Molecular structure of self-assembled pyrrolidone monolayers on the Au (111) surface: formation of hydrogen bond-stabilized hexamers

Adsorption of 2-pyrrolidone and 3-methyl 2-pyrrolidone molecules onto Au (111) surfaces has been studied by using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). In photoemission spectra, 1s N peak shows two components at 398.1 and 399.8 eV for the pyrrolidone overlayer while one peak grows up at 398.3 eV for the methyl pyrrolidone layer. STM images show a disordered methyl pyrrolidone overlayer, while the pyrrolidone layer exhibits area with a periodic arrangement, which can be described either as a 21×21 R±11° structure with respect to Au (111) lattice or as a two-dimensional close packing of hexamers. We propose a model based on semi-empirical calculations for optimization of the geometry of an unsupported hexamer formed by six pyrrolidone molecules interacting with their two nearest neighbors through hydrogen bonds. In this model, the plane of each molecule is tilted with respect to the substrate, which yields a molecule–molecule distance equal to 0.5 nm in good agreement with STM measurements. Finally, the nature of the bonding between pyrrolidone and methyl pyrrolidone molecules and gold atoms is discussed from STM data and XPS spectra.