Knowledge Bank

Molecular structures of protonated formamide-water clusters are investigated by vibrational predissociation spectroscopy in combination with density functional theory calculations. We produce the cluster ions by a supersonic expansion and identify their structures from a comparison of the observed and calculated NH and OH stretch spectra at $3400-3800cm-1$. The structural identification confirms theoretical predictions that O-protonation occurs in preference to N-protonation. The proton can be either localized at a site closer to formamide or at a site closer to water, depending sensitively on the structure and solvation number of cluster isomers. Further studies on the protonated N-methylformamide-water cluster ions reveal a characteristic free-NH stretch absorption at $3480cm-1$. We demonstrate in this work that a combined investigation of cluster ions by infrared spectroscopy and ab initio calculations may allow for detailed structural interrogation of protonated peptide ions at early stages of hydration in the gas phase.