Protonated hydrogen clusters

The effect of protonation of pure hydrogen clusters is investigated at low temperature using a combination of path integral simulations and first-principles density functional electronic structure calculations. These odd n Hn+ clusters are shown to lose the quantum-liquid properties of their unprotonated counterparts. The added proton gets trapped as a very localized and strongly bound H3+ impurity in the cluster core, surrounded by stable shells of more spatially delocalized solvating H2 molecules. The clusters are frozen with respect to the translational degrees of freedom, while the H2 ligands undergo large-amplitude rotations. The rotational delocalization is found to increase in successive solvation shells. The combination of translational rigidity and rotational floppiness, which is akin to plastic behavior in crystals, is a quantum induced phenomenon absent in the classical approximation for the nuclei.