Plasmon-Driven Chemical Transformation of a Secondary Amide Probed by Surface Enhanced Raman Scattering

Plasmon-driven chemical conversion is gaining burgeoning interest in the field of heterogeneous catalysis. Herein, we study the reactivity of N-methyl-4-sulfanylbenzamide (NMSB) at the nanocavities of gold and silver nanoparticle aggregates under plasmonic excitation to gain an understanding of the reaction mechanism. NMSB is a secondary amide, which is a frequent binding motive found in peptides and common coupling products of organic and biomolecules. Surface-enhanced Raman scattering (SERS) is used as a two-in-one in-situ spectroscopic tool to initiate the molecular transformation process and simultaneously monitor and analyse the reaction products. Supported by dissociative electron attachment (DEA) studies with the gas phase molecule, a hot electron-mediated conversion of NMSB to p-mercaptobenzamide and p-mercaptobenzonitrile is proposed at the plasmonic nanocavities. Importantly, the reaction rate showed negligible dependence on the external temperature excluding the dominant role of heat in the chemical transformation at the plasmonic interface. This is also reflected by the lack of superlinear dependence of the reaction rate constant on the laser power density. Although DEA studies and the nature of power dependence on the reaction rate reflect the hot-electron mediated pathway, the overall reaction rate is limited by the electron transfer probability to the NMSB molecule generating the reaction products.