Frontier Hybrid Orbitals . III . Study on SN 2 Reactions *

where Xis called the nucleophile, RjR2R3ZY the substrate and Ythe leaving group. The substrate structure, the nature of the nucleophile, of the reaction center Z and of the leaving group, and the nature of the solvent are important factors of the overall rate of reaction. Now, experimentsare available for studying solvated ions in the gas-phase, it is possible to explore the transition state in the kinetics of the ion-molecule reactions from the solvent-free case to the solution one, which shows that the reaction proceeds with inversion of configuration at the carbon atom in the gas phase just like in solution. Such experiments often offer a valuable starting point for theoretical ab initio computations. A variety of theoretical methodsš"!' have been used to check if the theory is able to give a correct picture of the experimental results. In terms of Salem's" and Fukui's'' treatment of the Wald en inversion, on the basis of the frontier orbital approximation, it is postulated that the reaction evolution is mainly determined by the interaction between the highest occupied molecular orbital (HOMO) of the nucleophilic reagent Xand the lowest unoccupied molecular orbital (LUMO) of the substrate and in this way it is possible to explain the switch from the inversion to retention of configuration by increasing the overlap of the HOMO with the orbital of the reaction center involved in the LUMO. The more complicated influence of the leaving group Y is also explained. But the limits of such a model are clear: it cannot give energy values and reaction barrier values comparable with the experimental ones.