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.