The Investigation\non the Reactivity and the Formation\nof Halogen Bond Complexes for the Reactions of α‑Nucleophiles\nXO<sup>–</sup> (X = F, *Cl, Br, I) and CH<sub>3</sub>CH<sub>2</sub>Cl

Precise prediction of reactivity and accurately identifying\nthe\ntypes of reaction complexes are prerequisites for delineating the\nmicroscopic mechanisms of ion–molecule reactions, which remain\nunclear for reactions involving α-nucleophilic reagents. Here,\nwe investigate the potential energy surface of the multiatomic reactions\nXO<sup>–</sup> (X = F, *Cl, Br, I) + CH<sub>3</sub>CH<sub>2</sub>Cl to elucidate the optimal descriptors for reaction reactivity and\nthe origin of halogen bond/hydrogen bond compounds. Through analyzing\nthe orbital composition and the relationship of energy barriers with\nthe proton affinity and nucleophilic index, the local nucleophilic\nindex is ultimately determined to be the optimal descriptor for predicting\nthe reactivity of the reactions with α-nucleophilic reagents.\nFurthermore, it is found that the type of a reaction complex is closely\nrelated to the initial relative orientations of ions and molecules,\nthe functional group substitutions, and the electrostatic potential\nextreme points of the halogen tops in the reactive substrates. In\nterms of the above factors, we can design suitable reaction substrates\nto generate intermolecular interactions with specific types, which\nis important in areas such as drug synthesis.