A MP2(full) and CCSD(T) theoretical investigation on unusual cation-π interaction between OCBBCO and H+, Li+, Na+, Be2+ or Mg2+

The unusual and strong cation-π interactions are found between cations (H+, Li+, Na+, Be2+, and Mg2+) and the electron-deficient BB bond using MP2(full) and CCSD(T) methods at 6-311+G(2df) (6-311++G(2df,2p) for H+ complexes) and aug-cc-pVTZ levels, accompanied by the BB bond contraction. A comparison with those between the corresponding cations and HCCH are also carried out. The binding energies follow the order of OCBBCO⋯H+ ≈ OCBBCO⋯Be2+ > OCBBCO⋯Mg2+ >> OCBBCO⋯Li+ > OCBBCO⋯Na+ and OCBBCO⋯cations > HCCH⋯cations at four levels. The atoms in molecules (AIM) theory has been applied to convince covalent interaction in the H+ complexes and confirm that the electron-deficient BB triple bond can be as the strong π-electron donor in the cation-π interaction. The analyses of natural bond orbital (NBO) and electron density shifts have revealed the nature of the cation-π interaction and explained the origin of BB bond contraction.