The hydrated proton H+(H2O)n. I. A single crystal neutron diffraction study of the oxonium ion in <i>p</i>-toluenesulfonic acid monohydrate, H3O+CH3C6H4SO3−

Abstract

The pyramidal nonplanar conformation of the oxonium ion has been confirmed in the first single crystal neutron diffraction study of a salt containing discrete H3O+ cations in an isolated state, p-toluenesulfonic acid monohydrate (H3O+CH3C6H4SO3−). All H atoms have been located to high precision and the cation has molecular dimensions O–H=1.011(8) (the numbers within parentheses here and throughout the paper are the estimated standard deviations in the least significant digits), 1.013(8), and 1.008(8) Å with H–O–H angles of 110.7(5), 109.2(5), and 111.2(5)°. The oxygen atom is 0.322(4) Å out of the plane of the three hydrogen atoms. Although unrestricted by crystal symmetry, the ion has essentially C3ν symmetry. O–H separations are characteristic of those found in similarly strongly H-bonded systems where the O...O separations are fairly short at 2.53 Å. The structure was determined using 1732 independent three-dimensional intensities measured using a neutron wavelength of 1.142 Å. The crystals are monoclinic, space group P21/c, with unit-cell dimensions (x ray) a=5.881, b=7.432, c=20.085 Å, and β = 97.95°. The structure was refined by full-matrix least-squares procedures, using anisotropic thermal parameters for all atoms, to an R(F2) of 0.067.