The composition, charge and architecture of hydronium ions as observed in the crystalline state

The structures of hydronium ions present in single crystals have, by and large, been thought as being relatively simple and characterized by the classical formulae H3O+, H5O2+, H7O3+ and H9O4+. All of them are thought to be quasi-planar, acyclic entities of charge +1, exclusively. In this review, we show that such is not the case in view of the fact that there exist documented examples of cyclic hydronium ions, that their charge can be larger than +1 and that they exist in aggregates containing more than four oxygens. Moreover, it is documented that hydronium ions exist in the form of geometrical isomers or conformers. To cite this article: I. Bernal, C. R. Chimie 9 (2006).