The hydrated proton H+(H2O)n. I. A single crystal neutron diffraction study of the oxonium ion in p-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 $({\mathrm{H}}_3{\mathrm{O}}^{+}{\mathrm{CH}}_3{\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{SO}}_3^{-})$ . 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 C_3ν 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.