Theoretical Analysis of Gas-Phase Front-Side Attack Identity SN2(C) and SN2(Si) Reactions with Retention of Configuration

Gas-phase front-side attack identity SN2(C) and SN2(Si) reactions, CH3X1 + X2− → CH3X2 + X1− and SiH3X1 + X2− → SiH3X2 + X1− (X = F, Cl), are investigated by the ab initio method and molecular face (MF) theory. The computations have been performed at the CCSD(T)/aug-cc-pVTZ//MP2/6-311++G(3df,3pd) and CISD/aug-cc-pVDZ levels. Front-side attack identity SN2 reactions for both SiH3X and CH3X have double-well potential energy surfaces (PESs), but their transition-state positions are different relative to the positions of reactants and products: it is lower for SiH3X, and it is higher for CH3X. The minimum energy path for an SN2(Si) reaction with retention of configuration proceeds from a stable pentacoordinated anion intermediate of Cs symmetry (TBP) via a Cs transition state (SP) to a complementary pentacoordinated intermediate (TBP) and finally up to separate products. Berry pseudorotation has been observed in the front-side attack identity SN2(Si) reactions with F− and Cl− along the intrinsic reaction coordinate (IRC) routes. In addition, the geometrical transformations of front-side attack identity SN2(C) and SN2(Si) reactions based on the IRC calculations at the MP2/6-311++G(3df, 3pd) level of theory are described compared with those of corresponding back-side attack reactions. The difference between front-side attack identity SN2(C) and SN2(Si) reactions has been demonstrated. In MF theory, the potential acting on an electron in a molecule (PAEM) is an important quantity; in particular, its Dpb can measure the strength of a chemical bond in a molecule. It is found that the difference between Dpb values of reactant and transition state may be related to the activation energy for front-side and back-side attack SN2(C) and SN2(Si) reactions, and the Dpb curves along the IRC routes have features similar to those of the potential energy profiles for all of the back-side attack SN2 reactions and front-side attack SN2(Si) reaction with F−. Furthermore, according to the MF theory, the spatial dynamic changing features of the molecular shapes and the face electron density are vividly depicted for the course of the reactions.