Electronic structure, stability, and nature of bonding of the complexes of C2H2 and C2H4 with H+, Li+, and Na+ ions.: Extensive ab initio and density functional study

The ground-state geometries of the complexes Of C2H2 and C2H4 with H+, Li+, and Na+ ions have been optimized at the B3LYP and MP2(full) levels of theory using several basis sets'. The difference in the direction of tilting of the terminal H atoms noticed in C2H3+ and C2H5+ seems to be an artifact of the methods of calculation and basis sets. The dissociation energies (DEs) of the complexes have been calculated using B3LYP, MP2(full) and CCSD(T)=full methods. When ZPE (zero-point energy) and BSSE (basis set superposition error) corrections are included the DEs at the CCSD(T)=full level of theory are obtained for a number of basis sets in very good agreement with the experimental values, wherever available. The nature of bonding of the complexes has been deduced on the basis of charge transfer, bond indices, localized MOs, and topological properties of electron density. Both bond indices and LMOs indicate the presence of three-center bonding in all the complexes. In the protonated species the bonding is found to be predominantly covalent; in the Li+ and Na+ complexes also the covalent interaction plays a fairly important role.