On the Photoabsorption by Permanganate Ions in Vacuo and the Role of a Single Water Molecule. New Experimental Benchmarks for Electronic Structure Theory

We report electronic spectra of mass‐selected MnO 4 − and MnO 4 − ⋅H 2 O using electronic photodissociation spectroscopy. Bare MnO 4 − fragments by formation of MnO 3 − and MnO 2 − , while the hydrated complex predominantly decays by loss of the water molecule. The band in the visible spectral region shows a well‐resolved vibrational progression consistent with the excitation of a MnO stretching mode. The presence of a single water molecule does not significantly perturb the spectrum of MnO 4 − . Comparison with the UV/Vis absorption spectrum of permanganate in aqueous solution shows that complete hydration causes a small blueshift, while theoretical models including a dielectric medium have predicted a redshift. The experimental data can be used as benchmarks for electronic structure theory methods, which usually predict electronic spectra in the absence of a chemical environment.