Structures and Energy Landscapes of Hydrated Sulfate Clusters

The
sulfate ion is the most kosmotropic member of the Hofmeister
series, but the chemical origins of this effect are unclear. We present
a global optimization and energy landscape mapping study of microhydrated
sulfate ions, SO42–(H2O)n, in the size range 3 ≤ n ≤ 50. The clusters are modeled using a rigid-body empirical
potential and optimized using basin-hopping Monte Carlo in conjunction
with a move set including cycle inversions to explore hydrogen bond
topologies. For clusters containing a few water molecules (n ≤ 6) we are able to reproduce ab initio global minima, either as global minima of the empirical potential,
or as low-energy isomers. This result justifies applications to larger
systems. Experimental studies have shown that dangling hydroxyl groups
are present on the surfaces of pure water clusters, but absent in
hydrated sulfate clusters up to n ≈ 43. Our
global optimization results agree with this observation, with dangling
hydroxyl groups absent from the low-lying minima of small clusters,
but competitive in larger clusters.