Open Access Version

Abstract:
This thesis presents a combined study of the (microscopic) electronic structure and (macroscopic) redox properties of permanganate (MnO4−) and manganate (MnO42-) ions in aqueous solution using liquid jet soft X-ray photoelectron spectroscopy (XPS). MnO4−(aq.) is a versatile, strong oxidizing agent and redox precursor of manganese species with a range of oxidation states. MnO42-(aq.) is a transient species that acts both as an oxidizing or reducing agent, forming a highly-reversible redox pair with MnO4−(aq.) in alkaline environments. Such redox properties make these transition metal complexes (TMCs) attractive for sustainable (electro-)chemical applications. The goal of this work was to gain insight into the absolute energetics and redox behavior of MnO4−(aq.) and MnO42-(aq.) from electronic structure information. Alternative sample sources – a micro-mixing scheme and an electrolysis cell liquid jet – were developed to generate and study MnO42 (aq.) transient ions. Within a single-configuration and single-active-electron electronic structure picture, binding energy-scaled molecular orbital (MO) diagrams were produced from MnO4−(aq.) and MnO42-(aq.) XPS data. Mn 2p and O 1s resonantly-enhanced photoelectron spectroscopy (RPES) measurements revealed intramolecular Auger processes and valence electron binding energies that were not accessible from the XPS experiments, as well as hybridization of and electronic coupling between the valence electrons. In addition, the O 1s RPES experiments revealed intermolecular coulombic decay (ICD) processes, signatures of electronic coupling between solute and solvent molecules in the first solvation shell. For MnO4−(aq.) at 0.2 M concentration, similar electronic energetics were observed at the gas-solution interface and in the solution bulk, independently of the nature of the counter ion (Na+(aq.) or K+(aq.)). Depth profiling experiments at 0.2 M and 1.0 M concentration highlighted a tendency of MnO4−(aq.) to accumulate in the solution bulk and away from the interface, with non-linear accumulation behavior occurring for the higher concentration. Through comparison to the gas-phase ionization energetics, the Gibbs free energy of hydration (ΔGhyd) for isolated MnO4− was also calculated. For MnO42-(aq.), valence band features could only be isolated in surface-sensitive XPS experiments and RPES experiments were relied on to study bulk-solution energetics. These results were applied to infer thermodynamic parameters of half redox reactions involving the MnO4−(aq.) / MnO4•(aq.) and MnO42- (aq.) / MnO4−(aq.) redox pairs, including oxidative reorganization energies, adiabatic ionization energies / Gibbs free energy of oxidation (ΔGox) and vertical electron affinities. For the MnO42- (aq.) / MnO4−(aq.) redox pair, the ΔGox value purely extracted from spectroscopic data was shown to match the reported electrochemical value. Overall, the extracted redox parameters demonstrate how insights into the macroscopic (redox) properties of chemical systems can be built up from microscopically (molecularly) sensitive measurements. The methodology can be extended to aqueous redox-active species that cannot be probed by conventional electrochemical methods.