Abstract
Permanganate aqueous solutions, MnO4-(aq.), were studied using liquid-micro-jet-based soft X-ray non-resonant and resonant photoelectron spectroscopy to determine valence and core-level binding energies. To identify possible differences in the energetics between the aqueous bulk and the solution-gas interface, non-resonant spectra were recorded at two different probing depths. Similar experiments were performed with different counter ions, Na+ and K+, with the two solutions yielding indistinguishable anion electron binding energies. Our resonant photoelectron spectroscopy measurements, performed near the Mn LII,III- and O K-edges, selectively probed valence charge distributions between the Mn metal center, O ligands, and first solvation shell in the aqueous bulk. Associated resonantly-enhanced solute ionisation signals revealed hybridisation of the solute constituents' atomic orbitals, including the inner valence Mn 3p and O 2s. We identified intermolecular coulombic decay relaxation processes following resonant X-ray excitation of the solute that highlight valence MnO4-(aq.)-H2O(l) electronic couplings. Furthermore, our results allowed us to infer oxidative reorganisation energies of MnO4˙(aq.) and adiabatic valence ionisation energies of MnO4-(aq.), revealing the Gibbs free energy of oxidation and permitting estimation of the vertical electron affinity of MnO4˙(aq.). Finally, the Gibbs free energy of hydration of isolated MnO4- was determined. Our results and analysis allowed a near-complete binding-energy-scaled MnO4-(aq.) molecular orbital and a valence energy level diagram to be produced for the MnO4-(aq.)/MnO4˙(aq.) system. Cumulatively, our mapping of the aqueous-phase electronic structure of MnO4- is expected to contribute to a deeper understanding of the exceptional redox properties of this widely applied aqueous transition-metal complex ion.
Highlights
IntroductionThe aforementioned applications rely on the same fundamental process, that is, an electron-transfer reaction between a reduced solute or an electrode (acting as the electron donor) and MnO4À(aq.) (acting as the electron acceptor)
The aforementioned applications rely on the same fundamental process, that is, an electron-transfer reaction between a reduced solute or an electrode and MnO4À(aq.)
Existing theoretical calculations highlight the retention of an average tetrahedral anion symmetry in moving from the gas-54 to the aqueous-phase.§ a Td point group is utilised along with existing molecular orbital diagrams[54,55,56] to assign and label the MnO4À(aq.) photoelectron spectroscopy (PES) signals
Summary
The aforementioned applications rely on the same fundamental process, that is, an electron-transfer reaction between a reduced solute or an electrode (acting as the electron donor) and MnO4À(aq.) (acting as the electron acceptor). During this type of reaction, the electronic and nuclear geometric configuration of both the donor and acceptor, as well as the dielectric and microscopic electronic polarization of the surrounding solvent molecules, are generally changed.[11] The energetics associated with these changes can be expressed as a reorganization energy which can be broken down into vibrational, li, and solvational, l0, components. Paper required to calculate l0, detailed knowledge of the electronic structure of the reactants in the reaction medium is a prerequisite to calculate li.[12,13,14,15,16] Experimentally, such electronic structure information may be garnered using PES techniques.[17,18,19]
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