Difference spectra generated by subtracting DRIFTS data collected upon warming (100 to 300 K) a sample of Cu2.4-MFU-4ldosed with18O2(negative features) from the corresponding data for Cu2.4-MFU-4ldosed with O2(positive features). Peaks corresponding to a secondary, less activated superoxide species appear at 200 K. The lower and higher energy features in both sets of spectra are assigned to superoxide species bound to copper(II) in a side-on and end-on fashion, respectively. Insets depict superoxide overtones.
