Does NHC Directly Participate in the CO₂ Insertion into the U^III–N Bond? A Density Functional Theory Study

The mechanisms of CO₂ insertion into the U–N bond of a silylamido NHC (N-heterocyclic carbene) U­(III) complex were investigated theoretically. An earlier reported mechanism involving a reversible NHC carboxylation was found to be energetically too demanding, and a novel four-step mechanism featuring a direct CO₂ insertion into the U–N bond is proposed

Reductive Half-Reaction of Aldehyde Oxidoreductase toward Acetaldehyde: A Combined QM/MM Study

We report a combined QM/MM study on the mechanism of the reductive half-reaction of aldehyde oxidoreductase. Five possible pathways are explored concerning the binding mode of acetaldehyde and the catalytic effect of the nearby glutamic acid (Glu869), taking both possible protonation states into account. In the most favorable pathway, Glu869 participates and acts as a Lewis base to deprotonate the labile hydroxide group. This proton transfer is essential for the high activity of the enzyme toward substrate because it increases the nucleophilicity of the migrating O atom and strengthens the electrophilicity of the target C atom in the substrate. The subsequent product-forming reactions occur in two discrete steps, first nucleophilic attack and then hydride transfer, which implies that the oxidation of aldehyde is a two-electron process. A variant of this mechanism, with an additional water molecule bridging the Glu869 side chain and the substrate, has similar barriers. Judging from previous gas phase calculations and our present QM/MM data, the catalytic effect of Glu869 mainly lowers the barrier of the nucleophilic attack so that the hydride transfer becomes the rate-determining step in the reductive half-reaction

Deep Potential Molecular Dynamics Study of Propane Oxidative Dehydrogenation

Oxidative dehydrogenation (ODH) of light alkanes is a key process in the oxidative conversion of alkanes to alkenes, oxygenated hydrocarbons, and COx (x = 1,2). Understanding the underlying mechanisms extensively is crucial to keep the ODH under control for target products, e.g., alkenes rather than COx, with minimal energy consumption, e.g., during the alkene production or maximal energy release, e.g., during combustion. In this work, deep potential (DP), a neural network atomic potential developed in recent years, was employed to conduct large-scale accurate reactive dynamic simulations. The model was trained on a sufficient data set obtained at the density functional theory level. The intricate reaction network was elucidated and organized in the form of a hierarchical network to demonstrate the key features of the ODH mechanisms, including the activation of propane and oxygen, the influence of propyl reaction pathways on the propene selectivity, and the role of rapid H2O2 decomposition for sustainable and efficient ODH reactions. The results indicate the more complex reaction mechanism of propane ODH than that of ethane ODH and are expected to provide insights in the ODH catalyst optimization. In addition, this work represents the first application of deep potential in the ODH mechanistic study and demonstrates the ample advantages of DP in the study of mechanism and dynamics of complex systems

Density Functional Theory Investigation of the Remarkable Reactivity of Geminal Dizinc Carbenoids (RZn)₂CHI (R = Et or I) as Cyclopropanation Reagents with Olefins Compared to Mono Zinc Carbenoids RZnCHI₂, EtCHIZnR (R = Et or I)

Density functional theory calculations for the cyclopropanation reactions of several mono zinc carbenoids and their corresponding gem-dizinc carbenoids with ethylene are reported. The mono zinc carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier to reaction in the 20−25 kcal/mol range similar to other Simmons−Smith type carbenoids previously studied. In contrast, the gem-dizinc carbenoids react with ethylene via a synchronous attack on both CH2 groups of ethylene and substantially lower barriers to reaction (about 15 kcal/mol) compared to their corresponding mono zinc carbenoid. Both mono zinc and gem-dizinc carbenoid reactions can be accelerated by the addition of ZnI2 groups as a Lewis acid, and this lowers the barrier by another 1.0−5.1 kcal/mol and 0.0−5.5 kcal/mol, respectively, for addition of one ZnI2 group. Our results indicate that gem-dizinc carbenoids react with CC bonds with significantly lower barriers to reaction and in a noticeably different manner than Simmons−Smith type mono zinc carbenoids. The three gem-dizinc carbenoids have a substantially larger positive charge distribution than those in the mono zinc carbenoids and, hence, a stronger electrophilic character for the gem-dizinc carbenoids

A Theoretical Study of the Mechanism of the Water-Catalyzed HCl Elimination Reactions of CHXCl(OH) (X = H, Cl) and HClCO in the Gas Phase and in Aqueous Solution

A systematic ab initio investigation of the water-assisted decomposition of chloromethanol, dichloromethanol, and formyl chloride as a function of the number of water molecules (up to six) building up the solvation shell is presented. The decomposition reactions of the chlorinated methanols and formyl chloride are accelerated substantially as the reaction system involves additional explicit coordination of water molecules. Rate constants for the decomposition of chlorinated methanols and formyl chloride were found to be in reasonable agreement with previous experimental observations of aqueous phase decomposition reactions of dichloromethanol [CHCl2(OH)] and formyl chloride. For example, using the calculated activation free energies in conjunction with the stabilization free energies from the ab initio calculations, the rate constant was predicted to be 1.2−1.5 × 104 s-1 for the decomposition of formyl chloride in aqueous solution. This is in good agreement with the experimental rate constant of about 104 s-1 reported in the literature. The mechanism for the water catalysis of the decomposition reactions as well as probable implications for the decomposition of these chlorinated methanol compounds and formaldehydes in the natural environment and as intermediates in advanced oxidation processes are briefly discussed

A Theoretical Study of Divalent Lanthanide (Sm and Yb) Complexes with a Triazacyclononane-Functionalized Tetramethylcyclopentadienyl Ligand

A density functional theory (DFT) study of the divalent lanthanide complexes [C5Me4SiMe2(iPr2-tacn)]LnI (Ln = Sm, Yb; tacn = 1,4-diisopropyl-1,4,7-triazacyclononane) is presented. A methodological study was done with various density functionals that employ large-core ECPs for the lanthanide atoms. The DFT results were compared with recent experimental X-ray structures for the compounds investigated here. The B3PW91 functional was found to give the best description of the complexes at an affordable level of computational effort. The geometry of the [C5Me4SiMe2(iPr2-tacn)]LnI complexes was found to be a distorted trigonal bipyramidal and the essential structural features are correctly reproduced from the DFT calculations. Further model studies show that the computations can be simplified by replacing the methyl groups (which do not interact with the lanthanide center directly) with hydrogen atoms to still provide reasonable predictions for the structure of the complex

Theoretical Study of Samarium (II) Carbenoid (ISmCH₂I) Promoted Cyclopropanation Reactions with Ethylene and the Effect of THF Solvent on the Reaction Pathways

A computational study of the cyclopropanation reactions of divalent samarium carbenoid ISmCH2I with ethylene is presented. The reaction proceeds through two competing pathways:  methylene transfer and carbometalation. The ISmCH2I species was found to have a “samarium carbene complex” character with properties similar to previously investigated lithium carbenoids (LiCH2X where X = Cl, Br, I). The ISmCH2I carbenoid was found to be noticeably different in structure with more electrophilic character and higher chemical reactivity than the closely related classical Simmons−Smith (IZnCH2I) carbenoid. The effect of THF solvent was investigated by explicit coordination of the solvent THF molecules to the Sm (II) center in the carbenoid. The ISmCH2I/(THF)n (where n = 0, 1, 2) carbenoid methylene transfer pathway barriers to reaction become systematically lower as more THF solvent is added (from 12.9 to 14.5 kcal/mol for no THF molecules to 8.8 to 10.7 kcal/mol for two THF molecules). In contrast, the reaction barriers for cyclopropanation via the carbometalation pathway remain high (>15 kcal/mol). The computational results are briefly compared to other carbenoid reactions and related species

Samarium(III) Carbenoid as a Competing Reactive Species in Samarium-Promoted Cyclopropanation Reactions

The trivalent samarium carbenoid I2SmCH2I-promoted cyclopropanation reactions with ethylene have been investigated and are predicted to be highly reactive, similarly to the divalent samarium carbenoid ISmCH2I. The methylene transfer and carbometalation pathways were explored and compared with and without coordination of THF solvent molecules to the carbenoid. The methylene transfer was found to be favored, with the barrier to reaction going from 12.9 to 9.2 kcal/mol compared to barriers of 15.4−17.5 kcal/mol for the carbometalation pathway upon the addition of one THF molecule

Recognition of Actinides by Siderocalin

Plain simulations and enhanced sampling unveil a novel siderocalin (Scn) recognition mode for An–Ent (where An = actinides and Ent = enterobactin) complexes and identify a “seesaw” relationship between actinide affinity to Ent and Scn recognition to an An–Ent complex. Electrostatic interactions predominantly govern competitive binding in both processes. Additionally, hydrolysis-induced negative charge, water expulsion-driven entropy, and Ent’s conformational adaptability collectively enhance high-affinity recognition

C(sp³)–H Amination Catalyzed by Ir(Me)-Porphyrin: A Computational Study

In recent years, a series of elegant studies demonstrated the potential of engineered cytochrome enzymes to catalyze C­(sp3)–H amination efficiently. This calls for an extensive understanding of the underlying mechanisms to assist rational design and optimization of engineered variants. This work reported a computational mechanistic study of C­(sp3)–H amination catalyzed by iridium porphyrin (IrIII-Por). Two Ir-Por model systems were investigated, differing from each other in their proximal ligand (methide or methylthiolate). The results showed that the C­(sp3)–H amination encompasses two stages: the azido dissociation and the nitrene insertion (combination of two elementary steps concertedly or in a stepwise manner: hydrogen migration and C–N coupling). A typical feature of this reaction is the formation of a nitrenoid intermediate upon the azido dissociation, and the proximal ligand may influence the azido dissociation and modulate the electronic structure of the nascent nitrenoid intermediate. Site selectivity relied on the hydrogen migration step. This work may enrich our understanding of the mechanisms of enzymatic reactions in the cytochrome family and benefit protein engineering for catalytic systems