Solvation Properties of Microhydrated Sulfate Anion Clusters: Insights from <i>ab</i> <i>Initio</i> Calculations

Sulfate–water clusters play an important role in environmental and industrial processes, yet open questions remain on their physical and chemical properties. We investigated the smallest hydrated sulfate anion clusters believed to have a full solvation shell, with 12 or 13 water molecules. We used <i>ab initio</i> molecular dynamics and electronic structure calculations based on density functional theory, with semilocal and hybrid functionals. At both levels of theory we found that configurations with the anion at the surface of the cluster are energetically favored compared to fully solvated ones, which are instead metastable. We show that infrared spectra of the anion with different solvation shells have similar vibrational signatures, indicating that a mixture of surface and internally solvated geometries are likely to be present in the experimental samples at low temperature. In addition, the computed electronic density of states of surface and internally solvated clusters are hardly distinguishable at finite temperature, with the highest occupied molecular orbital belonging to the anion in all cases. The equilibrium structure determined for SO₄^2–·(H₂O)₁₃ differs from that previously reported; we find that the addition of one water molecule to a 12-water cluster modifies its hydration shell and that water–water bonds are preferred over water–anion bonds

Electronic Structure of Aqueous Sulfuric Acid from First-Principles Simulations with Hybrid Functionals

We carried out the first ab initio molecular dynamics simulations of aqueous sulfuric acid solutions using hybrid density functionals and a concentration (∼1 mol/L) similar to that of electrolyte solutions used in photocatalytic water splitting experiments. We found that while the semilocal functional PBE greatly overestimates the degree of dissociation of the HSO₄^– ion, the hybrid functional PBE0 yields results in qualitative agreement with those of recent Raman measurements. Our findings highlight the importance of using hybrid functionals in the description of anion solvation. We further analyzed the electronic structure of the solution and found that the energy of the highest occupied molecular orbital of the anion is above that of the water valence band maximum only in the case of SO₄^2–. This indicates that SO₄^2– may be kinetically favored, instead of HSO₄^–, in scavenging photoexcited holes from photoanodes in water oxidation reactions

Raman Spectra of Liquid Water from <i>Ab Initio</i> Molecular Dynamics: Vibrational Signatures of Charge Fluctuations in the Hydrogen Bond Network

We report the first <i>ab initio</i> simulations of the Raman spectra of liquid water, obtained by combining first principles molecular dynamics and density functional perturbation theory. Our computed spectra are in good agreement with experiments, especially in the low frequency region. We also describe a systematic strategy to analyze the Raman intensities, which is of general applicability to molecular solids and liquids, and it is based on maximally localized Wannier functions and effective molecular polarizabilities. Our analysis revealed the presence of intermolecular charge fluctuations accompanying the hydrogen bond (HB) stretching modes at 270 cm^–1, in spite of the absence of any Raman activity in the isotropic spectrum. We also found that charge fluctuations partly contribute to the 200 cm^–1 peak in the anisotropic spectrum, thus providing insight into the controversial origin of such peak. Our results highlighted the importance of taking into account electronic effects in interpreting the Raman spectra of liquid water and the key role of charge fluctuations within the HB network; they also pointed at the inaccuracies of models using constant molecular polarizabilities to describe the Raman response of liquid water