A theoretical study of the gas-phase ion pair S N2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

Abstract

Identity and non-identity ion pair S N2 reactions, LiX+CH 3X, LiY+CH 3X (Y, X=F, Cl, Br and I) were investigated using CCSD(T) calculations. Two possible reaction mechanisms, inversion and retention, were discussed. Introduction of lithium cation will significantly raise the inversion barriers and may lower the retention barriers. The analysis of barrier gaps between the two channels indicates that the retention mechanism is favorable for all of the reactions involving fluorine, in contrast to the anionic S N2 reactions at carbon where inversion reaction pathway is much more favorable for all halogens. The stabilization energies for dipole–dipole complexes CH 3X⋯LiY (Y=F–I) are found to have a good correlation with the electronegativity of X. The CCSD(T) central barriers and overall barriers show good agreement with the predictions of Marcus equation and its modification, respectively. Further interesting feature of the non-identity ion pair S N2 reactions is a good correlation between inversion central barriers and the composite geometric looseness (%L).