Figure shows the CIVP spectrum of this cluster along with the lowest energy calculated spectra. The experimental spectrum is broadly similar to the first two. Of the two peaks in the free O-H stretching region, the OH S mode (at 3620 cm -1 ) is more intense than the OH G , perhaps due to there being more SA than G in this cluster. Two low-intensity peaks at 3500 cm -1 and 3548 cm -1 were revealed to originate from tag-induced effects (see Figure ). The carbonyl stretching mode is seen at 1773 cm -1 . The other distinct modes in the fingerprint region are the S-OH and SO 3 symmetric stretches.