Isomers and Energy Landscapes of Perchlorate–Water Clusters and a Comparison to Pure Water and Sulfate–Water Clusters

Abstract

Hydrated ions are crucially important in a wide array of environments, from biology to the atmosphere, and the presence and concentration of ions in a system can drastically alter its behavior. One way in which ions can affect systems is in their interactions with proteins. The Hofmeister series ranks ions by their ability to salt-out proteins, with kosmotropes, such as sulfate, increasing their stability and chaotropes, such as perchlorate, decreasing their stability. We study hydrated perchlorate clusters as they are strongly chaotropic and thus exhibit different properties than sulfate. In this study we simulate small hydrated perchlorate clusters using a basin-hopping geometry optimization search with empirical potentials. We compare topological features of these clusters to data from both computational and experimental studies of hydrated sulfate ions and draw some conclusions about ion effects in the Hofmeister series. We observe a patterning conferred to the water molecules within the cluster by the presence of the perchlorate ion and compare the magnitude of this effect to that observed in previous studies involving sulfate. We also investigate the influence of the overall ionic charge on the low-energy structures adopted by these clusters.

Investigation of the Structures and Energy Landscapes of Thiocyanate-Water Clusters

“…Candidate transition states were then optimised using hybrid eigenvector-following [38,39]. The resultant energy landscapes were As in our previous studies [4,5], the water molecules in the thiocyanate-water clusters are modelled using the TIP4P potential (with parameters listed in Table 1), with the water molecule represented by a rigid four-body molecule with a H-O-H bond angle of 104.52 • and O-H bond lengths of 0.9572 Å, and with an oxygen "lone pair" site (located 0.15 Å from the O atom along the bisector of the H-O-H angle, on the H side of the molecule) which carries the oxygen's charge but which has no LJ interaction (the LJ interaction is centred on the O atom itself) [15,26]. The computationally inexpensive TIP4P model has been widely used to model water clusters [19,27,28], though it should be noted that the simple TIP4P model fails to reproduce some properties of bulk water and ice [29].…”

“…Closed cycles of hydrogen bonds within the graph are identified; one of the cycles is inverted and the graph is then projected back onto the hydrogen bonding network of the original cluster [4]. In our previous studies of sulfate-and chlorate-water clusters, these cycle inversions were found to have a greater effect on total energy for larger hydrogen bonded cycles and, hence, to be more beneficial for the global optimisation of larger clusters [4,5].…”

“…While in the sulfate-water clusters, the more highly charged sulfate anion was found to lie in the centre of the cluster, generating structures quite different from pure water clusters, the chlorate anion (with only a single negative charge) was found to occupy surface sites in the cluster, with overall structures similar to those of the pure water clusters [5]. This led to a greater number of dangling water O-H bonds compared to the sulfate-water case, where the suppression of dangling O-H bonds (compared to pure water clusters) has been confirmed by infra-red multiphoton dissociation (IRMPD) measurements of Williams et al [6,7].…”

“…In our previous studies of water-sulfate and water-perchlorate clusters, we enumerated the number of dangling O-H bonds (i.e., those bonds which are not involved in hydrogen bonding either to other water molecules or to the anion) [4,5]. We found that, although the presence of both anions result in a reduction in the number of dangling O-H bonds, compared to pure water clusters, the effect is much more marked for the higher charged (and centrally located) SO 4 2− ion, which acts as a sink for hydrogen bonds.…”

“…We have previously studied sulfate (SO 4 2− ) (kosmotropic) [4] and perchlorate (ClO 4 − ) (chaotropic) [5] ions solvated in finite water clusters. While in the sulfate-water clusters, the more highly charged sulfate anion was found to lie in the centre of the cluster, generating structures quite different from pure water clusters, the chlorate anion (with only a single negative charge) was found to occupy surface sites in the cluster, with overall structures similar to those of the pure water clusters [5].…”