Light-induced Reactions in Cryogenic Matrices - STER Spotlight talk

Abstract of the talk:
Studies on the photochemical reactivity and characterization of the primary photoproducts permit a deeper understanding of reaction mechanisms. In this talk, the experimental technique of low temperature matrix isolation allowing to experimentally test the incipient steps of photochemical reactions, and to characterize novel species with unusual functionalities, properties and behavior will be addressed.

We shall start with the fundamentals of the method and show its possibilities in the studies of structure and reactivity at cryogenic temperatures (~10 K). Typically, the molecules are embedded in solid inert matrices (Ar, Xe, N₂) and excited in situ either by a broadband light source, such as Hg/Xe lamp, or by narrowband light generated in an optical parametric oscillator or in a diode laser. The structures of reactants and photoproducts are characterized experimentally by infrared spectroscopy and theoretically by computation of vibrational spectra.

The potential of the method will be demonstrated using several conformational studies. Narrowband near-infrared irradiations, tuned at the frequencies of the OH or NH first overtone modes, result in conformational switching. Hereby, it becomes possible to characterize high-energy conformers, not accessible experimentally otherwise, and study processes of intramolecular vibrational energy transfer. The examples will include carboxylic acids, amino acids, nucleobase cytosine, among others. Besides, the isomerizations occurring in matrix-isolated molecules in dark (H-atom and heavy-atom tunneling) and those induced by the light source of the spectrometer will be discussed.

 Further examples concern the reactivity induced by frequency-tunable UV light. Here, H-atom transfer reactions, resulting in oxo-hydroxy, amine-imino, thiol-thione isomerism, for phenol, cytosines, thiophenol, and some heterocycles will be described.

 In general, this contribution will provide a selection of experimental and computational results, co-authored by the presenter [1], providing insight into the observed reactivity.

All Warsaw-4-Phd students (and others) very welcome!