Base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) reactions are of significant importance in physical organic chemistry. The textbook example of the retardation of SN2 reactivity by bulky alkyl substitution is widely accepted based on the static analysis of molecular structure and steric environment. However, the direct dynamical evidence of the steric hindrance of SN2 from experiment or theory remains rare. Here, we report an unprecedented full-dimensional (39-dimensional) machine learning-based potential energy surface for the 15-atom F− + (CH3)3CI reaction, facilitating the reliable and efficient reaction dynamics simulations that can reproduce well the experimental outcomes and examine associated atomic-molecular level mechanisms. Moreover, we found surprisingly high “intrinsic” reactivity of SN2 when the E2 pathway is completely blocked, indicating the reaction that intends to proceed via E2 transits to SN2 instead, due to a shared pre-reaction minimum. This finding indicates that the competing factor of E2 but not the steric hindrance determines the small reactivity of SN2 for the F− + (CH3)3CI reaction. Our study provides new insight into the dynamical origin that determines the intrinsic reactivity in gas-phase organic chemistry.
An accurate global potential energy surface (PES) for the HBr2 system has been constructed using the fundamental invariant neural network fitting method based upon 11 698 ab initio energies at the UCCSD(T)/CBS level of theory, with the spin–orbit coupling of the 2P3/2 orbit of the Br atom properly included. The time-dependent wave packet calculations have been performed to study the H + Br2 → HBr + Br reaction on the new PES. The total reaction probabilities for total angular momentum J = 0 for the ground initial state show no threshold due to the submerged barrier height (−0.351 kcal/mol) of the PES. The total integral cross sections (ICS) for reactant Br2 in ro-vibrational states (v 0 = 0, j 0 = 0, 10, 20, 30; v 0 = 1–5, j 0 = 0) were calculated for collision energy of up to 0.5 eV. It is found that the initial rotational excitation has a negligible effect on the ICS, and the initial vibrational excitation depresses the reactivity to some extent. The thermal rate constants for the title reaction in the temperature range of 100–1000 K were calculated from the Boltzmann averaging of the v 0 = 0–5 rate constants, which overestimated the experimental results to some extent.
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