Dioxygen can coordinate to first-row transition metals (in different oxidation and spin states) with several protonation states, and hapticity, leading to e.g. peroxo, superoxo, hydroperoxo or bis-µoxo species (see Scheme 1). Although crystal structures exist for all of these species, the Few of the computed frequencies were observed experimentally, either due to too low intensity or not being able to assign these specifically to one of the manifolds of possible compounds that might be present in the experiments. For instance, in the reaction of AlF with O2 the authors observed (at least) three species: 1, 3 and a third one involving four oxygens (assigned as a di-superoxo species [FAl(O2 • -)2] 0 ). Based on isotope effects when treating all species with either 16 O2, O2 or a mixture of these, they were able to assign most of the observed peaks to one of these three species.