Radial and orientational solvation structure of the sodium chloride ion pair in dimethyl sulfoxide

Abstract

The structures of the solvation shells around each ion of the Na+–Cl− ion pair in liquid dimethyl sulfoxide (DMSO) have been studied in terms of the ion–solvent radial distribution functions (RDFs) and the ion–solvent orientational distribution functions (ODFs) at the three interionic separations of 2.6 Å, 4.9 Å, and 7.2 Å. The solvation shell around the sodium ion consists of only three DMSO molecules at the ion–ion separation of 2.6 Å and this number grows to five DMSO molecules at interionic separation 4.9 Å and beyond. These are in contrast with the octahedral solvation shells around sodium ion in water at all ion–ion separations, where the chloride ion replaces a molecule of water only at a short interionic distance of 2.7 Å. The orientational structure of the solvent around the ion pair has been probed by dividing the DMSO solvent into five spatial regions and analyzing the angular distributions in each region. In the shell near the Na+ ion, the orientation of the sulphur–oxygen vector in DMSO is sharply peaked about 155° away from the sodium–sulphur vector for all the three interionic distances. Similarly, the orientation of the DMSO dipole vector is also sharply peaked about 155° away from the sodium–DMSO center of mass (COM) vector. In the shell near the Cl− ion, the orientation of the sulphur–oxygen vector with respect to the chloride-sulphur vector shows broader peaks in the range 20°–100°. The solvent dipole vector gets oriented in a similar fashion with respect to the chloride–COM vector in this shell. In the regions far from the Na+ and Cl− solvation shells, both the sulphur–oxygen vector and the solvent dipole vector have broad distributions covering all the angles except the parallel and the antiparallel alignments. The angles between the Na+–S–O plane (or the Cl−–S–O plane) and the S–Na+–Cl− plane do not show a preference for any specific inclination, in any of the spatial regions around the ion pair. These broad distributions are indicative of a weaker second shell around the ion pair in DMSO than the second shell found in water and are a consequence of the near absence of hydrogen bonding in DMSO.