Determination of orders of relative alkali metal ion affinities of crown ethers and acyclic analogs by the kinetic method

Ladders of relative alkali ion affinities of crown ethers and acyclic analogs were constructed by using the kinetic method. The adducts consisting of two different ethers bound by an alkali metal ion, (M ₁ + Cat + M ₂) ⁺, were formed by using fast atom bombardment ionization to desorb the crown ethers and alkali metal ions, then collisionally activated to induce dissociation to (M ₁ + Cat) ⁺ and (M ₂ + Cat) ⁺ ions. Based on the relative abundances of the cationized ethers formed, orders of relative alkali ion affinites were assigned. The crown ethers showed higher affinities for specific sizes of metal ions, and this was attributed in part to the optimal spatial fit concept. Size selectivties were more pronounced for the smaller alkali metal ions such as Li ⁺, Na ⁺, and K ⁺ than the larger ions such as Cs ⁺ and Rb ⁺. In general, the cyclic ethers exhibited greater alkali metal ion affinities than the corresponding acyclic analogs, although these effects were less dramatic as the size of the alkali metal ion increased.