Theoretical study on the mechanisms of the nucleophilic substitution reactions of hydrosulfide ion and halomethanes

Abstract

Three species involved in the nucleophilic substitution reaction of hydrosulfide ion and halomethanes are investigated by ab initio calculations. Geometries for stationary structures along the reaction paths are fully performed with the second-order Møller-Plesset perturbation approximation with the cc-pVDZ basis set. The monomer geometries determined by the MP2 method match the experimental results very well. Single point energy calculations are carried out at the coupled cluster with perturbative triple excitations CCSD (T) theory with aug-cc-pVDZ basis set. Halomethanes have three conformers here, which lead to the three product channels,, and HSCH 3 þ Br À1 . The investigation encompasses the six complexes formed among three channels, respectively. By selecting the six complexes as the model, we investigate the binding energy, topological property of the electron charge density and their Laplacian in detail theoretically. Electrostatic density potential maps of halomethanes are generated for the determination of attractive interaction sites. It is proved that the similar misshaped electron clouds of the three halogen atoms result in the similar properties of the carbon-halogen bonds, and reveals that the product ion-dipole complexes interactions are predominantly electrostatic in nature. The calculated results predict the binding energy of the most stable complex in six complexes is À47.06 kcal/mol at the MP2 level of theory. The second channel has the lowest energy barrier, which is À3.63 kcal/mol at the CCSD (T) levels of theory, is expected to be the most important pathway. It occurs via CACl cleavage accompanied by CAS bond formation. The other two channels have higher energy barriers and are expected to have smaller rates.