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 + , NH 4 + , NH 3 OH + , and (NH 2 ) 2 CNH 2 + 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).