On the search for H5O2+-centered water clusters in the gas phase

In searching for H5O2+-centered water clusters, we employed vibrational predissociation spectroscopy and ab initio calculations. Structures of the clusters were characterized by the free- and hydrogen-bonded-OH stretches of ion cores and solvent molecules. Systematic examination of H+(H2O)(5.7) in a supersonic expansion reveals the presence of both cyclic and noncyclic forms of H5O2+-centered water clusters. The proton transfer intermediate H5O2+(H2O)(4) was identified, for the first time, by its characteristic hydrogen-bonded-OH stretches of the ion core at 3178 cm(-1). Also discovered at n = 7 is the H5O2+-containing five-membered ring isomer, whose existence is evidenced by the observation of a bonded-OH stretching doubler at 3544 and 3555 cm(-1) of the solvent molecules. The observations are in accord with ab initio calculations which forecast that H5O2+(H2O)(4) and H5O2+(H2O)(5) are, respectively, the lowest-energy isomers of protonated water hexamers and heptamers.