Theoretical Studies On Structures In Ground State And IR Vibrational Spectra Of Hydrated Sodium Sulfate Ion Clusters

When the sodium sulfate ionic pair microsolvates into water, the sodium ion and the sulfate ion separate from each other induced by water molecules interacting. But it is still not clear that how it happens. In this article, theoretical study has been carried out on the microsolvation of sodium sulfate ion pair solvating in water by using quantum theory calculation and molecular vibrational analysis.The properties, including geometrical configurations, energies, electron density distribution and vibrational frequencies, of some stable hydrated clusters of sodium sulfate ion pair, NaSO4- (H2O)n(n=0-5), were investigated using MP2 method with aug-cc-pvdz basis set. The result indicates that the bonding form of ion pair in dominance changes from a tripod shape, which three oxygen atoms in sulfate ion coordinate with sodium ion, to a doubted-teeth shape, which two oxygen atoms in sulfate ion coordinate with sodium ion, with the water molecules increasing. Meanwhile, the two obvious variations also represents on moving in the peaks of IR vibrational spectra. First, the splitting of the peaks in ～800-1300 cm-1 region of the IR spectra, which contains the information of S-O2 bond symmetric stretching and asymmetric stretching of the anion, indicates the deviation of the cation from the C3v axis as well as the asymmetric distribution of the water molecules around the sodium sulfate ionic pair. The frequency of the stretching vibration peak of hydroxyl existing in water molecules and strong hydrogen bonding with sulfate ion in the ～2300-3000 cm-1 window, on the other hand, provides the information on the position of the sodium ion being further away with the strong H-bond water molecule and the interaction changing from direct coordination to indirect network hydrogen-bonds between them. To some extent, this article describes the microsolvation in water for sodium sulfate ion pair and provides abundant structural information on the early stage of the microsolvation and has the potential to become a reference to experiment for infrared photon dissociation spectroscopy.