Counterion effects on the cation-π interactions between alkaline earth cations and pyrrole

Systematic theoretical calculations were performed on the interactions between pyrrole and a variety of MX+ cations (M=Be2+, Mg2+, and Ca2+; X=H−, F−, Cl−, OH−, SH−, CN−, NC−, and NH2−) in order to expand the scope of the cation-π interaction. The basis set and electron correlation effects on the geometry optimization and energy calculation were investigated. It was found that the cation-π interaction between pyrrole and a naked alkaline earth cation is much stronger than the pyrrole-alkali cation interaction. In comparison, when a counterion is introduced into the pyrrole⋯M2+ complex, the pyrrole⋯MX+ interaction is very similar to the pyrrole-alkali cation interaction in many aspects.