Unlike in the (2, 1, 1) cluster case, all three lowest energy isomers (A, B, and C) are quite similar in energy. However, the experimental spectrum has several key features that allow the assignment of the experimentally present isomer to be relatively straightforward. One feature is the presence of the free OH stretch in the experimental spectrum. Another feature is the two sets of doublets between 1400 and 1500 cm -1 in the NH 2 / NH 3 bending/wagging mode region. The final feature is the presence of a free OH bend. Using these three vibrational mode regions, both isomers B and C can be eliminated almost immediately. Only isomer Figure : The experimental CIVP spectrum (top) is compared to the computed harmonic IR spectra (bottom) of the (2, 2, 1) cluster. Isomer A, B, and C are all relatively close in energy, with isomer A marginally lowest in energy. The main features involve OH stretching and bending modes (peaks in red), NH stretching and NH 2 / NH 3 bending modes (peaks in blue), and IO stretching modes (peaks in purple). Isomer A appears to be the experimentally present isomer as it has a free OH stretching mode, a singular NH stretching mode, as well as a relative peak intensity pattern that is well replicated in the fingerprint region.