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Geometries, electronic states, charge distributions, and stability of Cu n−1 Ag ͑n =2-8͒ neutral and cationic clusters have been investigated using density functional theory. Structural optimization and frequency analyses were performed with the basis set of Lanl2dz. Our results reveal that all neutral and charged Cu n−1 Ag ͑n =2-8͒ clusters can be derived from a substitution of the peripheral position occupied by Cu atom with an Ag atom in the corresponding Cu n ͑n =2-8͒. The atoms at peripheral positions have a preference for bearing the most negative or least positive charges in the smaller clusters ͑n Ͻ 8͒. All atomic charges depend on their atomic positions and ability to obtain or lose electrons in the cluster. In various copper-silver species, Cu n−1 Ag ͑n = even number͒ and Cu n−1 Ag + ͑n = odd number͒ clusters are predicted to be more stable, which can be attributed to difficultly removing an electron from the doubly occupied HOMO of a closed-shell system.

Three archetypal ion pair nucleophilic substitution reactions at the methylsulfenyl sulfur atom LiX+CH3SX-->XSCH3+LiX (X=Cl, Br, and I) are investigated by the modified Gaussian-2 theory. Including lithium cation in the anionic models makes the ion pair reactions proceed along an SN2 mechanism, contrary to the addition-elimination pathway occurring in the corresponding anionic nucleophilic substitution reactions X-+CH3SX-->XSCH3+X-. Two reaction pathways for the ion pair SN2 reactions at sulfur, inversion and retention, are proposed. Results indicate the inversion pathway is favorable for all the halogens. Comparison of the transition structures and energetics for the ion pair SN2 at sulfur with the potential competition ion pair SN2 reactions at carbon LiX+CH3SX-->XCH3+LiXS shows that the SN2 reactions at carbon are not favorable from the viewpoints of kinetics and thermodynamics.