For the (I 2 O 5 )(IO - 3 ) cluster, we find two nearly isoenergetic structures. The computed spectrum of isomer 1a shows very good agreement with the experimental spectrum, reproducing the three peaks in the 800-900 cm -1 range nearly quantitatively. It also captures the position of the peak near 700 cm -1 accurately. The intensity of this peak is likely suppressed in the experimental spectrum due to the binding energy of the N 2 adduct. This binding energy is typically ∼800 cm -1 , so a single photon of less than 800 cm -1 is not always sufficient to induce tag dissociation. The spectrum of isomer 1c 71 is also broadly consistent with the experimental spectrum, but the relative intensity patterns and peak positions in the 800-900 3 ) versus their lowest three energy computed spectra. All relative energies are reported in kcal/mol. Structure 1c has been reported before in a previous study. We assign structures 1a and 2b as the experimentally-present structures. None of the current computed spectra satisfactorily replicate the experimentally present one, but similarities between the experimental spectra of (I 2 O 5 ) 2 (IO - 3 ) and (I 2 O 5 ) 3 (IO - 3 ) suggest comparable structures for these clusters. cm -1 range display a worse match than Isomer 1a. Isomer 1c is also significantly higher in energy; as a result we do not expect it to be present in the experiment.