Simulating the Favorable Aggregation of Monolacunary Keggin Anions.

We here present a series of classical molecular dynamics simulations (MD) on aqueous solutions of the salts Li5 AlW12 O40 and Li9 AlW11 O39 , providing us with valuable insight on their aggregative behavior. Analysis of relative dipole moment orientation in pairs of aggregated [AlW11 O39 ]9- excludes that their large dipole moment is behind their greater propensity to aggregate. On the other hand, MD simulations of the aqueous Li+ salt of the fictitious [AlW12 O40 ]9- -as high in charge as [AlW11 O39 ]9- , but lacking dipole moment and tetrahedral in shape like [AlW12 O40 ]5- -reveal that it is in fact the higher negative charge itself that promotes aggregation, by allowing to recruit a higher number of Li+ countercations, which then act as an electrostatic glue. The lower charge on [AlW12 O40 ]5- , on the other hand, is not able to muster enough Li+ countercations for it to aggregate favorably.