Ionic clathrates from aqueous solutions detected with laser induced liquidbeam ionization/desorption mass spectrometry

Clathratelike structures of the alkali metal ions M+ = Li+, Na+, K+ Cs+, an d ammonium with water molecules have been investigated with laser induced l iquid beam ionization/desorption (LILBID) mass spectrometry. The LILBID pro cess implies the desorption of ions from a thin jet of supercooled water in high vacuum by means of an IR laser pulse. Magic numbered water clusters a re observed in the cation mass spectra of K+ and Csi halides, but not for L i+ and Na+. Predominant cluster sizes for Ki(+) (H2O)(n) are n = 16, 20, 25 , 27, acid for Cs+ (H2O)(n) n = 18, 20, 22, 24, 27, 29, with the highest si gnal intensity for n = 20. Water clusters of this size correspond to deform ed dodecahedral cage structures (clathrates) with a cation in the cavity, w hich are known to exist in the gas phase and in the solid state. The possib le occurrence of clathratelike clusters in the liquid phase is discussed in the context of the physical chemistry of electrolyte solutions and the LIL BID experiment. (C) 1999 Elsevier Science B.V.