Theoretical study of M(+)-RG2 (M+ = Li, Na, Be, Mg; RG = He-Rn)

Ab initio calculations were employed to determine the geometry (MP2 level), and dissociation energies [MP2 and RCCSD(T) levels], of the M(IIa)(+)-RG2 species, where M(IIa) is a group 2 metal, Be or Mg, and RG is a rare gas (He-Rn). We compare the results with similar calculations on M(Ia)(+)-RG2, where M(Ia) is a group 1 metal, Li or Na. It is found that the complexes involving the group 1 metals are linear (or quasilinear), whereas those involving the group 2 metals are bent. We discuss these results in terms of hybridization and the various interactions in these species. Trends in binding energies, D(e), bond lengths, and bond angles are discussed. We compare the energy required for the removal of a single RG atom from M(+)-RG2 (D(e2)) with that of the dissociation energy of M(+)-RG (D(e1)); some complexes have D(e2) > D(e1), some have D(e2) < D(e1), and some have values that are about the same. We also present relaxed angular cuts through a selection of potential energy surfaces. The trends observed in the geometries and binding energies of these complexes are discussed. Mulliken, natural population, and atoms-in-molecules (AIM) population analyses are performed, and it is concluded that the AIM method is the most reliable, giving results that are in line with molecular orbital diagrams and contour plots; unphysical amounts of charge transfer are suggested by the Mulliken and natural population approaches.