Size-selected infrared predissociation spectroscopy of neutral and cationic formamide-water clusters: Stepwise growth of hydrated structures and intracluster hydrogen transfer induced by vacuum-ultraviolet photoionization

Vibrational spectroscopy of size-selected formamide-water clusters, FA-(H₂O)_n, n = 1 -4, prepared in a supersonic jet is performed with vacuum-ultraviolet-ionization detected-infrared predissociation spectroscopy (VUV-ID-IRPDS). The cluster structures are determined through comparisons of the observed IR spectra with theoretical calculations at the MP2/6-31++G** level. The FA-(H₂O)_n, n = 1-3, clusters have ringtype structures, where water molecules act as both single donor and single acceptor in the hydrogen-bond network between the amino and carbonyl groups of FA. For FA-(H₂O)₄, on the other hand, the absence of the free NH stretching vibration indicates formation of a double ring type structure, where two NH bonds of the amino group and the carbonyl oxygen of FA form hydrogen bonds with water molecules. An infrared spectrum of the formamide-water cluster cation, [FA-H₂O]⁺, is also observed with infrared predissociation spectroscopy of vacuum-ultraviolet-pumped ion (IRPDS-VUV-PI). No band is observed for the free OH stretches of neutral water. This shows [FA-H₂O]⁺ has such a structure that one of the hydrogen atoms of the water moiety is transferred to the carbonyl oxygen of FA⁺.