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 H-n(+) 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 H-3(+) impurity in the cluster core, surrounded by stable shells of more spatially delocalized solvating H-2 molecules. The clusters are frozen with respect to the translational degrees of freedom, while the H-2 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. (C) 1997 American Institute of Physics.