Fluxional Hydrogen Bonds in Small Water Clusters (H2O)n (n = 2–6)

Understanding the fluxional bonding nature of hydrogen bonds (HBs) and their networks in small water clusters and bulk water remains a huge challenge in both chemistry and physics. Detailed natural bonding orbital (NBO) and non-covalent interactions-reduced density gradient (NCI-RDG) analyses performed in this work at ab initio theoretical level unveil the existence of fluxional hydrogen bonds (FHBs) in small neutral water clusters (H2O)n (n = 2–6). The fluxional mechanisms of linear water dimer (H2O)2 and cyclic water trimer (H2O)3, tetramer (H2O)4, and pentamer (H2O)5 from their transition states (TSs) to ground states (GSs) mainly involve one 3c-2e FHB which fluctuates backward and forward through a weak 4c-2e van der Waals interaction in the TS, while that of the triangular prism water hexamer (H2O)6 involves two 3c-2e FHBs on two neighboring edges perpendicular to each other in a concerted mechanism. It is the FHBs that facilitate the structural fluxionalities of water clusters and their assemblies.