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

Abstract

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.

A Surprisingly Simple Electrostatic Model Explains Bent Versus Linear Structures in M+-RG2Species (M = Group 1 Metal, Li–Fr; RG = Rare Gas, He–Rn)

“…17 since the emphasis was on the larger complexes. These bent structures are, of course, in contrast to the (quasi)linear structures we 21 and others 16,20 have reported for the lighter species (M = Li and Na). In the present work, we complete the overall study of alkali metal M + -RG 2 complexes by reporting the results on the remaining twenty four systems involving M = K-Fr and RG = He-Rn.…”

“…Both AIM (values in bold in Table 3) and NPA (values in square brackets in Table 3) results are in line with the expected very small charge movement for physical interactions; additionally, these results are consistent with those for the Li + -RG 2 and Na + -RG 2 complexes. 21 The general trend for the values in Table 3 is that there is a very slight decrease in atomic charge on the metal with increasing atomic number of the rare gas atom which is consistent with the increasing polarizability and decreasing ionization energy of the RG atom. The AIM and NPA charges are very close to each other and look reasonable, with essentially all of the charge still being located on the M + cation, consistent with previous deductions that the Group 1 diatomic M + -RG complexes are described well by an essentially physical model.…”

“…21. Note that in Table 1, the R e1 values have been obtained in the present work from optimizations using the same MP2/aV5Z method as used for the M + -RG 2 complexes.…”

“…The interactions in the Group 1 M + -RG and M + -RG 2 systems are in contrast to the corresponding Group 2 complexes with their facile sp and sd hybridization, which may be viewed as chemical effects. 7,8,9,21,42 Such hybridization is not possible for the Group 1 complexes, as there are no low-lying p or d orbitals 43 (see Table 4, and comments below).…”

“…We suspect it arises owing to the small size of He and its sensitivity to the electronic environment, such as the ability for its electrons to access the Fr + virtual orbitals (albeit to a very limited extent) -angular plots were presented in ref. 21 for the two lightest Group 1 M + -RG 2 species.…”