Cation•••π interactions: QTAIM and NBO studies on the interaction of alkali metal cations with heteroaromatic rings

The cation center dot center dot center dot pi interactions of alkali metal cations (Li+, Na+, and K+) with five-membered heteroaromatic rings [furan(C4H4O), thiophene(C4H4S), pyrrole(C4H5N)] were examined by high level ab initio calculations, to investigate the different roles of C4H4O, C4H4S, and C4H5N as the electron donor, the influential factors that affect these interactions, the nature of this kind of cation center dot center dot center dot pi interaction, and to determine topological and energetical properties to characterize these interactions. The sulfur atom in C4H4S plays a certain role in the cation center dot center dot center dot pi interactions except the C-C pi bond, which is different from C4H4O and C4H5N. The size of cation and the character of heteroaromatic ring are two influential factors that affect the cation center dot center dot center dot pi interactions. The studied cation center dot center dot center dot pi interactions can be classified as "closed-shell" and noncovalent interactions. The electron density and its Laplacian at the bond critical points and ring critical points generated upon complexation are useful measurements for the strength of cation center dot center dot center dot pi interactions.