Structure and dynamics of sulfate ion in aqueous solution - An ab initio QMCF MD simulation and large angle X-ray scattering study

The hydrated sulfate ion has been characterized in aqueous solution in structural and dynamic aspects using ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation and large angle X-ray scattering (LAXS) methods. The LAXS data show an average coordination number of the sulfate ion of up to 12 water molecules bound through hydrogen bonding, while the QMCF MD simulation displays a wide range of coordination numbers between 8 and 14 with an average value of similar to 11. The O-s center dot center dot center dot O-w distance cannot be distinguished from the O-w center dot center dot center dot O-w distance in the LAXS experiment; the weighted mean O center dot center dot center dot O distance is 2.880(10) A. In the simulation, the O-s center dot center dot center dot O-w and O-w center dot center dot center dot O-w distances are found to be very similar, namely, 2.86 and 2.84 A, respectively. The S-O-s bond and S center dot center dot center dot O-w distance have been determined by the LAXS experiment as 1.495(6) and 3.61(2) A, respectively, indicating an average nearly tetrahedral S-O-s center dot center dot center dot O-w angle. The similar to 5% deviations of simulation distances (1.47 and 3.82 A) from the experimental ones can probably be ascribed to the neglect of correlation energy in the quantum mechanical method. The mean residence time of water ligands at O atoms, 2.57 ps, is longer than that in pure water, 1.7 ps, characterizing the sulfate ion as a weak structure maker.