Structures, interactions, and vibrations of an intramolecular hydrogen bond (IHB)-containing protonated ion(water)3 complex, g-enH(H2O)(3) (g-enH(+) = gauche-protonated ethylenediamine) and t-enH(+)(H2O)(3) (t-enH(+) = traus-protonated ethylenediamine) with no IHB were investigated by combined vibrational predissociation spectroscopy and ab initio calculations. The observed vibrational feature at 3552 cm(-1) (signature of cyclic ion-(water)(3) structure), consistent with ab initio results on the stabilities of structural isomers, suggested the IHB-assisted bicyclic structure for g-enH(+)(H2O)(3). Ab initio many-body analyses further indicated that the C Greater stability of bicyclic isomer versus monocyclic and bicyclic tripod isomers originates from the destabilization of the latter by the large relaxation energies and repulsive nonadditive interactions. The close correlation between the observed and calculated vibrational spectra for g-enH(+)(H2O)(3) revealed the coexistence of bicyclic isomers and monocyclic open isomers in our beam, consistent with the trend of the calculated Gibbs free energies at 150 K.