Density, Elasticity, and Stability Anomalies of Water Molecules with Fewer than Four Neighbors

*S Supporting Information ABSTRACT: Goldschmidt−Pauling contraction of the H−O polar-covalent bond elongates and polarizes the other noncovalent part of the hydrogen bond (O:H− O), that is, the O:H van der Waals bond, significantly, through the Coulomb repulsion between the electron pairs of adjacent oxygen (O−O). This process enlarges and stiffens those H2O molecules having fewer than four neighbors such as molecular clusters, hydration shells, and the surface skins of water and ice. The shortening of the H−O bond raises the local density of bonding electrons, which in turn polarizes the lone pairs of electrons on oxygen. The stiffening of the shortened H−O bond increases the magnitude of the O1s binding energy shift, causes the blue shift of the H−O phonon frequencies, and elevates the melting point of molecular clusters and ultrathin films of water, which gives rise to their elastic, hydrophobic, highly-polarized, ice-like, and low-density behavior at room temperature. SECTION: Physical Processes in Nanomaterials and Nanostructures Under-coordinated water molecules refer to those withfewer than the ideal four neighbors as in the bulk o