Among the complexes depicted in Figure , mono(μoxo) dimer species (IV, V, and VI) most closely satisfy these criteria. These complexes exhibit strong absorbance between 8,000 -20,000 cm -1 correspond to d-d transitions. Frequency calculations show that these species also possess a significant Raman scattering features between 590 -620 cm -1 that do not show significant deformation of the Cu-O bonds (Supporting Information, Videos). In comparison, our calculations show that Cu dimers I -III (Figure ) do not have significant UV-Vis absorbance features near 18,800 cm -1 and do not possess Raman features near 603 cm -1 that correspond to vibrations with significant Cu-O or O-O bond deformations (which is inconsistent with Δ 18 O2 = 0). Consequently, the CuxOy species that form over Cu-CHA-I and Cu-CHA-M over long time scales (0 -15 ks) are active for PMO and that are responsible for the 603 cm -1 feature likely correspond to mono(μ-oxo)dicopper intermediates. We note, however, that the precise and unambiguous assignment of the 603 cm -1 Raman scattering feature to a multinuclear vibration of mono(μ-oxo)dicopper species remains difficult as DFT+U calculations are performed off-resonance and do not account for the preferential excitation of vibrations due to induced electronic transitions. Consequently, we tentatively assign the species responsible for the observed Raman feature at 603 cm -1 and for PMO reactivity as a mono(μ-oxo)dicopper species but do not exclude any of the plausible isomers (species IV, V, or VI). For brevity, we refer to mono(μ-oxo)dicopper as ZCuOCuZ in the following sections.