Theoretical Analysis of Gas-Phase Front-Side Attack Identity S_N2(C) and S_N2(Si) Reactions with Retention of Configuration

Abstract

Gas-phase front-side attack identity S(N)2(C) and S(N)2(Si) reactions, CH(3)X1 + X2(-) --> CH(3)X2 + X1(-) and SiH(3)X1 + X2(-) --> SiH(3)X2 + 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 S(N)2 reactions for both SiH(3)X and CH(3)X 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 SiH(3)X, and it is higher for CH(3)X. The minimum energy path for an S(N)2(Si) reaction with retention of configuration proceeds from a stable pentacoordinated anion intermediate of C(s) symmetry (TBP) via a C(s) 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 S(N)2(Si) reactions with F(-) and Cl(-) along the intrinsic reaction coordinate (IRC) routes. In addition, the geometrical transformations of front-side attack identity S(N)2(C) and S(N)2(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 S(N)2(C) and S(N)2(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 D(pb) can measure the strength of a chemical bond in a molecule. It is found that the difference between D(pb) values of reactant and transition state may be related to the activation energy for front-side and back-side attack S(N)2(C) and S(N)2(Si) reactions, and the D(pb) curves along the IRC routes have features similar to those of the potential energy profiles for all of the back-side attack S(N)2 reactions and front-side attack S(N)2(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.

Insight into Markovnikov regioselectivity rule via molecular face and ABEEM‐σπ theory

“…In this section, we briefly describe how to define and obtain a MF [6,[12][13][14][15][16][17][18]. First, to introduce the definition of the MF, we should formulate the PAEM.…”

“…MF theory [6,[12][13][14][15][16][17][18] has recently been defined by Yang and coworkers. It is a new model to represent both molecular shape and face ED, based on the potential acting on an electron in a molecule (PAEM).…”

“…Recently, according to the Molecular Face theory, they have investigated an organic chemical reaction about FþC 2 H 2 [15]. More recently, they have discussed the gas-phase front-side attack identity S N 2(C) and S N 2(Si) reactions with retention of configuration [18] and electrophilic addition reactions of alkenes employing the Molecular Face model [6].…”

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

“…Besides the well-known Walden inversion mechanism, there is a front-side attack pathway, which goes over a high energy barrier and results in retention of configuration. This front-side attack mechanism is much less studied than the Walden inversion [10][11][12][13] . Due to the high barrier of the former, the S N 2 reactions are known to proceed via Walden inversion at low collision energies (E coll ) and the front-side attack pathway may open at higher E coll .…”

New investigation of potential acting on an electron in a molecule to draw molecular faces

“…However, these surfaces have not provided an intrinsic and unique representation to molecular shape because of different van der Waals radius [3] and the cutoff values of electron density(ED) [4,5] . Recently, Yang et al [6][7][8][9][10][11][12] have proposed the molecular face theory(MFT) in terms of the classical turning point of electron movement in a molecule. MF is an iso-PAEM(the potential acting on an electron in a molecule) contour where PAEM equals the negative value of ionization energy(-IE) of the molecule.…”