The Structure of Benzoquinone Chemisorbed on Pd(111): Simulation of EC-STM Images and HREELS Spectra by Density Functional Theory

Earlier studies on the chemisorption of hydroquinone (H(2)Q) on well-defined Pd(111) surfaces based on electrochemistry, high-resolution electron energy loss spectroscopy, and in situ scanning tunneling microscopy revealed that H(2)Q undergoes oxidative chemisorption to generate an adlayer of benzoquinone oriented flat, albeit with a slight tilt. Certain structural details, however, such as the actual adsorbate structure and the surface coordination site could not be unambiguously confirmed solely from the experimental measurements. Density functional theory was thus employed not only to calculate the total adsorption energies of the likely configurations but also to simulate their respective vibrational spectra. The results suggest that: (1) the flat-adsorbed quinone ring is centered on a bridge site in which the C-2 axis that points along the para-oxygen atoms is rotated 30A degrees from the [110] direction of the Pd(111) substrate; (2) the p-oxygen atoms are located above twofold sites; and (3) quinonoid ring is slightly puckered with the C-H bonds tilted away from the surface, at an angle of approximately 20A degrees.