Observing Elusive Reaction Intermediates with Raman Spectroscopy via Melt-React-Trap

Understanding reaction pathways relies on detecting and characterizing reactive and short-lived intermediate species. Melt-React-Trap (MRT) is introduced as a technique to generate and identify such elusive species via Raman microscopy. This method combines the advantages of low-temperature spectroscopy, rendering convenient the measurement of unstable molecules, with localized heating to allow thermal reactions. Using this approach, reagents are sequentially frozen, mixed in a cryomill and held under cryogenic conditions. A high-power focused Raman laser can locally melt this mixture and allow the reagents to react. Reaction intermediates are then trapped after rapid refreezing and observed via resonance Raman enhancement. Importantly, multiple Melt-React-Trap cycles at a single spot can reveal the growth and decay of small spectral changes to construct a "spectroscopic movie". To validate this technique, MRT cycles were performed for the generation of amylose-stabilized polyiodide species formed by the oxidation of I-. Using MRT, we successfully followed the growth and decay of I2, en route to I3- and I5-. Careful examination of small spectral variations also leads to the first identification of I4•- and I6•- as reactive intermediates in this process. These results illustrate the potential of Melt-React-Trap to monitor short-lived intermediates during a reaction.