Manifolds of low energy structures for a magic number of hydrated sulfate: SO₄^2-(H₂O)₂₄

Low energy structures of SO₄^2-(H₂O)₂₄ have been obtained using a combination of classical molecular dynamics simulations and refinement of structures and energies by quantum chemical calculations. Extensive exploration of the potential energy surface led to a number of low-energy structures, confirmed by accurate calibration calculations. An overall analysis of this large set was made after devising appropriate structural descriptors such as the numbers of cycles and their combinations. Low energy structures bear common motifs, the most prominent being fused cycles involving alternatively four and six water molecules. The latter adopt specific conformations which ensure the appropriate surface curvature to form a closed cage without dangling O-H bonds and at the same time provide 12-coordination of the sulfate ion. A prominent feature to take into account is isomerism via inversion of hydrogen bond orientations along cycles. This generates large families of ca. 100 isomers for this cluster size, spanning energy windows of 10-30 kJ mol^-1. This relatively ignored isomerism must be taken into account to identify reliably the lowest energy minima. The overall picture is that the magic number cluster SO₄^2-(H₂O)₂₄ does not correspond to formation of a single, remarkable structure, but rather to a manifold of structural families with similar stabilities. Extensive calculations on isomerization mechanisms within a family indicate that large barriers are associated to direct inversion of hydrogen bond networks. Possible implications of these results for magic number clusters of other anions are discussed.