| Abstract
| - The gas-phase identity methyl transfer reactions, X- + CH3X ⇌ XCH3 + X-, have been investigated withX = H, F, Cl and Br at the MP2, B3LYP, QCISD and QCISD(T) levels by geometry and energy optimizationsusing the 6-311++G(3df,2p) basis sets at each level. Energy barriers, Δ, Δ, ΔH⧧ and ΔG⧧, arereported relative to both the reactants (ΔG⧧) and ion−dipole complex levels (Δ). The electron correlationenergy (−Ecorr) decreases in the MP2, QCISD and QCISD(T) results as the size (number of electron) of thesystem becomes larger (X = F → Cl → Br). The MP2 and QCISD methods underestimate the electroncorrelation effects relative to the highest level QCISD(T) results, which are, in general, in good agreementwith the available experimental values. The lowest and highest activation barriers obtained with X = F andH, respectively, are found to be the consequences of the strong electrostatic interaction energies in the TS(ΔEes ≪ 0 and ΔEes ≫ 0, respectively), in contrast to small differences between nucleophiles, X, in theproximate σ−σ* charge transfer and deformation energies. The gas-phase barrier heights are in the order X= F < Br < Cl < H, and hence the reactivity and the gas-phase nucleophile strength are in the reverse order.Moreover, the extent of bond formation in the transition state, as expressed by the percentage bond orderchange, %Δn⧧, is also in the order of intrinsic nucleophilicity. Thus the stronger the nucleophile, the greateris the bond formation in the transition state for the intrinsic barrier controlled reactions.
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