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 MnO4 and MnO4H2O using electronic photodissociation spectroscopy. Bare MnO4 fragments by formation of MnO3 and MnO2, 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 MnO4. 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.