A B3LYP and MP2 theoretical investigation on unusual cation-π interaction between the singlet state HBBH (1Δg) and H+, Li+, Na+, Be2+ or Mg2+

The nature of unusual cation-π interaction has been investigated by carrying out B3LYP and MP2(full) theoretical calculations of the complexes of cations (H+, Li+, Na+, Be2+ and Mg2+) with the electron-deficient BB double bond. The interaction energies have been calculated using MP2(full) and B3LYP methods at 6-311++G(2df,2p) and aug-cc-pVTZ levels, with the order of HBBH…H+ > HBBH…Be2+ > HBBH…Mg2+ ≫ HBBH…Li+ > HBBH…Na+. Furthermore, the interaction energies between HBBH (1Δg) and cations are stronger than those of the HCCH complexes with the corresponding cations. The analyses of natural bond orbital (NBO), atoms in molecules (AIM) theory and electron density shifts have revealed that the origin of the cation-π interaction is that many of the lost densities from the π-orbital of BB are shifted toward the cation, leading to the electron densities accumulation and the formation of the cation-π interaction. Differently from the other complexes, the complex of H+ is indicative of covalent interaction, as is agreement with the interaction between HCCH and H+.