Cation complexation of quinocrown ethers in electrospray ionization mass spectrometry. A comparison with benzocrown ethers

The alkali metal complexation of 15- to 18-membered ring quinocrown ethers Q15–18, 18- and 24-membered diquinocrown ethers DQ18,24, and 18-membered quinobenzocrown ether QB18 is compared with that of the corresponding benzocrown ethers B15–18 and dibenzo crown ethers DB18,24 in electrospray ionization mass spectrometry (ESI-MS). The quinocrown ethers exhibit lower cation affinity than the benzocrown ethers, although the two crown series essentially show the same ion selectivity except 18-membered DQ18 which preferentially binds Na+. The reduced binding ability of the quinocrowns is interpreted in terms of the deformation of the crown rings by the steric repulsion with the quinone carbonyls as well as the electron withdrawing ability of the fused quinone.