Chorand crown ether macrocycle-alkali metal cation 1:1 complexes -: 'Host-guest' effect investigated directly by mass spectrometry

An electrospray mass spectrometer is used to investigate the interaction between chorand crown ether macrocycles (12C4, 15C5, 18C6) and alkali metal cations (M+ : Li+, Na+, K+, Rb+, Cs+) in solution. The 'host-guest' effect can be investigated for the first time directly from solution. It is observed that the best fit 'hole-size' rule is not obeyed, although comparison with calculations show that, in competition, the relative abundances of different complexes correlate well with their relative stabilities. Structures and binding energies of the 1:1 complexes are determined by the empirical force field CHARMm and are compared to ab initio calculations.