Inequivalent molecules in a two-dimensional crystal

Physisorbed monolayers formed at solution-solid interfaces are two-dimensional crystals sharing many structural characteristics, such as packing motifs and reactivity, with their three-dimensional counterparts. Study of these monolayers with scanning tunneling microscopy (STM) offers a promising tool for exploring crystallization phenomena. Although the analogy between the structures of two-dimensional and three-dimensional crystals is becoming clearer with the imaging of increasing numbers of molecules with STM, the occurrence of inequivalent molecules in a unit cell has been limited to three-dimensional crystals. We report that the monolayer of 1,3-dinonadecanoyl benzene formed at the solution-HOPG interface possesses a unit cell with 1.5 inequivalent molecules (Z′ = 1.5) demonstrating that, as in the case of three-dimensional crystals, simple molecules can give rise to inequivalent packing in the solid state. Copyright © 2002 American Chemical Society.