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,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.