Structures, Thermochemical Properties, and Bonding of Mixed Alkaline-Earth-Metal Silicon Trimers Si<sub>3</sub>M<sup>+/0/–</sup> with M = Be, Mg, Ca

The ground state geometries, electronic structures, and thermochemical properties of binary alkaline-earth-metal silicon clusters Si<sub>3</sub>M with M = Be, Mg, Ca in neutral, cationic, and anionic states were investigated using quantum chemical computations. Lowest-lying isomers of the clusters were determined on the basis of the composite G4 energies. Along with total atomization energies, thermochemical parameters were determined for the first time by means of the G4 and coupled-cluster theory with complete basis set CCSD­(T)/CBS approaches. The most favored equilibrium formation sequences for Si<sub>3</sub>M clusters emerge as follows: all Si<sub>3</sub>M<sup>+/0/–</sup> clusters are formed by attaching the M atom into the corresponding cation, neutral and anion silicon trimer Si<sub>3</sub><sup>+/0/–</sup>, except for the Si<sub>3</sub>Mg<sup>+</sup> and Si<sub>3</sub>Ca<sup>+</sup> where the metal cations are bound to the neutral Si<sub>3</sub>. The resulting mixed tetramers exhibit geometrical and electronic features similar to those of the pure silicon tetramer Si<sub>4</sub><sup>+/0/–</sup>. Electron localization function (ELF) and ring current analyses point out that the σ-aromatic character of silicon tetramer remains unchanged upon substituting one Si atom by one alkaline-earth-metal atom.