Using Surface-Enhanced\nRaman Spectroscopy to Unravel\nthe Wingtip-Dependent Orientation of N‑Heterocyclic Carbenes\non Gold Nanoparticles

Abstract

N-Heterocyclic carbenes (NHCs) are an attractive alternative\nto\nthiol ligands when forming self-assembled monolayers on noble-metal\nsurfaces; however, relative to the well-studied thiol monolayers,\ncomparatively little is known about the binding, orientation, and\npacking of NHC monolayers. Herein, we combine surface-enhanced Raman\nspectroscopy (SERS) and first-principles theory to investigate how\nthe alkyl “wingtip” groups, i.e., those attached to\nthe nitrogens of N-heterocyclic carbenes, affect the NHC orientation\non gold nanoparticles. Consistent with previous literature, smaller\nwingtip groups lead to stable flat configurations; surprisingly, bulkier\nwingtips also have stable flat configurations likely due to the presence\nof an adatom. Comparison of experimental SERS results with the theoretically\ncalculated spectra for flat and vertical configurations shows that\nwe are simultaneously detecting both NHC configurations. In addition\nto providing information on the adsorbate geometry, this study highlights\nthe extreme SERS enhancement of vibrational modes perpendicular to\nthe surface.