Ab initio investigation of the structure and spectroscopy of hydronium-water clusters

Ab initio (MP2, CASSCF, CASPT2) and DFT/B3LYP calculations have been performed to explore the structures and the electronic and vibrational spectra of hydronium−water clusters as well as the corresponding cluster cations. Minimum-energy and transition-state structures have been optimized at the MP2 and DFT/B3LYP levels. Whereas protonated water clusters can exist both in Eigen-type structures, H3O+(H2O)n, as well as in Zundel-type structures, H5O2+(H2O)n, the neutral radical clusters are found to prefer Eigen-type structures, H3O(H2O)n. While H3O(H2O)3, like the hydronium radical, is a metastable species, it exhibits a significantly higher barrier for hydrogen detachment (7 kcal/mol), indicating the possibility of kinetic stability of larger H3O(H2O)n clusters at low temperatures. Remarkably, hydronium−water clusters are charge-separated species, consisting of a hydronium cation and a localized electron cloud, which are connected by a water network. The vertical electronic excitation energies of the vario...