Electrostatic and steric effects in the selective complexation of cations in nonactin.

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).