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.