The hydrated proton H+(H2O)n. V. Observation of two diaquohydrogen ions, (D2O⋅D⋅OD2)+, in 5-sulphosalicylic acid dideuterate. A neutron diffraction study

The crystal and molecular structure of 5-sulphosalicylic acid dideuterate has been studied using three-dimensional single crystal neutron diffraction data. The triclinic unit cell, space group P1̄, contains two formula units, with dimensions a=7.028(5), b=7.007(5), c=11.678(8) Å, α=71.49(4), β=94.07(2), and γ=109.35(2) °. A total of 2883 unique reflections were collected and full matrix least-squares refinement on F0 2 yielded an R=0.079. Two almost identical D5O2+ cations were found, and both reside on centers of symmetry, suggesting that the cations are best formulated as (D2O⋅D⋅OD2)+ species. The bridging deuterium atoms have their largest vibrational amplitudes along the O–O direction, which is indicative of a broad–flat anharmonic single-minimum potential energy well. The slight deviations from this direction are correlated with the terminal oxygen atom vibrations. X-ray versus neutron difference density maps show pronounced polarization of the C–H and O–D bonds.