Comparative Static and Dynamic Study of a Prototype SN2 Reaction.

Ab initio molecular-dynamic simulations, using density functional theory (DFT) and the recent atom-centered density-matrix propagation method (ADMP), were used to study the bond formation process in a prototypical SN2 reaction, namely the Walden inversion. Using the real space partition schemes of both electronic density and electron localization function gradient fields, we analyzed different quantum chemical topology (QCT) properties along the ADMP trajectory. In particular, atomic charges derived from the Bader's atoms-in-molecules (AIM) theory were used to analyze intra- and intermolecular charge transfers between atoms, while the electronic population of the forming bonding basin obtained from the electron localization function (ELF) gradient field was employed to describe the bond formation process. These results were compared to the corresponding QCT properties issuing from a static approach based on the intrinsic reaction path (IRP). Although similar features are found for both static and dynamic approaches, the dynamic QCT analysis provides some explanation of the differences observed during the formation of the ion-molecule complex. In particular, it suggests a stronger electron exchange leading to an effective maximization of both covalent and noncovalent interactions.