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

Significance Understanding the mechanics underlying the diffuse OH stretching spectrum of water is a grand challenge for contemporary physical chemistry. Water clusters play an increasingly important role in this endeavor, as they allow one to freeze and isolate the spectral behavior of relatively large assemblies with well-defined network morphologies. We exploit recently developed, hybrid instruments that integrate laser spectroscopy with cryogenic ion trap mass spectrometry to capture the H ₃O ⁺ and Cs ⁺ ions in cage structures formed by 20 water molecules. Their infrared spectra reveal a pattern of distinct transitions that is unprecedented for water networks in this size range. Theoretical analysis of these patterns then reveals the intramolecular distortions associated with water molecules at various sites in the 3D cages. 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, along with isotopic substitution, to quench water clusters attached to the H ₃O ⁺ and Cs ⁺ ions into structures that yield well-resolved vibrational bands over the entire 215- to 3,800-cm ⁻¹ range. The magic H ₃O ⁺(H ₂O) 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.