Selective cation binding of crown ether acetals in electrospray ionization mass spectrometry

Diphenyl-substituted 11- to 20-membered ring crown ether acetals 1a–d exhibit selective cation complexation with alkali metal ions and NH4+ in electrospray ionization mass spectrometry (ESI-MS). In addition, cation-bound crown ether dimers (sandwiches) and trimers have also been observed, especially for the smaller crown ethers. However, their binding ability is completely lost upon acetal hydrolysis. It has been found that these crown ether acetals tend to preferentially bind smaller ions as compared with the corresponding simple crown ethers possessing the same number of ring oxygen atoms. These results are discussed on the basis of ring-contracted deformation, hydrophobic effects of the diphenyl group and the reduced solvation energies of the complexes.