Metal and mineral catalyzed organic photochemistry in modern and prebiotic environments
Minerals and metals serve important roles in the organic geochemistry of natural environments. Mobility of organics, catalysis of degradation, and redox catalysis are among the processes affected by minerals. With the addition of ultraviolet light a new suite of photo-induced redox reactions is possible including reductive and oxidative ligand-to-metal/mineral charge transfer. Such reactions allow for novel chemistry that has relevance to the modern Earth as well as the pre-biotic origin of life.
This thesis describes processes by which electrons transfer between minerals/metals and organic ligands relevant to natural systems as well as the origins of life. I present evidence of ultrafast electron transfer and the production of radical intermediates essential to deducing redox reaction mechanisms. I also present methods for communicating understanding of interfacial chemistry to the public that promote engagement in science. This thesis is broadly applicable to those interested in mineral organic photochemistry, electron transfer, the origin of life, and science teaching methods.
I probed the chemistry between organic molecules and minerals/metals, using pump/probe transient absorption (TA) spectroscopy to observe the dynamics of electrons and vibrational modes at timescales ranging from picoseconds to nanoseconds. This technique can be conducted in solution and can be highly sensitive to intermediate reaction products.
I examined the photolysis of the metal carboloto, ferric oxalate, under UV irradiation using mid-infrared TA spectroscopy in both D2O and H2O. Ferric oxalate is a model molecule for natural systems and is used to measure photo flux due to its well-characterized quantum efficiency. However, the mechanism of its photolysis is debated. This was the first time the intermediates of ferric oxalate photolysis were observed using techniques sensitive to the vibrational states of organic molecules. I observed the rapid intramolecular charge transfer and the production of CO2 and tentatively CO2•–. Additionally, we observed intermediate states that we interpret to be CO2 disassociating from ferrous iron, a signature never before reported.
Investigations of photo-induced electron transfer were expanded to ZnS nanoparticles and fumarate. Fumarate is an intermediate metabolite in the tricarboxilic acid (TCA) cycle, which is a part of core metabolism in modern organisms. It undergoes a two-electron reduction to form succinate. Reductive versions of the TCA cycle may have been important for the origin of prebiotic metabolism. I measured the effect of adsorbed fumarate on the electronic states of photo-excited ZnS and observed electron transfer both at short (<1 ps) and long (>1 ns) timescales. Additionally, I observed an electronic signature tentatively attributed to fumarate radical, which persisted for at least 8 nanoseconds. The appearance of a long-lived radical intermediate product and the rapid initial electron transfer from the mineral to the organic suggests that ZnS could be a viable catalyst for prebiotic metabolism on the early Earth.
To better educate students on the importance of mineral surface chemistry I designed and implemented a classroom experiment wherein students performed electrolysis of water using mineral electrodes. The experiment emphasized both the mineral catalysis and mineral redox chemistry, which occur at the solid/liquid interface. Concepts in interfacial chemistry are often difficult to exhibit, making this teaching tool unique and useful. Students were guided through a set of investigations and constructed their understanding through observations and sharing of ideas. The experiment was successfully implemented in a college level mineralogy course.