Isomers and energy landscapes of micro-hydrated sulfite and chlorate clusters

Abstract

We present putative global minima for the micro-hydrated sulfite SO32−(H2O)N and chlorate ClO3−(H2O)N systems in the range 3≤N≤15 found using basin-hopping global structure optimization with an empirical potential. We present a structural analysis of the hydration of a large number of minimized structures for hydrated sulfite and chlorate clusters in the range 3≤N≤50. We show that sulfite is a significantly stronger net acceptor of hydrogen bonding within water clusters than chlorate, completely suppressing the appearance of hydroxyl groups pointing out from the cluster surface (dangling OH bonds), in low-energy clusters. We also present a qualitative analysis of a highly explored energy landscape in the region of the global minimum of the eight water hydrated sulfite and chlorate systems.This article is part of the theme issue ‘Modern theoretical chemistry’.

Manifolds of low energy structures for a magic number of hydrated sulfate: SO42−(H2O)24

“…Thus we use "cubicity" as a trend property of structures to exhibit cube-like motifs which can be high, low or absent (see Table 2). Similar trends have already been documented for hydrated, oxygen-containing polyatomic anions including sulfate, 30 perchlorate, 31 chlorate and sulfite, 33 iodate, 6,69 CO − 2 70 and also hydrated thiocyanate SCN − . 6,32 Another commonly found motif for SO 2− 4 (H 2 O) 24 is a series of fused rings including 4 and 6 water oxygens in alternation.…”

“…The latter type of isomerism has been discussed before for water clusters, 27,67 small hydrated sulfate ions 8 and for several ions embedded in water clusters with n=3 -50, including sulfate. [30][31][32][33] It was therefore deemed important to take this type of isomerism into account in a systematic way. As in reference 30 , the NetworkX package 68 was used to generate directed graphs where "directed" refers to the orientation of hydrogen bonds.…”

“…A track was opened by Hey et al in the case of ClO − 3 (H 2 O) 8 . 33 The two most stable isomers found differ by inversion along a 4-cycle and their interconversion was suggested to follow a three-step mechanism which only involves relative motions of intact water molecules, rather than exchange of hydrogens between water molecules as considered herein. The three steps involve breaking hydrogen bonds and forming others.…”

“…25,29 However, an extensive, even more an exhaustive, PES exploration for these medium-size clusters is likely to be achieved only using a classical representation of the energy with a force field. 24,[30][31][32][33][34] The quality of the force field has an influence on the relative energies of the various isomers in such non-covalent systems and polarization effects have been included to improve energetics. [34][35][36][37] The TTM2.1-F and TTM3-F flexible polarizable potentials have been developed to reproduce accurately infrared spectra of liquid water and ice by improving the description of the hydrogen bond network.…”

Modern theoretical chemistry: the legacy of Prof. John N. Murrell

“…One of the products, a protonated water cluster, has long been proposed as a precursor to cloud formation at very high altitudes. Clusters of water molecules as models for ion solvation are the subject of a paper by Hey et al [5]. Empirical potential energy surfaces have been constructed to compare the sulfite and chlorate ions in solution, and the calculations show that the former ion preferentially favours hydrogen bond formation.…”