Magnesium dication plays many significant roles in biochemistry. While it is available to the environmentfrom both ocean waters and mineral salts on land, its roles in environmental and atmospheric chemistry arestill relatively unknown. Several pieces of experimental evidence suggest that contact ion pairing may notexist at ambient conditions in solutions of magnesium chloride up to saturation concentrations. This is nottypical of most ions. There has been disagreement in the molecular dynamics literature concerning the existenceof ion pairing in magnesium chloride solutions. Using a force field developed during this study, we show thatcontact ion pairing is not energetically favorable. Additionally, we present a concentration-dependent Ramanspectroscopic study of the Mg-Owater hexaaquo stretch that clearly supports the absence of ion pairing inMgCl2 solutions, although a transition occurring in the spectrum between 0.06x and 0.09x suggests a changein solution structure. Finally, we compare experimental and calculated observables to validate our force fieldas well as two other commonly used magnesium force fields, and in the process show that ion pairing ofmagnesium clearly is not observed at higher concentrations in aqueous solutions of magnesium chloride,independent of the choice of magnesium force field, although some force fields give better agreement toexperimental results than others.
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