Theoretical studies on H-M•••π (M=H, Li, Na, K) interactions involving the π-electron donors, C2H2, C2H4 and C6H6

The intermolecular H-M center dot center dot center dot pi (M=H, Li, Na, K) interactions involving the pi-electron donors C2H2, C2H4 and C6H6 have been investigated using the DFT-B3LYP, MP2(full) and M06-2X methods with the 6-311++G(3df,2p) basis set. A comparison with the M center dot center dot center dot pi interaction in the corresponding M center dot center dot center dot pi complex has also been carried out. The results show that the H-M center dot center dot center dot pi interaction energies follow the orders: HLi center dot center dot center dot pi > HNa center dot center dot center dot pi >HK center dot center dot center dot pi >> HH center dot center dot center dot pi and HM center dot center dot center dot C6H6 > HM center dot center dot center dot C2H4 approximate to HM center dot center dot center dot C2H2. The H-M center dot center dot center dot pi interaction is stronger than the M center dot center dot center dot pi interaction. The analyses of the natural bond orbital, atoms in molecules and electron density shifts reveal the nature of the H-M center dot center dot center dot pi interactions and explain why C6H6 can form stronger complexes with HM. The reduced density gradient analysis indicates that except for the H-Li center dot center dot center dot pi and Li center dot center dot center dot pi interactions, there is no clear distinction of bonding nature between the H-M center dot center dot center dot pi and M center dot center dot center dot pi interactions. In the complexes, an unusual green RDG isosurface appears above the Li atom, like a "cap" on the Li atom.