Hydrogen-Bond Studies. XXIX. Crystal Structure of Perchloric Acid Dihydrate, H5O2+ClO4−

The crystal structure of HClO₄·2H₂O has been determined from single‐crystal x‐ray data obtained at −190°C. The crystals are orthorhombic, space group Pnma, with four formula units in a cell of dimensions: $\text{a\hspace{0.17em}=\hspace{0.17em}5.819}$ , $\text{b\hspace{0.17em}=\hspace{0.17em}10.598}$ , $\text{c\hspace{0.17em}=\hspace{0.17em}7.369 Å}$ . The water molecules are bonded to each other into pairs by a very short hydrogen bond (2.424 Å), forming H₅O₂⁺ ions. With the space group chosen there is a center of symmetry in the middle of this hydrogen bond; the H₅O₂⁺ ion is perfectly staggered. The H₅O₂⁺ ions are hydrogen bonded to the ClO₄⁻ ions into layers, which are held together by van der Waals forces. The average Cl–O distance in the perchlorate ion is 1.438 Å.