Moving on to the larger (1, 2, 0) cluster in Figure , we begin to see more vibrational modes appear, most notably above 3400 cm -1 . Since this cluster contains two iodic acid molecules and is singly protonated, we expect both iodic acid molecules to remain protonated resulting in the presence of OH modes in the experimental spectrum. We do see both free OH stretching and NH 2 asymmetric and symmetric stretching modes implying that the experimentally present isomer contains at least one free iodic acid OH as well as two nonhydrogen bonded NHs. We also see what looks to be two peaks between 1000 and 1200 cm -1 , which are below the typical NH bending and wagging mode region and generally indicative of either a free or hydrogen bonded OH bending mode. The hydrogen bonded OH bend in the previously studied (NH 3 ) 4 (HSO 4 ) 3 H + dry and hydrated clusters was found to lie between 1300 and 1400 cm -1 , 14,15 which would imply that the higher energy mode closer to 1200 cm -1 is presumably a hydrogen bonded OH bending mode and the lower energy, lower intensity peak is possibly a free OH bend. These features allow us to eliminate the likelihood of the presence of either higher energy isomer, B or C, in the experimental spectrum. Isomer C, can also confidently be eliminated as a candidate, as this structure is significantly higher in energy than the lowest energy isomer, isomer A. The experimentally present structure is therefore likely isomer A, where there are both a free NH 2 and OH component, as well as a hydrogen bonded OH component. The relative intensity pattern within the fingerprint region of the experimental spectrum is also strikingly similar to that of isomer A, with the IO stretching modes around 800 cm -1 most closely reproduced in the calculated spectrum of isomer A too.