Determination of alkali metal cation selectivities of dibenzo-16-crown-5 lariat ethers with ether pendant groups by using electrospray ionization quadrupole ion trap mass spectrometry

The alkali metal cation selectivities of six lariat ethers with ether pendant groups were evaluated by electrospray ionization mass spectrometry in four methanolic solvent systems. The observed binding selectivities are affected by the number of oxygen atoms in the pendant ether group, the presence of a gerninal propyl group, and to a lesser extent the polarity of the solvent environment. The presence of a dioxapentyl group in conjunction with a propyl sidearm yields the most Na+-selective lariat ether. A longer trioxaoctyl pendant group exhibits a preference for complexation of K+ over Na+ due to the optimization of the interactions between the metal ion and the oxygen atoms of the trioxaoctyl group. Ab initio calculations suggest that the addition of a dioxapentyl or trioxaoctyl group pulls the Na+ above the crown ether oxygens, increasing interaction with the former at the expense of interaction with the latter. (C) 2001 Elsevier Science B.V.