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 prop
erties of their unprotonated counterparts. The added proton gets trapp
ed as a very localized and strongly bound H-3(+) impurity in the clust
er core, surrounded by stable shells of more spatially delocalized sol
vating H-2 molecules. The clusters are frozen with respect to the tran
slational degrees of freedom, while the H-2 ligands undergo large-ampl
itude rotations. The rotational delocalization is found to increase in
successive solvation shells. The combination of translational rigidit
y and rotational floppiness, which is akin to plastic behavior in crys
tals, is a quantum induced phenomenon absent in the classical approxim
ation for the nuclei. (C) 1997 American Institute of Physics.