| Sumario: | In order to characterize the oxidation of metallic surfaces, the reactions of O with a number of Al and, for the first time, Ga clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O pressures of ~10 mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [MO], proceeds via the less stable peroxo species, [M...O], which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O. This could be one of the main reasons why O reactions with metal surfaces proceed more slowly than Cl reactions with such surfaces, even though O reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl with them. Furthermore, our results indicate that the spin-forbidden reactions of 3O with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [Mx+...O] are the root cause for the observed even/odd differences in reactivity.
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