Gas-phase identity S N 2(N), S N 2(Si) and S N 2(P) versus S N 2(C) reactions with Clare investigated by the ab initio method. Front-side attack identity S N 2 reactions considered have all double-well potential energy surfaces (PES), and back-side attack identity S N 2(C) and S N 2(N) reactions have also double-well PES, while back-side attack identity S N 2(Si) and S N 2(P) have single-well PES. In addition, the geometrical transformations, potential energy profiles of front-side and back-side attack identity S N 2(N), S N 2(Si) and S N 2(P) versus S N 2(C) reactions based on the IRC calculations are described, the differences between them for the front-side or back-side attack reactions have been demonstrated.Keywords: Ab initio method; Identity S N 2(C), S N 2(N), S N 2(Si) and S N 2(P) reactions; Front-side attack; Back-side attack.

Electrophilic additions of HCl to a series of asymmetric alkenes in the gas phase are investigated by the Molecular Face (MF) theory and ABEEM-rp model. The interesting features of regioselectivity for these reactions are characterized by the electron density (ED) encoded on the MF of alkenes and charge distribution of alkenes obtained via the ABEEM-rp model, respectively. It is then demonstrated that for a series of alkenes, the Hammett constant r p (substituent constant) has a good linear correlation with K ED , where K ED is character of the ED at the p region in the initial state of alkenes. Comparison between investigations using MF, ABEEM-rp, molecular electrostatic potential, and DFT theories, in essence, give similar conclusions for explaining the regioselectivity of the electrophilic additions to alkenes, although from different points of view.

The SN2 reactions at C and Si have been investigated by ABEEM-σπ method compared with the potential acting on an electron in a molecule (PAEM). The ABEEM-σπ model demonstrates the charge distributions of the atom region, bond region, and lone-pair electron region during the course of the reaction. The differences between title reactions have been demonstrated. Studies indicate that there are good linear correlations among the charges obtained by ABEEM-σπ method, the force constants, the relative energies, the ionization potentials, etc. This study enhances our understanding of the electron transfer from the nucleophile to the leaving group.

According to the local hard-soft acidbase (HSAB) principle, the generalized local softness has been investigated based on the electrophilic reactions of H 2 O, HF, HCl and HBr to alkenes, and the effect of the softness on the chemical reactivity was studied in terms of the ab initio and atom-bond electronegativity equalization method plus σπ (ABEEM-σπ) method. Studies show that the finite difference approximation with ab initio method cannot always be used to predict the regioselectivity of the investigated reactions in terms of the local HSAB principle. But ABEEM-σπ model could well describe chemical reactivity according to the local HSAB principle. A significant characteristic of the ABEEM-σπ method is that the double bond is explicitly considered and partitioned. The square sums of differences of the local softness of the reaction center atoms, that is, Δs, or ΔS G with the generalized local softness and Δs χ with the electronegativity, have been studied on the considered electrophilic additions. Studies indicate that ΔS G can be utilized not only to predict regioselectivity rules but also to rationalize the chemical reactivity of these addition reactions, and the generalized local softness is more suitable for the investigation of chemical reactivity between two chemical species.