POTENTIAL-ENERGY SURFACES OF THE GAS-PHASE S(N)2 REACTIONS X(-)-) (X=F, CL, BR, I) - A COMPARATIVE-STUDY BY DENSITY-FUNCTIONAL THEORY AND AB-INITIO METHODS(CH(3)X=XCH(3)+X()

We have explored the potential energy surfaces of the title reactions by density functional theory (DFT) and ab initio methods. The DFT calc ulations were based on the local density approximation (LDA) as well a s the more sophisticated approach, NL-SCF, in which nonlocal correctio ns are included self consistently. The ah initio methods made use of t he Hartree-Fock (HF) scheme as well as up to fourth-order Moller-Pless et perturbation theory (MP4). We have systematically characterized the geometries, frequencies, and energies for the reactants, ion-dipole c omplexes, and the transition states. Our study shows that the DFT meth ods offer overall better geometries and frequencies than the HF and MP 2 schemes in comparison with the experimental results. In predicting t he C-X bond energies of the reactants, CH(3)X, the NL-SCF scheme is su perior to ah other methods applied in this study. The NL-SCF and MP4 c omplexation energies are similar and in good agreement with the experi mental results for all but the fluorine system, for which the NL-SCF v alue is about 6 kcal/mol larger than the MP4 estimate. For the transit ion state energies, i.e., the barrier heights, the ab initio and DFT r esults turn out to be qualitatively different in the order HF >> MP2 > MP4 >> NL-SCF >> LDA. The experimental data seem to fall into the reg ion with the MP4 and NL-SCF values as the upper and lower bounds, resp ectively. Within the DFT approaches, the relativistic effects on the g eometries, frequencies, and energies were discussed, and the intrinsic reaction coordinate (IRC) method was utilized to provide further info rmation about the potential energy surfaces, and to rationalize the re action mechanism. We finally carried out bond energy decomposition and population analyses on the X-C bonds formed or broken during the reac tion processes studied here.