Geometries and Binding Energies of Hn+

Ab initio calculations were made for Hn+(n=4, 5, 6), with simple Gaussian orbital wavefunctions. Their geometries were determined by minimizing the total energy with respect to relative proton positions. The stable ions, their geometries, electronic symmetries, and estimated binding energies are: H4+, planar C2v, 2A1, 0.54 eV; and H5+, D2d, 1A1, 0.81 eV: H6+ is unstable. The electron densities and vibration frequencies of H4+ and H5+ were also calculated. A hydrogen-bond model is proposed for the binding of H4+ and H5+ (with extensions to H6+ and H7+). The importance of this model for kinetics of certain ion reactions is discussed.