π⋅⋅⋅H⁺ ⋅⋅⋅π Hydrogen Bonds and Their Lithium and Gold Analogues: MP2 and CASPT2 Calculations

Molecular systems in which two simple π-electron species, acetylene and ethylene, are linked by a cation located between them are analyzed in this study. In particular, the C₂ H₂ ⋅⋅⋅M⁺ ⋅⋅⋅C₂ H₂ , C₂ H₄ ⋅⋅⋅M⁺ ⋅⋅⋅C₂ H₂ , and C₂ H₄ ⋅⋅⋅M⁺ ⋅⋅⋅C₂ H₄ complexes (M⁺ =H⁺ , Li⁺ , Au⁺ ) are calculated with the use of MP2 and CASPT2 methods. The Quantum Theory of Atoms in Molecules (QTAIM), energy decomposition analysis (EDA), and Natural Bond Orbital (NBO) approaches are applied to deepen the understanding of the nature of M⁺ ⋅⋅⋅π interactions in these complexes. It is found that the interactions in gold and proton complexes are characterized by at least partial covalency, whereas interactions in lithium complexes are rather electrostatic in nature.