Theoretical understanding on the v 1 ‐SO 4 2− band perturbed by the formation of magnesium sulfate ion pairs

The factors determining the spectroscopic characteristics of the v 1 ‐SO 4 2− band of the MgSO 4 ion pairs are discussed via ab initio calculation, including coupling effect, hydrogen bonding effect, and direct contact effect of Mg 2+ with SO 4 2− . With the calculation of the heavy water hydrated contact ion pairs (CIP), the overlap between the librations of water and the v 1 ‐SO 4 2− band can be separated, and thus the coupling effect is abstracted, and this coupling effect leads to a blue shift for the v 1 ‐SO 4 2− band of 5.6 cm −1 in the monodentate CIP and 3.6 cm −1 in the bidentate CIP. The hydrogen bonding between each water molecule without relation to Mg 2+ and the sulfate ion makes the v 1 ‐SO 4 2− band blue shift of 3.7 cm −1 . When the outer‐sphere water around Mg 2+ are hydrogen bonded between SO 4 2− and Mg 2+ , it will make the largest disturbance to the v 1 ‐SO 4 2− band. Moreover, the inner‐sphere water can affect the v 1 ‐SO 4 2− band conjunct with the direct contact of Mg 2+ with SO 4 2− , showing a blue shift of 14.4 cm −1 in the solvent‐shared ion pair, 22.6 cm −1 in the monodentate CIP, 4.3 cm −1 in the bidentate CIP, and 21.4 cm −1 in the tridentate CIP. At last, the Raman spectral evolution in the efflorescence production process is tried to be rationalized. The shoulder at 995 cm −1 is attributed to the monodentate CIP with 2–3 outer‐sphere water molecules, whereas the new peak at 1021 cm −1 at high concentration is assigned to the formation of aqueous triple ion. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009