The spectrum of AAH + -H 2 O-CO is presented in Figure alongside the two lowest energy computed structures. The experimental spectrum presents two primary features, an OH stretch consistent with that of the fragment of the first water loss, and an intense peak in the region previously ascribed to CO stretching. The computed harmonic vibrational spectrum of the lowest energy structure (A), a six-membered ring, is in good agreement with the observed CIVP spectrum, capturing both the prominent OH stretch and the CO stretches. Recalling that this fragment is produced from the putative seven-membered ring discussed above, rather than the five-membered ring resulting from the second loss of water, this structure seems mechanistically plausible. In fact, the spectra are nearly identical, lacking only the higher energy CO stretching band that is presumably lost as neutral CO upon dissociation. The second lowest energy structure (B) is a five-membered ring significantly higher in energy, and predicts two unique OH stretches that are not observed experimentally.