Revisiting the Gold-Carbon Stretching Frequency: Spectroscopic and Computational Analysis of N-Heterocyclic Carbene Monolayers on Nanostructured Gold Surfaces.

N-Heterocyclic carbenes (NHCs) are notable ligands in surface and nanomaterial chemistry due to their improved stability compared with thiol self-assembled monolayers (SAMs). This stability originates from a strong carbon-gold bond; hence, insights into binding of NHCs on metal surfaces are valuable, with implications in many fields. In this work, 5-functionalized 1,3-diisopropyl benzimidazolium NHCs were deposited on nanostructured gold and analyzed using surface-enhanced Raman spectroscopy (SERS) to study the tuning of the carbon-gold stretching frequency (ca. 800 cm-1) based on 11 derivatives. Density functional theory (DFT) analysis demonstrated that the C-Au bond strength is not reflected in the SERS vibration, but the R-group does influence the ligand-Au interaction, with more electron-donating groups associated with increased bond strength (up to 4.63 kcal mol-1). Therefore, the C-Au stretching frequency from the SERS spectra is not a direct measure of the carbon-gold bond strength.