Theoretical studies on identity S_N2 reactions of lithium halide and methyl halide: A microhydration model

摘要

Reactions of lithium halide (LiX, X = F, Cl, Br and I) and methyl halide (CH_3X, X = F, Cl, Br and I) have been investigated at the B3LYP/6-31G(d) level of theory using the microhydration model. Beginning with hydrated lithium ion, four or two water molecules have been conveniently introduced to these aqueous-phase halogen-exchange S_N2 reactions. These water molecules coordinated with the center metal lithium ion, and also interacted with entering and leaving halogen anion via hydrogen bond in complexes and transition state, which to some extent compensated hydration of halogen anion. At 298 K the reaction profiles all involve central barriers ΔE _(cent) which are found to decrease in the order F > Cl > Br > I. The same trend is also found for the overall barriers (ΔE _(ovr)) of the title reaction. In the S_N2 reaction of sodium iodide and methyl iodide, the activation energy agrees well with the aqueous conductometric investigation.