Li Yang

The reaction of the peroxide ion with CH3Br was investigated by direct dynamics simulation. A variety of reaction mechanisms, i.e., front-side complex, proton exchange, and H-migration mechanisms, were found, and the nature of these mechanisms was discussed in detail, which is essential for the dynamical behavior of diatomic or triatomic nucleophilic substitution reactions involving an α-nucleophile. Furthermore, the dynamic characteristics of X– + CH3Y reactions (X= HOO and OH; Y = Cl, Br, and I) were compared to reveal the influences of the nucleophile and leaving group on the α-nucleophile reactions. Atomistic reaction mechanism analyses find that HOO– + CH3Br collisions show similar reaction probabilities to the HO– + CH3Br reaction, although the proton affinity of HO– is larger than that of HOO–, which reveals the enhanced reactivity of HOO– and the presence of an α-effect relative to the normal nucleophiles. For the HOO– + CH3Br reaction, the indirect mechanisms are dominant at low collision energy, and a dramatic transition from complex-mediated indirect to direct occurs when changing HOO– to the normal nucleophile HO–. Besides, with the leaving group ability trend of Cl < Br, the branching ratio of the direct mechanism increases from 7 to 17%, and simultaneously, the ratio of the indirect mechanism decreases, which is attributed to the lower central barrier and an “earlier” transition state for the HOO– + CH3Br reaction.

The indirect mechanism is typically dominant at room temperature for most S_N2 reactions X^- + CH₃Y (X = F, Cl; Y = Cl, Br, I) due to a dynamic bottleneck formed by the prereaction minimum, which prevents the direct collisions. However, recent studies have found that when X = OH, the S_N2 reaction shifts to be dominated by the direct mechanism. In this work, direct dynamic simulations with the B3LYP/ECP/d method were employed to thoroughly investigate the cause of this transition by exploring the OH^- + CH₃Br reaction. And the dynamic properties of X^- + CH₃Y (X = OH, F; Y = Cl, Br, I) reactions were further compared to investigate the mechanism-changing regularities of this series of S_N2 reactions. For the X = F reactions, the indirect mechanisms were found to be dominant, although their proportion decreases along the leaving group changing from Cl to I. In contrast, a reaction mechanism conversion was found for X = OH. When the leaving group changes from Cl to Br or I, the dominant mechanism of the S_N2 reactions alters from indirect to direct, which implies the critical influence of the nucleophile and leaving group on the dominant mechanism of S_N2 reaction and the importance of the kinetic and dynamic factors, i.e., the energy barrier, electronegativity, and dipole moment. This work clearly summarizes the dynamical behaviors in the seemingly similar X^- + CH₃Y (X = F, OH; Y = Cl, Br, I) reactions and provides insights into the impact of the leaving group and the nucleophile on the dominant mechanism of S_N2 reactions.

This study presents the experimental characterization of pre-reactive intermediates in the furan-acrolein Diels-Alder reaction using high-resolution rotational spectroscopy. Three distinct isomers were identified, with π-π stacking unexpectedly prevailing over hydrogen-bonded structures, and the exo structure being slightly more stable than endo. The computationally affordable yet accurate XYG3 method, validated against benchmark CCSD(T) calculations, accurately reproduced the energetic ordering of the isomers. Solvent effects were investigated through microsolvation models, revealing that hexafluoro-2-propanol preserves π-π stacking and enhances exo selectivity via strong hydrogen-bond networks. These findings provide molecular-level insights into the stereocontrol of Diels-Alder reactions, outlining a generalizable role of π-π stacking and solvent-induced noncovalent forces in determining stereoselectivity in solution.

It is essential but difficult to monitor the underlying atomistic mechanisms for the competitive nucleophilic substitution (SN2) and base-induced elimination (E2) reactions. Especially, the dynamic characters of bulky alkyl substitution halides remain unclear due to its complexity. Here, we present direct dynamics simulations of the fluoride anion reaction with isopropyl chloride, uncovering distinct dynamical behaviors compared to its ethyl chloride counterpart. Reaction dynamics simulation capture the key trends observed in differential scattering experiment, demonstrating predominant preference for the direct E2 pathway through stripping mechanisms at 1.9 eV of collision energy. Notably, an enhancement of indirect mechanisms emerges with increasing methyl substitution (from ethyl to isopropyl chloride), even at the high collision energy where such pathways are typically suppressed. This phenomenon arises from the synergistic effects of ion-induced dipole forces and van der Waals interactions between the reactants, coupled with the alkyl substitution-induced stabilization of the entrance-channel complex, which collectively prolong the interaction timescales. This work deepens an atomistic dynamics understanding of sterically hindered systems and highlights the role of the entrance channel on the chemical dynamics.

When the nucleophilic reagent changes from F⁻ to HO⁻, the dominant mechanism changes from an indirect to a direct mechanism; when the nucleophilic reagent is HO⁻, the competitiveness of S_N2 is enhanced as the leaving group changes from Cl to Br.

Solvents play a crucial role in ion–molecule reactions and have been used to control the outcome effectively. However, little is known about how solvent molecules affect atomic-level mechanisms. Therefore, we executed direct dynamics simulations of the HO⁻(H₂O_w) + CH₃CH₂Br system to elucidate the dynamics behavior of chemical reactions in a microsolvated environment and compared them with previous gas-phase data. Our results show that the presence of a single water solvent molecule significantly suppresses the direct mechanism, reducing its ratio from 0.62 to 0.18, thereby promoting the indirect mechanism. Spatial effects and prolonged ion–molecule collisions combine to drive this mechanism shift. Among them, water molecules impede the reactive collisions of HO⁻ and CH₃CH₂Br, while at the same time, the attractive interaction of hydrogen bonds between ions and molecules produces long-lived intermediates that favor the indirect mechanism. On the other hand, microsolvation also affects the reaction preference of the S_N2 and E2 channels, which is more conducive to stabilizing the transition state of the S_N2 channel due to the difference in solute–solvent interactions, thus increasing the competitiveness of this pathway. These results emphasize the profound influence of solvent molecules in regulating reaction selectivity and underlying microscopic mechanisms in more complex systems.

E2 elimination and SN2 substitution reactions are of central importance in preparative organic synthesis due to their stereospecificity. Herein, atomistic dynamics of a prototype reaction of ethyl chloride with hydroxide ion are uncovered that show strikingly distinct features from the case with fluoride anion. Chemical dynamics simulations reproduce the experimental reaction rate and reveal that the E2 proceeding through a direct elimination mechanism dominates over SN2 for the hydroxide ion reaction. This unexpected finding of a pronounced contribution of direct reaction dynamics, even at a near-thermal energy, is in strong contrast to the complex-mediated indirect mechanism for the fluoride case that characterizes the low-energy ion–molecule reactions. The entrance channel structures are found to be crucial and the differences are attributed to subtle changes in the hydrogen-bonding interaction of the approaching reactants. This effect presents in E2/SN2 reactions of different bases and alkyl halides and might play a role in complex chemical networks and environments.

Precise prediction of reactivity and accurately identifying the types of reaction complexes are prerequisites for delineating the microscopic mechanisms of ion-molecule reactions, which remain unclear for reactions involving alpha-nucleophilic reagents. Here, we investigate the potential energy surface of the multiatomic reactions XO- (X = F, *Cl, Br, I) + CH3CH2Cl to elucidate the optimal descriptors for reaction reactivity and the origin of halogen bond/hydrogen bond compounds. Through analyzing the orbital composition and the relationship of energy barriers with the proton affinity and nucleophilic index, the local nucleophilic index is ultimately determined to be the optimal descriptor for predicting the reactivity of the reactions with alpha-nucleophilic reagents. Furthermore, it is found that the type of a reaction complex is closely related to the initial relative orientations of ions and molecules, the functional group substitutions, and the electrostatic potential extreme points of the halogen tops in the reactive substrates. In terms of the above factors, we can design suitable reaction substrates to generate intermolecular interactions with specific types, which is important in areas such as drug synthesis.

Graphitic phase carbon nitride (g-C₃N₄, abbreviated as CN) can be used as a photocatalyst to reduce the concentration of atmospheric carbon dioxide. However, there is still potential for improvement in the small band gap and carrier migration properties of intrinsic materials. K-B co-doped CN (KBCN) was investigated as a promising photocatalyst for carbon dioxide reduction via the Density Functional Theory (DFT) method. The electronic and optical properties of CN and KBCN indicate that doping K and B can improve the catalytic performance of CN by promoting charge migration and separation. In terms of the Gibbs free energy change, the CO₂ reduction reaction catalysed by KBCN results in CH₃OH, and its optimal pathway is CO₂ → *CO₂ → *COOH → CO → *OCH → HCHO → *OCH₃ → CH₃OH. Compared with CN, the doping elements K and B shift the rate-determining step from CO₂ → *CO₂ to *CO₂ → *COOH. The K and B elements co-doping tuned the charge distribution between the catalyst and the adsorbate and reduced the Gibbs free energy of the rate-determining step from 1.571 to 0.861 eV, suggesting that the CO₂ reduction activity of KBCN is superior to that of CN. Our work provides useful insights for the design of metallic–nonmetallic co-doped CN for photocatalytic CO₂ reduction (CO₂PR) reactions.