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

Abstract

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.