Correlation between electron delocalization and structural planarization in small water rings

The discovery of the covalent-like character of the hydrogen bonding (H-bonding) system [Science 342, 611(2013)] has promoted a renewal of our understanding of the electronic and geometric structures of water clusters. In this work, based on density functional theory calculations, we show that the preferential formation of a stable quasiplanar structure of (H₂O)_n(n-=-3-6) is closely related to three kinds of delocalized molecular orbitals (MOs; denoted as MO-I, II, and III) of water rings. These originate from the 2p lone pair electrons of oxygen (O), the 2p bond electrons of O and the 1s electrons of H and the 2s electrons of O and 1s electrons of H, respectively. To maximize the orbital overlaps of the three MOs, geometric planarization is needed. The contribution of the orbital interaction is more than 30% in all the water rings according to our energy decomposition analysis, highlighting the considerable covalent-like characters of H-bonds.