and the bisulfate free O-H to the transition at 3609 cm -1 (denoted as "OH G " and "OH S " in the spectrum, respectively). The small, low intensity peak at 3505 cm -1 has no clear structural origin. In order to identify the spectral manifestations of tag-induced red-shifts, we collected the D 2 tagged CIVP spectrum of this cluster shown in Figure . As shown in S9, D 2 tagging does not significantly affect the peak at 3505 cm -1 , suggesting that the origin of this peak cannot be explained as tag-effect alone. Lower in energy is a set of three peaks belonging to the stretching motion of the free N-H of both ammonia and glycine. The broad, relatively intense feature between 2000 -3100 cm -1 corresponds to the set of anharmonic features arising from hydrogen bonded moieties in the cluster. Also based on literature precedent, we assign the dominant modes below 1900 cm -1 as the carbonyl C=O stretch at 1785 cm -1 , the N-H scissor/twist and N-H in-phase bending modes at 1633 cm -1 and 1546 cm -1 , the SO 3 asymmetric stretch at 1234 cm -1 and symmetric stretch at 1055 cm -1 , and finally the S-OH stretch at 842 cm -1 . The carbonyl C=O stretching transition of glycine has been previously observed at 1790 cm -1 , suggesting that the observed small (∼5 cm -1 ) shift is likely due to a hydrogen bonding interaction between the carbonyl group and a hydrogen bond donor in the cluster. Computed spectra and structures for the lowest energy isomers are shown in Figure . Inspecting the free O-H stretching region of the calculated spectra, we note that structures A and C feature only one peak, while the experimental spectrum has two. However, on closer inspection it is clear that the OH G and OH S are essentially isoenergetic in structure C but are separated by ∼10 cm -1 in structure A. Structure B does reproduce the two free O-H peaks found in the experimental spectrum. However, the complex pattern of peaks in the fingerprint region are not unambiguously reproduced by any calculated spectrum. We hypothesized that the presence of isomers could explain the observed spectral complexity of this region. The IR-IR hole burning spectra shown in Figure make it clear, however, that there is only one isomer experimentally present. The calculated spectra of structures A and B are consistent with the experimental spectrum, although neither spectrum is an exact match.