Evolution of Hydrogen-Bond Networks in Protonated Water Clusters H⁺(H₂O)_<i>n</i> (<i>n</i> = 1 to 120) Studied by Cryogenic Ion Mobility-Mass Spectrometry

Cryogenic (80 K) ion mobility-mass spectrometry (cryo-IM-MS) is employed to study structural transitions of protonated water clusters in both the small, H⁺(H₂O)_<i>n</i> (<i>n</i> = 1 to 30), and large, (<i>n</i> = 31 to ∼120), size regions. In agreement with previous studies, we find compelling evidence of regions of uniform cluster decay in the small size region, accompanied by sharp transition points whereby the loss of a single water monomer induces a different H-bonding motif. The investigation of the isomeric distribution of each species at 80 K reveals experimental evidence supporting the notion that H⁺(H₂O)_<i>n</i> (<i>n</i> = 6) is the smallest system to possess both Eigen- (H₃O⁺) and Zundel- (H₅O₂⁺) centered structures. Cryo-IM-MS is particularly well-suited for studying clusters in the large size region, for which previous spectroscopic experimental studies are scarce