Impact of Nuclear Quantum Effects on the Structural Properties of Protonated Water Clusters

Nuclear
quantum effects (NQEs) play a crucial role in hydrogen-bonded
systems due to quantum tunneling and proton fluctuation. Our understanding
of how NQEs affect microstructures mainly focuses on bulk phases of
liquids and solids but remains deficient for water clusters, including
their hydrogen nuclei, hydrogen-bonded configurations, and temperature
dependence. Here, we conducted ab initio molecular dynamics (MD) and
path integral MD simulations to investigate the influence of NQEs
on the structural properties of protonated water clusters H+(H2O)n (n = 3, 6, 9, 12). The results reveal that the NQEs become less evident
as the cluster size increases due to the competition between NQEs
and electrostatic interactions. Simulations of several H+(H2O)6 isomers at different temperatures indicate
that the effect of elevated temperature on proton transfer is related
to the initial structure. Interestingly, the process of proton transfer
also involves the interconversion between Zundel-type and Eigen-type
isomers. These findings significantly deepen our understanding of
ion–water and water–water interactions, opening new
avenues for the study of hydrated ion clusters and related systems.