Phosphorus-centered ion-molecule reactions: benchmark ab initio characterization of the potential energy surfaces of the X- + PH2Y [X, Y = F, Cl, Br, I] systems

Dynamics of sterically hindered F- + i-C3H7Cl reaction: An enhancement of indirect mechanisms

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.

Mechanisms and dynamics of the halophilic reaction between CH2CN- and CCl4

In gas phase experiments [H. Chen, R. G. Cooks, E. C. Meurer and M. N. Eberlin, J. Am. Soc. Mass Spectrom., 2005, 16, 2045], the reaction of the CH2CN- ion with CCl4 was observed to proceed predominantly via a halophilic reaction, where the nucleophile attacks a Cl atom, displacing the CCl3- ion, along with minor products from SN2 reactions and H/Cl exchange. In this study, the energetics of the three reaction pathways were investigated using the DFT, MP2, and DLPNO-CCSD(T) methods. The B3LYP/6-311++G** level of theory accurately described the reaction pathways compared to the DLPNO-CCSD(T)/CBS benchmark while remaining computationally efficient. At the B3LYP/6-311++G** level, the halophilic pathway was found to be barrierless and energetically favorable, whereas the SN2 pathway exhibited an energy barrier of 4.34 kcal mol-1 relative to the reactants. The H/Cl exchange reaction is proposed to occur through sequential steps: an initial halophilic pathway followed by proton transfer, due to the high energy barrier of 11.71 kcal mol-1 for the direct reaction. The reaction dynamics, investigated through bimolecular ab initio trajectory simulations at the B3LYP/6-311++G** level, revealed the formation of major halophilic products, consistent with experimental findings. Additionally, the dynamics of the SN2 reaction were explored by analyzing the post-transition state trajectories.

Mechanistic Insights into the Proton Transfer and Substitution Dynamics of N-Atom Center Reactions: A Study of CH3O- with NH2Cl

Bimolecular substitution reactions involving N as the central atom have continuously improved our understanding of substitution dynamics. This work used chemical dynamics simulations to investigate the dynamics of NH2Cl with N as the central atom and the multiatomic nucleophile CH3O- and compared these results with the F- + NH2Cl reaction. The most noteworthy difference is in the competition between proton transfer (PT) and the SN2 pathways. Our results demonstrate that, for the CH3O- + NH2Cl system, the PT pathway is considerably more favorable than the SN2 pathway. In contrast, no PT pathway was observed for the F- + NH2Cl system at room temperature. This can be attributed to the exothermic reaction of the PT pathway for the CH3O- + NH2Cl reaction and is coupled with a more stable transition state compared to the substitution pathway. Furthermore, the bulky nature of the CH3O- group impedes its participation in SN2 reactions, which enhances both the thermodynamic and the dynamic advantages of the PT reaction. Interestingly, the atomic mechanism reveals that the PT pathway is primarily governed by indirect mechanisms, similar to the SN2 pathway, with trajectories commonly trapped in the entrance channel being a prominent feature. These trajectories are often accompanied by prolonged and frequent proton exchange or proton abstraction processes. This current work provides insights into the dynamics of N-centered PT reactions, which are useful in gaining a comprehensive understanding of the dynamics behavior of similar reactions.