The hydrated proton H+(H2O)n. III. A neutron diffraction study of deuterated paratoluene sulfonic acid monohydrate, D3O+CH3C6H4SO3−, and an analysis of the effect of deuteration on oxonium ion geometry

Crystals of ${\mathrm{D}}_3{\mathrm{O}}^{+}{\mathrm{CH}}_3{\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{SO}}_3^{-}$ (DPTSA) are isomorphous with those of ${\mathrm{H}}_3{\mathrm{O}}^{+}{\mathrm{CH}}_3{\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{SO}}_3^{-}$ (PTSA), and the detailed molecular geometries of the two compounds are very similar. Deuteration causes an increase in the O–O distance in each of the three (H₃O⁺ · · · 0₃S⁻) hydrogen bonds, ef. 2.529(8) (the numbers within parentheses here and throughout the paper are the estimated standard deviations in the least significant digits) to 2.537(4), 2.545(8) to 2.549(4), and 2.522(8) to 2.537(4) Å. The height of the oxonium ion pyramid increased from 0.322(4) to 0.334(3) Å, and the H–O–H bond angles decreased an average of 0.6° to 108.6(3), 110.3(4), and 110.2°(4)°, respectively. Deuteration does not change the average O–H distance in the oxonium ion. The individual changes are small enough to be of doubtful significance. A total of 1732 independent, three dimensional intensities were measured using a neutron wavelength of 1.142(1) Å. The crystals are monoclinic, space group P2₁/c, with unit cell dimensions a = 5.885(2), b = 7.461(3), c = 20.148(7) Å, and β = 97.89(1)°. The unit cell contains four molecules. The structure was refined by full‐matrix least‐squares procedures varying anisotropic thermal parameters for all atoms, an isotropic extinction correction factor, the scale factors (S), and the deuterium scattering amplitudes. The final deuterium scattering amplitudes indicated that the oxonium ion was 90% deuterated. It was also shown that only the oxonium ion hydrogen atoms were replaced by deuterium. Final values of 0.053 for ${\mathrm{R(F}}_{\mathrm{O}}^2)$ and 1.24 for σ₁, the ``goodness of fit'' parameter, were obtained.