GAS-PHASE SELECTIVITIES OF CROWN ETHERS FOR ALKALI-METAL ION COMPLEXATION

Gas-phase size selectivities of crown ethers have been investigated by application of the kinetic method. Ion complexes of crown ethers with two different alkali metals were generated by liquid secondary ion mass spectrometry. Upon gas-phase isolation and high-energy dissociation, these complexes predominantly produced crown ether/alkali metal adduct ions. The abundance ratios of product ions were correlated to the alkali metal ion selectivities of the crown ether. The concept of "maximum contact point" best described gas-phase selectivities because binding preferences were for a smaller metal diameter than what is predicted by the "best fit" concept. The following orders of selectivities were observed for alkali metal ions: 15-crown-5, Li+ >> Na+ > K+ > Cs+; 18-crown-6, Na+ greater-than-or-equal-to K+ > Li+ > Rb+ > Cs+; and 21-crown-7, K+ > Na+ greater-than-or-equal-to Rb+ > Li+ > Cs+. Theoretical studies agree with the present selectivity of 18-crown-6 for sodium ion in the absence of solvation effects. Also, steric and/or pseudo-solvation effects on gas-phase selectivities were evaluated. Complexes of crown ether dimers with two different alkali metal ions were examined, and 18-crown-6 showed a binding preference for potassium over sodium ion.