Unexpected steric hindrance failure in the gas phase F + (CH)CI S2 reaction.

Base-induced elimination (E2) and bimolecular nucleophilic substitution (S2) reactions are of significant importance in physical organic chemistry. The textbook example of the retardation of S2 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 S2 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 + (CH)CI 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 S2 when the E2 pathway is completely blocked, indicating the reaction that intends to proceed via E2 transits to S2 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 S2 for the F + (CH)CI reaction. Our study provides new insight into the dynamical origin that determines the intrinsic reactivity in gas-phase organic chemistry.