The Investigation on the Reactivity and the Formation of Halogen Bond Complexes for the Reactions of α-Nucleophiles XO^– (X = F, *Cl, Br, I) and CH₃CH₂Cl

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 α-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 α-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.