X-ray structure refinement of CsHSO₄ in phase II

An accurate room temperature structure refinement of CsHSO₄ in phase II, the thermodynamically stable phase at temperatures below 414 K, is reported. The compound is monoclinic, space group P2₁/c, with lattice parameters a = 7.781(2), b = 8.147(2), c = 7.722(2) angstroms, and β = 110.78(1)°, as determined by single crystal X-ray diffraction. There are four formula units per unit cell and seven atoms in the asymmetric unit. The compound, comprised of zigzag chains of SO₄ groups alternating with zigzag rows of Cs atoms, is isostructural to CsDSO₄. The single, crystallographically distinct proton forms a `symmetry-free' hydrogen bond between O(1) and O(2). The S-O bond lengths are consistent with the presence of this bond. The hydrogen bond geometries in the protonated and deuterated compounds are within experimental error of one another, after taking due account of the difference in proton (or deuterium atom) positions as measured by X-ray and neutron diffraction.