Pawley fits against the diffraction data for activated Cu2.4-MFU-4l collected under vacuum and the sample dosed with 8 mbar of O2 revealed a unit cell contraction upon O2 binding from 31.2090(14) to 31.0044(3) Å (cubic space group Fm-3m, see Figures and), while a smaller unit cell contraction was characterized upon dosing the material with 9 mbar of N2 at 195 K (from 31.2090 (14) to 31.0997(11) Å, Figure ). These results suggest a greater perturbation of the local electronic structure around the copper(I) ions upon O2 binding versus N2 binding. Rietveld refinement of the diffraction data collected for activated Cu2.4-MFU-4l revealed a structure consistent with that reported previously based on neutron powder diffraction data (Figure ). Rietveld refinement of the diffraction data for O2-dosed Cu2.4-MFU-4l using the structure of the activated framework as a starting model revealed electron density above the copper sites that was refined as O2, yielding an occupancy of 2.8(4) molecules per node (Figures and). However, the structural disorder of the O2 motif about the C3 axis as well as the crystallographic superposition of the zinc and copper ions preclude meaningful commentary on structural metrics or the nature of O2 binding. We turned to variable-temperature in situ DRIFTS to further elucidate the nature of O2 binding in Cu2.4-MFU-4l. Dosing the material with up to 1 bar of O2 at 300 K resulted in clear changes in the fingerprint region (600 to 1400 cm -1 ) (Figure ). To elucidate the features arising from bound O2, we conducted identical experiments in which Cu2.4-MFU-4l was dosed at 263 K with 8 mbar of either natural abundance O2 (99.8% 16 O2) or 18 O2 (97 atom % 18 O) to saturate the Cu I sites. Spectra were then collected at 5 K intervals as the sample was warmed from 263 to 298 K. A set of difference spectra generated by subtracting the 18 O2-dosed spectra from the O2-dosed spectra are plotted in Figure and clearly show positive and negative peaks corresponding to O2 and 18 O2 vibrations, respectively. Intriguingly, two sets of features were generated upon O2 dosing: one pair at 1131 and 1051 cm -1 and another at 1073 and 993 cm -1 for the O2-and 18 O2-dosed samples, respectively. The shift in the peak positions upon going from the O2-to the 18 O2-dosed sample is consistent with what is predicted based on the simple harmonic oscillator model for O2 (peaks for 18 O2-dosed material are predicted to be at 1066 and 990 cm -1 based on a simple harmonic oscillator model). As such, we assign both sets of features to adsorbed O2 species.