Microhydration Effects on the Intermediates of the SN2 Reaction of Iodide Anion with Methyl Iodide

Abstract

Reactions of halide anions with methyl halides (X- + CH3Y → XCH3 + Y-) are bimolecular nucleophilic substitution (SN2) reactions that have been well investigated in the last few decades.[1] Figure 1 shows typical potential energy surfaces (PESs) proposed for symmetric (X- + CH3X → XCH3 + X-) SN2 reactions along the reaction coordinate. In the gas phase, the PES has two minima corresponding to the stable X-(CH3X) complexes.[2] The PES is substantially distorted by the solvation. Since the negative charge is delocalized over the [X•••CH3•••X]- moiety at the transition state the stabilization energy gained by the solvation is smaller for the transition state than that for the (X- + CH3X) reactants or the X- (CH3X) complexes. In solution, a large potential barrier exists between the reactants and products. The rate constants of these reactions in protic solvents were reported to be a few orders of magnitude smaller than those in aprotic solvents; this trend was explained by the formation of solvation shells of protic molecules around the halide anions.[1,3] Morokuma has previously reported a theoretical study on the PES of the (Cl- + CH3Cl → ClCH3 + Cl-) SN2 reaction with a few H2O molecules. The attachment of H2Omore » molecules to the Cl-(CH3Cl) reactive system produces metastable isomers, which affect the reaction mechanism.[4] Johnson and coworkers extensively investigated the structure and reactions of halide anion complexes in the gas phase using photodissociation spectroscopy.« less