Probing the Intermediates of Catalyzed Dehydration Reactions of Primary Amide to Nitrile in Plasmonic Junctions

Visible light can effectively drive chemical reactions in plasmonic molecular junctions owing to the high reactivity of adatoms at the surface of plasmonic metal nanostructures and the localized surface plasmon resonance (LSPR)-induced energetic charge carriers (electrons and holes) and heat. Here, we investigated the dehydration reaction of primary amides, which is important to generate valuable nitrile molecules, in the visible-light-irradiated self-assembled gold nanoparticle-aromatic primary amide-gold nanoelectrode junctions in aqueous solution under ambient conditions. At present, the research on the dehydration reaction of the primary amide group is only at the macroscopic level, limiting the mechanistic study of reaction dynamics and intermediates. Using time-resolved surface enhanced Raman spectroscopy (SERS) with tens of millisecond time resolution, we successfully followed the evolution of the SERS spectra along with various transient spectral changes during the rise of the nitrile vibration peak. Combined with density functional theory and a picocavity model, we revealed that most pronounced transient spectral changes were from the gold surface adatom-coupled reaction intermediates. The adatoms produced picocavities with a strong atomic size local field, which strongly enhanced the SERS signals of the intermediates down to sub-single- molecule resolution. The active adatoms played critical roles in producing, interacting, and stabilizing the intermediates. We have determined the complex reaction pathway involving multiple proton transfer steps and intermediates with signature carbon-nitrogen double and triple bonds.