechanism in depth. This could be done, in principle, by measuring the relevant distances in the optimized TS, but often distances are not accurate enough, especially in the presence of many simultaneous processes. Here, we have applied the Extended Transition State‐Natural Orbital for Chemical Valence‐method (ETS‐NOCV), also in combination with the Activation Strain Model (ASM) and Energy Decomposition Analysis (EDA), to separate and quantify these molecular events at the TS of both organometallic and organic reactions. For the former, we chose the decomposition of formic acid to CO₂ by an iridium catalyst, and for the latter, a CO₂‐mediated transamidation and its chemical variations (hydro‐ and aminolysis of an ester) as case studies. We demonstrate that the one‐to‐one mapping between the “molecular events” and the ETS‐NOCV components is maintained along the entire lowest energy path connecting reactants and products around the TS, thus enabling a detailed picture on the relative importance of each interacting component. The methodology proposed here provides valuable insights into the effect of different chemical substituents on the reaction mechanism and promises to be generally applicable for any concerted TSs.

A Density Functional Theory Study toward Ring-Opening Reaction Mechanisms of 2,4,6-Trinitrotoluene's Meisenheimer Complex for the Biodegradation of Old Yellow Enzyme Flavoprotein Reductase

The subsequent degradation pathway of the dihydride-Meisenheimer complex (2H^--TNT), which is the metabolite of 2,4,6-trinitrotoluene (TNT) by old yellow enzyme flavoprotein reductases of yeast and bacteria, was investigated computationally at the SMD/TPSSH/6-311+G(d,p) level of theory. Combining the experimentally detected products, a series of protonation, addition, substitution (dearomatization), and ring-opening reaction processes from 2H^--TNT to alkanes were proposed. By analyzing reaction free energies, we determined that the protonation is more advantageous thermodynamically than the dimerization reaction. In the ring-opening reaction of naphthenic products, the water molecule-mediated proton transfer mechanism plays a key role. The corresponding activation energy barrier is 37.7 kcal·mol^-1, which implies the difficulty of this reaction. Based on our calculations, we gave an optimum pathway for TNT mineralization. Our conclusions agree qualitatively with available experimental results. The details on transition states, intermediates, and free energy surfaces for all proposed reactions are given and make up for a lack of experimental knowledge.

A theoretical insight into the reaction mechanisms of a 2,4,6-trinitrotoluene nitroso metabolite with thiols for toxic effects

2,4,6-Trinitrotoluene (TNT) is a class C carcinogen as rated by the Environmental Protection Agency. One of the toxicity mechanisms of TNT is the covalent binding of its metabolites to critical proteins. However, knowledge about their molecular reaction mechanisms is scarce. Herein, we have provided density functional theory (DFT) simulation evidences for the reaction mechanisms of the nitroso metabolite of TNT with the sulfhydryl group of model thiols for the first time. The results show that the solvent-mediated proton-transfer mechanism plays a significant role in the entire process. For the formation of semimercaptal, the mechanism is slightly different from the previous one where the thiolate anion attacks the nitroso group. The rearrangement of semimercaptal needs to be triggered by an acid or hydrated ion (H3O+), which is consistent with the previous assumption. The other pathway, the conversion of semimercaptal to hydroxylamine, has to overcome a higher barrier, although it does not need the participation of an acid or a hydrated ion. In addition, the details on transition states, intermediates and free energy surfaces for three reactions are given, which make up for the lack of experimental knowledge. These conclusions can help to deeply understand the toxic effects of TNT and other nitroaromatic explosives.

New insight into the reaction mechanism of carbon disulfide hydrolysis and the impact of H2S with density functional modeling

One-step hydrolysis can easily take place without a catalyst. H₂S is beneficial in maintaining the stability of the intermediates.

Mechanistic and kinetic study on the catalytic hydrolysis of COS in small clusters of sulfuric acid

The catalytic hydrolysis of carbonyl sulfide (COS) and the effect of small clusters of H₂O and H₂SO₄ have been studied by theoretical calculations. The addition of H₂SO₄ could increase the enthalpy change (ΔH<0) and decrease relative energy of products (relative energy<0), resulting in hydrolysis reaction changed from an endothermic reaction to an exothermic reaction. Further, H₂SO₄ decreases the energy barrier by 5.25 kcal/mol, and it enhances the catalytic hydrolysis through the hydrogen transfer effect. The (COS + H₂SO₄-H₂O) reaction has the lowest energy barrier of 29.97 kcal/mol. Although an excess addition of H₂O and H₂SO₄ increases the energy barrier, decreases the catalytic hydrolysis, which is consistent with experimental observations. The order of the energy barriers for the three reactions from low to high are as follows: COS + H₂SO₄-H₂O < COS + H₂O + H₂SO₄-H₂O < COS + H₂O+(H₂SO₄)₂. Kinetic simulations show that the addition of H₂SO₄ can increase the reaction rate constants. Consequently, adding an appropriate amount of sulfuric acid promotes the catalytic hydrolysis of COS both kinetically and thermodynamically.

Catalytic hydrolysis of carbonyl sulphide and carbon disulphide over Fe2O3 cluster: Competitive adsorption and reaction mechanism

The competitive adsorption and reaction mechanism for the catalytic hydrolysis of carbonyl sulphide (COS) and carbon disulphide (CS₂) over Fe₂O₃ cluster was investigated. Compared with experimental results, the theoretical study was used to further investigate the competitive adsorption and effect of H₂S in the hydrolysis reaction of COS and CS₂. Experimental results showed that Fe₂O₃ cluster enhanced the catalytic hydrolysis effect. Meanwhile, H₂S was not conducive to the hydrolysis of COS and CS₂. Theoretical calculations indicated that the order of competitive adsorption on Fe₂O₃ is as follows: H₂O (strong) >CS₂ (medium) >COS (weak). In the hydrolysis process, the C=S bond cleavage occurs easier than C=O bond cleavage. The hydrolysis reaction is initiated via the migration of an H-atom, which triggers C=S bond cleavage and S–H bond formation. Additionally, we find the first step of CS₂ hydrolysis to be rate limiting. The presence of H₂S increases the reaction energy barrier, which is not favourable for COS hydrolysis. Fe₂O₃ can greatly decrease the maximum energy barrier, which decreases the minimum energy required for hydrolysis, making it relatively facile to occur. In general, the theoretical results were consistent with experimental results, which proved that the theoretical study was reliable.

Influence of the preparation conditions of MgAlCe catalysts on the catalytic hydrolysis of carbonyl sulfide at low temperature

The catalytic hydrolysis of carbonyl sulfide (COS) at 50 °C over Mg–Al–Ce mixed oxides derived from hydrotalcite-like compounds was studied.