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

摘要

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-TRPDS).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 ring-type 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~+.