Mixed cluster ions as a structure probe: Experimental evidence for clathrate structure of (H2O)2H+ and (H2O)21H+

Abstract

First direct experimental evidence for clathrate structures of (H2O)nH+ (n=20,21) is reported based on a technique allowing the number of nonhydrogen-bonded surface hydrogens to be counted. Neutral clusters (H2O)n⋅((CH3)3N)m, prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time-of-flight mass spectrometry technique. The magic numbers (n,m) in the ion intensity distributions of (H2O)n⋅((CH3)3N)m⋅H+ studied under various experimental conditions are well correlated to the stable hydrogen-bonding structures. For the mixed cluster ion (H2O)20⋅((CH3)3N)m⋅H+, the intensity distribution displays an abrupt intensity drop after the magic number at (20,11), while for (H2O)21⋅((CH3)3N)m⋅H+ the magic number appears at (21,10). The findings provide experimental evidence for a stable clathrate structure of (H2O)20H+, with the proton residing on the surface, while for (H2O)21H+, the H3O+ ion is encaged inside the clathrate structure of (H2O)20; the latter structure provides a total of 10 hydrogen-bonding sites for (CH3)3N.