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 doublet 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.