Four new hydronium ion structures are investigated by means of quantum mechanical calculations at the DFT/B3LYP6-311+G(2d,2p) level of theory. There exist experimental crystallographic hydronium cations (H11O5 +) of two different geometrical structures, one BEXFEQ (acyclic) and one IYEPEH (cyclic). Molecular calculations reveal their relative stability. Another hydronium cation NEBDII (H15O7 +) when optimized reveals a totally new and unexpected structure. All three optimized structures are shown to be quite stable as judged by their binding energies, and therefore may possibly be found in solution. A main result of this article is the discovery of three new optimized structures of hydronium ions, all of which are preferentially ring structures. The optimized structure of H15O7 + is a cube lacking a vertex. Putting a water molecule at the "empty" vertex leads by energy optimization to a structure of H17O8 + which has the approximate symmetry of a cube. This cubic structure, as judged by its fragments, is one of the most interesting of the hydronium ions studied in this paper. The addition of H3O+ to a group of seven neutral molecules in the hypothetical reaction H3O+ + 7 H2O → H17O8 + induces two water molecules to each capture a proton at the expense of two other water molecules (converting them into hydroxyl anions) leading to a cluster with the formula , where the superscripts are the integrated QTAIM atomic charges (in atomic units) on the respective species (inside the bracket) or on groups of a given species (outside the bracket). The cubic arrangement of 3H3O+.3H2O.2OH- is accompanied with a significant redistribution of charge: Each hydronium cation carries ca. +0.7 au, the hydroxyl anions only around -0.6 au each, while the water molecules remain quasi-neutral with a slight positive charge.
Keywords: Bond paths; Critical points; Hydronium ion; Optimized structures; Quantum calculations; Vibration frequencies.