Single-Molecule Vibrational Characterization of Binding Geometry Effects on Isocyanide–Metal Interactions

Isocyanide-metal binding is governed by σ-donation and π-back-bonding, which affects the isocyanide stretching energy, a characteristic probe for ligand-metal interactions. While extensive correlations exist between structure and spectroscopy in molecular isocyanide-metal systems, interactions of isocyanide with crystalline metallic surfaces, where ligands often bind in various geometries, remain underexplored. Conventional vibrational spectroscopies such as infrared and Raman spectroscopies lack the molecular-scale resolution to distinguish this binding inhomogeneity. In contrast, inelastic electron tunneling spectroscopy with scanning tunneling microscopy (STM-IETS) directly visualizes ligand adsorption geometries and their vibrational signatures. Using STM-IETS, we investigate a metal-adsorbed <i>m</i>-terphenyl isocyanide ligand and find that the adsorption geometry on Cu(100) induces a significant shift in isocyanide stretching frequency, more prominent than replacing Cu(100) with Ag(111). Density functional theory confirms that this shift arises from atomic-scale variations in coordination environments. This study elucidates how precise binding influences the vibrational fingerprints of isocyanide ligands, an often-overlooked factor in understanding the isocyanide-metal interplay.