To determine the origins of these resonances, we performed another set of experiments in which the activated framework was dosed with either 45 mbar O2 or 18 O2 at 300 K, cooled to 100 K, and then spectra were collected as the sample was incrementally warmed to room temperature (Figure ). Interestingly, a reversible color change from pink to gray-brown was observed upon warming the sample from 100 K to room temperature (Figure ). At 100 K, only the peaks at 1051 and 993 cm -1 were present, while above 200 K the peaks at 1131 and 1073 cm -1 were also apparent. These results suggest that the higher energy peaks are not arising from O2 bound at a secondary adsorption site but instead from O2 adsorbed at the copper sites with a different binding mode; in addition, the binding modes appear to be in thermal equilibrium at 200 K. We assign the lower energy features to a superoxide-bound side-on (η 2 ) to Cu II (typically 970-1100 cm -1 ) 30-32 and the higher energy features to a superoxide-bound end-on (η 1 ) to Cu II (typically 1100-1150 cm -1 ). Dosing Cu I -MFU-4l with air at room temperature results in an additional resonance at 2242 cm -1 associated with N2 adsorption. This resonance is red-shifted from the Raman active mode of free N2 (2331 cm -1 ), indicating the copper(I) sites function as a weak π-donor for N2. The spin state of a copper-O2 adduct depends on the O2 binding mode. In particular, η 2 -O2 adducts have been found to possess a singlet ground state (S = 0), while η 1 -O2 adducts possess a triplet ground state (S = 1). As such, to support the assignments made based on the in situ DRIFTS data, variable-temperature dc magnetic susceptibility data were collected at 1 T for a sample of Cu2.4-MFU-4l dosed with O2 at 195 K (see the Supporting Information for details and Figures and). Below 195 K, the magnitude of the molar magnetic susceptibility-temperature product (χMT) is close to zero. However, as the material is warmed above 200 K, χMT steadily increases to 0.25 emu•K/mol. Although the values of χMT should be treated qualitatively due to challenges with the sample diamagnetic correction and desorption of O2 at higher temperatures, these data are consistent with an equilibrium between the S = 0 and S = 1 species in which the higher spin state is at least partially accessed upon warming.