A Conductance Study of Alkali Metal Ion–18-Crown-6 Complexes in N,N-Dimethylformamide

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Abstract
The formation constants, KML+, for 1 : 1 complexes of 18-crown-6 (18C6) with alkali metal ions (Na+, K+, Rb+, and Cs+) and the limiting ionic molar conductivities, λ°, of the complexed cations in N,N-dimethylformamide have been determined conductometrically at 25 °C. The KML+ sequences of the alkali metal ions with 18C6 are K+>Rb+>Cs+>Na+. The stabilities and selectivities on complexation are governed by the solvent medium and the relative sizes of the cation and the 18C6 cavity. The λ° values of the 18C6 complexes with the alkali metal ions are all approximately equal except for Cs+, suggesting that, in the cases of Na+, K+, and Rb+, the charge of the alkali metal ion trapped in the 18C6 cavity is effectively screened by 18C6, while this is not true in the case of Cs+. The ratio of the size of the alkali metal ion to that of the 18C6 cavity appears to be an important factor in determining the magnitude of λ° for alkali metal ion-18C6 complexes.

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