The identity of the PMO-and Raman-active CuxOy species must be determined to generate a meaningful model that describes its genesis during O2 activation. Figure shows steady-state spectra contain several significant vibrational features between 300 -1,300 cm -1 . The sharp features at 330 and 475 cm -1 correspond to the ν(T-O-T) mode of the six-membered rings and νs(T-O-T), respectively. The broad features centered around 800 cm -1 and 1100 cm -1 correspond to νs(Si-O) and νas(Si-O) modes, which are typical of zeolite frameworks. The features at 580 (observed only for Cu-CHA-M) and 603 cm -1 have previously been assigned as ν(Cu-O) of a trans-μ-1,2-peroxo dicopper(II) intermediate and ν(Cu-O) of a mono-(μ-oxo)dicopper(II) species, respectively. Figure also displays a spectrum of Cu-CHA-P, intended as a control material to contain predominantly Z2Cu sites that have been reported previously to be PMO-inactive and thus, should not form multinuclear CuxOy species during O2 activation. Figure shows that the Raman spectrum of O2-activated Cu-CHA-P possesses a spectral lineshape distinct from Cu-CHA-I and Cu-CHA-M, which suggests that Raman-active species that form on Cu-CHA-P is not the same as the PMO-active species formed on the ZCuOH-containing materials. In the presence of H2O (0.1 -1 kPa H2O, 21 kPa O2, 723 K, Figure ), spectra of Cu-CHA-M show attenuation of the large vibrational feature at 580 cm -1 such that these spectra closely resemble those of Cu-CHA-I.