E M (E=C-Pb, M=Li-Cs) Clusters: The Smallest Molecular Stars.

Extensive potential energy surface explorations of twenty-five clusters with the formula E M (E=Group 14 element and M=Group 1 element) through density functional theory and high-level ab initio computations reveal that the lowest-energy isomer for all these systems corresponds to a non-classical D star-like structure in the singlet state, where three M atoms interact electrostatically with the triangular E core, occupying three bridging positions around it. More than 18 200 calculations were done in the search for the minima structures, starting with a first phase at the PBE0/LANL2DZ level and ending with an analysis of the most representative clusters at the CCSD(T)/def2-TZVP//PBE0/def2-TZVP level. The title clusters represent the smallest molecular stars with three planar tetracoordinate E atoms (E=Group 14 element). All these E M clusters behave like superalkali cations with small vertical electron affinities (smaller than Cs), large vertical electron detachment energies, and HOMO-LUMO energy gaps. Their energetics, bonding, and electron delocalization are discussed in detail. The high stability of these clusters is reflected from the large dissociation energy needed for different dissociation channels. The electron delocalization is confirmed by the presence of two delocalized π electrons over the E core and strong diatropic responses.