Electrostatic and Steric Effects in the Selective Complexation of Cations in Nonactin

Abstract

The ester carbonyl stretch frequencies of complexes of the macrotetrolide nonactin with Na⁺, K⁺, Rb⁺, Cs⁺, Tl⁺, NH₄⁺, NH₃OH⁺, and (NH₂)₂CNH₂⁺ have been measured. For the larger alkali cations and the polyatomic cations, the ester carbonyl stretch frequency is linearly proportional to the cation-ester carbonyl electrostatic interaction energy. This constitutes direct evidence that the cation-nonactin interaction is primarily electrostatic, rather than mechanical (steric).