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

Manifolds of low energy structures for a magic number of hydrated sulfate: SO42-(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.

Modeling microsolvation clusters with electronic-structure calculations guided by analytical potentials and predictive machine learning techniques

We propose a new methodology to study, at the density functional theory (DFT) level, the clusters resulting from the microsolvation of alkali-metal ions with rare-gas atoms. The workflow begins with a global optimization search to generate a pool of low-energy minimum structures for different cluster sizes. This is achieved by employing an analytical potential energy surface (PES) and an evolutionary algorithm (EA). The next main stage of the methodology is devoted to establish an adequate DFT approach to treat the microsolvation system, through a systematic benchmark study involving several combinations of functionals and basis sets, in order to characterize the global minimum structures of the smaller clusters. In the next stage, we apply machine learning (ML) classification algorithms to predict how the low-energy minima of the analytical PES map to the DFT ones. An early and accurate detection of likely DFT local minima is extremely important to guide the choice of the most promising low-energy minima of large clusters to be re-optimized at the DFT level of theory. In this work, the methodology was applied to the Li+Krn (n = 2-14 and 16) microsolvation clusters for which the most competitive DFT approach was found to be the B3LYP-D3/aug-pcseg-1. Additionally, the ML classifier was able to accurately predict most of the solutions to be re-optimized at the DFT level of theory, thereby greatly enhancing the efficiency of the process and allowing its applicability to larger clusters.

SN2 Reactions of N2O5 with Ions in Water: Microscopic Mechanisms, Intermediates, and Products

Reactions of dinitrogen pentoxide (N₂O₅) greatly affect the concentrations of NO₃, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N₂O₅ with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl^-, SO₄^2-, ClO₄^-, and RCOO^- (R = H, CH₃, C₂H₅). The following main results are obtained: (i) all the reactions take place by an S_N2-type mechanism, with a transition state that involves a contact ion pair (NO₂⁺NO₃^-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO₂⁺) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO₂ formation from N₂O₅ reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO₄NO₂^-) and RCOONO₂, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.