To probe the structural motifs leading to this unintuitive behavior, CIVP spectra were recorded of both the putative ammonium-adipic acid cluster, shown in Figure , and protonated adipic acid, shown in Figure . Included are the lowest energy structures found through quantum chemical calculations and the corresponding computed harmonic vibrational spectra. Focusing on the ammonium-bound cluster, we find a single sharp feature near 3600 cm -1 that is typical for a free OH stretch. At about 3400 cm -1 we see a doublet feature that is typical of free NH 2 symmetric and antisymmetric stretches. Between 2500 -3200 cm -1 we see relatively broad features that previous work has assigned to NH 2 stretches for NH 2 groups that are participating in a hydrogen bond. These observations are consistent with the hypothesis of a bridging ammonium. The spectra between 600 -2500 cm -1 are more complex, and here we turn to quantum chemical predictions to assign the rest of the features and confirm the hypothesized structure. Between 600 -1400 cm -1 we see features consisting primarily of OH bending character. Between 1400 -1700 cm -1 the spectral features are typically mixtures of NH 4 and CH 2 bending modes. Around 1800 cm -1 we see an intense feature that is likely reflective of CO stretches in the structure.