| dc.description | The properties of the electronic charge distribution are used to study the electronic structures, reactivities and intrinsic barriers of some model S$\sb{\rm N}$2 reactions. Computations were carried out at the Hartree-Fock and second-order Moller-Plesset (MP2) levels. The systems examined are N$\sp-$+CH$\sb3\rm X{\to}$CH$\sb3$N+X$\sp-$, where N and X are H, CCH, CN, NC, NH$\sb2$, OH, OOH, F, PH$\sb2$, SH and Cl. Using the integrated charges obtained from the molecular structure theory, a way of analysing the electronic structure of the transition state is proposed and is used to study the electronic structures of model S$\sb{\rm N}$2 reactions. Results at the MP2 level show that for asymmetric reactions, the reactant and product do not make equal contributions to the transition state wavefunction. However, in some of the reactions, valence-bond configurations N: R$\cdot\cdot$X and N$\cdot\cdot$R X:, which are constituents of the reactant and product wavefunctions, respectively, make similar contributions to the transition state. Thus, in these reactions N and X do have equal charges at the transition state. The charge development on the leaving group is related not only to the exothermicity of the reaction but also to the electronic structure of the transition state and to the electronegativity of the leaving group. The factors determining the height of the intrinsic barrier are discussed. For symmetric reactions, the type of C-X bonding affects the barrier significantly. Within the same type of reaction, the intrinsic barrier is related to the electronegativity of the X group. The Laplacian of the charge density is used to study the shell structures of free atoms and also the different reaction processes of nucleophilic substitution at carbon and at silicon. It is shown that electronegativity and polarizability play an important role in the reaction process. The stereochemistry of silicon is explained by the Laplacian of the charge density. | en_US |
