Vibrational spectral signature of the proton defect in the three-dimensional H+(H2O)21 cluster

Blackjack water cluster detected Spectroscopy of protonated water clusters has played a pivotal role in elucidating the molecular arrangement of acid solutions. Whereas bulk liquids manifest broad spectral features, the cluster bands tend to be sharper. The 21-membered water cluster has for decades inspired particular interest on account of its stability and its place in the transition from two-dimensional to three-dimensional hydrogen-bonding network motifs, but the spectral signature of its bound proton has proved elusive. Fournier et al. have now detected this long-sought vibrational feature by applying an innovative ion cooling technique. Science, this issue p. 1009 An ion-cooling technique enables detection of a long-sought motif in the study of acid structure. The way in which a three-dimensional network of water molecules accommodates an excess proton is hard to discern from the broad vibrational spectra of dilute acids. The sharper bands displayed by cold gas-phase clusters, H+(H2O)n, are therefore useful because they encode the network-dependent speciation of the proton defect and yet are small enough to be accurately treated with electronic structure theory. We identified the previously elusive spectral signature of the proton defect in the three-dimensional cage structure adopted by the particularly stable H+(H2O)21 cluster. Cryogenically cooling the ion and tagging it with loosely bound deuterium (D2) enabled detection of its vibrational spectrum over the 600 to 4000 cm−1 range. The excess charge is consistent with a tricoordinated H3O+ moiety embedded on the surface of a clathrate-like cage.