Site-specific vibrational spectral signatures of water molecules in the magic H₃O⁺(H₂O)₂₀and Cs⁺(H₂O)₂₀ clusters

Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, alongwith isotopic substitution, to quench water clusters attached to the H₃O⁺ and Cs⁺ ions into structures that yield well-resolved vibrational bands over the entire 215- to 3,800-cm^-1 range. The magic H₃O⁺(H₂O)₂₀ cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface.