The electronic structure of the aqueous permanganate ion: aqueous-phase energetics and molecular bonding studied using liquid jet photoelectron spectroscopy

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 L-II,L-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.)--H(2)O((l))electronic couplings. Furthermore, our results allowed us to infer oxidative reorganisation energies of MnO(4)((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 MnO(4)(-)was determined. Our results and analysis allowed a near-complete binding-energy-scaled MnO(4)((aq.))(-)molecular orbital and a valence energy level diagram to be produced for the MnO4(aq.)-/MnO(4)((aq.))system. Cumulatively, our mapping of the aqueous-phase electronic structure of MnO(4)(-)is expected to contribute to a deeper understanding of the exceptional redox properties of this widely applied aqueous transition-metal complex ion.