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Mass-density and Phonon-frequency Relaxation Dynamics of Under-coordinated Water Molecules

The interplay between intra-molecular H-O covalent bond contraction, due to molecular under-coordination, and inter-molecular O:H expansion, due to inter-electron pair Coulomb repulsion, has been shown to be the source of the anomalous behavior of under-coordinated water molecules in nanoclusters and in the surfaces of water. 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 H-O bond increases the magnitude of O1s binding energy shift, causes the blueshift of the H-O phonon frequencies, and furthermore, elevates the melting point of molecular clusters and ultrathin films of water, which gives rise to their ice-like behavior at room temperature. At the same time, the elongation of the entire O:H-O bond polarizes and enlarges the under-coordinated H$_2$O molecules.