Quantum chemical study of the thermodynamic and kinetic aspects of the S(N)2 reaction in gas phase and solution using a DFT interpretation

The S(N)2 reactions X- + CH3Y --> Y- + CH3X have been investigated in the g as phase and in solution using the Hartree-Fock level with 6-31+G* and 3-21 G* bases, respectively. In the gas phase the interpretation has been done i n the context of the HSAB principle using Gazquez's formalism using the pol arizability (proportional to softness) of the two minima corresponding to t he two ion-molecule complexes, and the TS, the softness of the nucleophile and the charge on the leaving group. The thermodynamic study shows that the reaction energy, DeltaE(r-p) and the energy differences between the two io n-molecule complexes, DeltaE(m), are close to the experimental data availab le. The application of the MHP indicates that in these reactions the molecu les arrange themselves to be as hard as possible. The polarizable continuum model (PCM) has been used to study the influence of the solvent on the kin etics of the S(N)2 reaction. The application of Gazquez's formula provides fundamental information in a case study, with X = Cl and Y = I: the activat ion energy calculated with the PCM model yields a very good correlation wit h the values obtained with Gazquez's formula. Finally, we concentrate on th e influence of solvation on the nucleophilicity and the kinetics using the (PCM) model with X = F, I and Y = Br and compare the results with the gas p hase. The order of reactivity in the gas phase is F- < I-, which is the sam e order as in the solvent using the polarizable continuum model, but oppose d to experiment. If, however, we take into consideration the interaction en ergy caused by the hydrogen bond, the order of reactivity in solution is re versed yielding the same results as experiment.