Pentagons and heptagons in the first water layer on Pt(111).

Scanning tunneling topography of long-unexplained "square root of 37" and "square root of 39" periodic wetting arrangements of water molecules on Pt(111) reveals triangular depressions embedded in a hexagonal H2O-molecule lattice. Remarkably, the hexagons are rotated 30° relative to the "classic bilayer" model of water-metal adsorption. With support from density functional theory energetics and image simulation, we assign the depressions to clusters of flat-lying water molecules. 5- and 7-member rings of H2O molecules separate these clusters from surrounding "H-down" molecules.