Time-Resolved Chemical Bonding Structure Evolution\nby Direct-Dynamics Chemical Simulations

Direct-dynamics simulations\nmonitor atomic nuclei trajectories\nduring chemical reactions, where chemical bonds are broken and new\nones are formed. While they provide valuable information about the\nongoing nuclear dynamics, the evolution of the chemical bonds is customarily\noverlooked, thus, hindering key information about the reaction mechanism.\nHere we examine the evolution of the chemical bonds for the three\nmain mechanisms of the F<sup>–</sup> + CH<sub>3</sub>CH<sub>2</sub>Cl reaction using quasi-classical trajectories for the nuclei,\nand global natural orbitals for the electrons. Key findings include\n(i) bimolecular nucleophilic substitution (S<sub>N</sub>2) resembles\na one-step bond breaking and formation process; (ii) the elimination\nmechanisms (<i>syn</i>- and <i>anti</i>-E2) feature\na sequential two-step process of proton abstraction and Cl<sup>–</sup> elimination; and (iii) the <i>anti</i>-E2 mechanism is\nslower, exhibits rebound effects, and gets activated by specific vibrational\nmodes. This study highlights the importance of correctly describing\nand thoroughly analyzing the dynamical evolution of chemical bonds\nfor chemical reaction mechanistic studies.