Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to H2O2 mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O–O coupling. This mechanism is predicted to be energetically feasible for Mn2O y + (y = 2–4) and the binary CaMn3O4 +, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides Mn4O y + (y = 4–6) under these experimental conditions. This is attributed to the high fluxionality of the tetramanganese clusters, resulting in the instability of the terminal oxyl radical as well as an energetically unfavorable change of the spin state required for H2O2 formation. Ca doping, yielding a symmetry-broken lower-symmetry three-dimensional (3D) CaMn3O4 + cluster, results in structural stabilization of the oxyl radical configuration, accompanied by a favorable coupling between potential energy surfaces with different spin states, thus enabling the cluster-mediated water oxidation reaction and H2O2 formation.

Infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations have been employed to elucidate the geometric structure of a series of di-manganese oxide clusters Mn2Ox+ (x = 4–7).

The interaction of manganese oxide clusters Mn x O y + (x = 2− 5, y ≥ x) with CO 2 is studied via infrared multiple-photon dissociation spectroscopy (IR-MPD) in the spectral region of 630−1860 cm −1 . Along with vibrational modes of the manganese oxide cluster core, two bands are observed around 1200−1450 cm −1 and they are assigned to the characteristic Fermi resonance of CO 2 arising from anharmonic coupling between the symmetric stretch vibration and the overtone of the bending mode. The spectral position of the lower frequency band depends on the cluster size and the number of adsorbed CO 2 molecules, whereas the higher frequency band is largely unaffected. Despite these effects, the observation of the Fermi dyad indicates only a small perturbation of the CO 2 molecule. This finding is confirmed by the theoretical investigation of Mn 2 O 2 (CO 2 ) + revealing only small orbital mixing between the dimanganese oxide cluster and CO 2 , indicative of mainly electrostatic interaction.

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