Vibrational spectra have been computed by means of the B3LYP hybrid density functional in order to interpret the experiments [J. Phys. Chem. B 2002, 106, 6358], particularly regarding a transient 1225 cm−1 absorption during accumulation of NO2 in MgO supported BaO. The degree of generality of results is tested by comparing (NO2)x(MO)9 for x=1, 2 and M=Mg, Ca. Finger prints are produced for the single bidentate M2+−[ONO]−−M2+ surface nitrite ion, a novel single monodentate [Oclus−NO2]2− ion, and the chemisorbed nitrite/nitrate ion pair, i.e., M2+ − [ONO]−−M2++ [Oclus−NO2]−. The results suggest the novel monodentate [Oclus− NO2]2− to be responsible for the experimentally observed 1225 cm-1 absorption, being a transient towards surface nitrate rather than nitrite formation. This result is consistent with a mechanistic periodic DFT study concerning the initial loading of NO2 in BaO.