Structure and water exchange of the hydrated thiosulfate ion in aqueous solution using QMCF MD simulation and large angle X-ray scattering
Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) has been applied in conjunction with experimental large angle X-ray scattering (LAXS) to study the structure and dynamics of the hydrated thiosulfate ion, S2O32−, in aqueous solution. The S–O and SC–ST bond distances have been determined to be 1.479(5) and 2.020(6) Å by LAXS and to be 1.478 and 2.017 Å by QMCF MD simulations, which are slightly longer than the mean values found in the solid state, 1.467 and 2.002 Å, respectively. This is due to the hydrogen bonds formed at hydration. The water dynamics show that water molecules are exchanged at the hydrated oxygen and sulfur atoms, and that the water exchange is ca. 50% faster at the sulfur atom than at the oxygen atoms with mean residence times, τ0.5, of 2.4 and 3.6 ps, respectively. From this point of view the water exchange dynamics mechanism resembles the sulfate ion, while it is significantly different from the sulfite ion. This shows that the lone electron-pair in the sulfite ion has a much larger impact on the water exchange dynamics than a substitution of an oxygen atom for a sulfur one. The LAXS data did give mean SC⋯Oaq1 and SC⋯Oaq2 distances of 3.66(2) and 4.36(10) Å, respectively, and SC–Othio and Othio⋯Oaq1, SC–ST and ST⋯Oaq2 distances of 1.479(5), 2.845(10), 2.020(6) and 3.24(5) Å, respectively, giving SC–Othio⋯Oaq1 and SC–ST⋯Oaq2 angles close to 110°, strongly indicating a tetrahedral geometry around the terminal thiosulfate sulfur and the oxygens, and thereby, three water molecules are hydrogen bound to each of them. The hydrogen bonds between thiosulfate oxygens and the hydrating water molecules are stronger and with longer mean residence times than those between water molecules in the aqueous bulk, while the opposite is true for the hydrogen bonds between the terminal thiosulfate sulfur and the hydrating water molecules. The hydration of all oxo sulfur ions is discussed using the detailed observations for the sulfate, thiosulfate and sulfite ions, and the structure of the hydrated peroxodisulfate ion, S2O82−, in aqueous solution has been determined by means of LAXS to support the general observations. The mean S–O bond distances are 1.448(2) and 1.675(5) Å to the oxo and peroxo oxygens, respectively.