Visualizing tilted binding and precession of diatomic NO adsorbed to Co-porphyrin on Au(111) using scanning tunneling microscopy

The axial bindings of diatomic molecules to metalloporphyrins play key roles in dynamic processes of biological functions. The basic knowledge of binding structures of NO adsorbed to Co-porphyrin remains unsettled; recent scanning tunneling microscopy showed center protrusion images, while X-ray diffraction and nuclear magnetic resonance spectroscopy revealed tilted binding structures of NO. Here we report high-resolution scanning tunneling microscopy images of NO adsorbed to Co-porphyrins which show perfect agreement with the latter. Upon NO exposure, three-lobed structures of Co-porphyrins were transformed to bright ring shapes on Au(111). With the help of density functional theory calculations, we reproduce the bright ring shapes in the simulated scanning tunneling microscopy image by considering tilted binding and precession motion of NO. Thus, our study directly visualizes the NO ligand of the NO-Co-porphyrin nitrosyl complexes that is tilted away from the axis normal to the porphyrin plane, and is undergoing precession. This journal is