Abstract
A neutron diffraction study of 2,5-dichlorobenzenesulphonic acid trihydrate has been undertaken in order to investigate the precise hydrogen atom arrangement within the ’’H7O+3’’ unit. The cationic species can be unambiguously formulated as H3O+⋅2H2O, the diaquooxonium ion, although the hydrogen bond distances are short and comparable to those found in diaquohydrogen ions, (H2O⋅H⋅OH2)+. Considerable asymmetry is observed in these hydrogen bonds. The oxonium ion geometry is highly distorted from trigonal symmetry, with dO–H of 1.08(2), 0.98(1), and 0.94(2) Å, and the ion is quite flat. The derived H3O+ ion geometry, and the high thermal motion of the oxygen atom, may indicate some sort of disorder involving oxonium ion inversion. The structure was determined using 1906 independent three dimensional data measured with a neutron wavelength of 1.142(1) Å. The crystals are monoclinic, space group P21/c with unit cell dimension (x-ray) a=7.350(2) Å, b=23.516(3) Å, c=6.862(1) Å, and β=100.68(4) °. The structure was refined by full matrix least-squares techniques on F2o, with anisotropic thermal parameters for all atoms, to R=0.089.
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