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 S_N2 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 S_N2 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 S_N2 reaction were explored by analyzing the post-transition state trajectories.