Spectroscopic properties and stability of the SH⋅H2O open shell complex

Radicals in aqueous solution: assessment of density-corrected SCAN functional

We study self-interaction effects in solvated and strongly-correlated cationic molecular clusters, with a focus on the solvated hydroxyl radical. To address the self-interaction issue, we apply the DC-r2SCAN method, with the auxiliary density matrix approach. Validating our method through simulations of bulk liquid water, we demonstrate that DC-r2SCAN maintains the structural accuracy of r2SCAN while effectively addressing spin density localization issues. Extending our analysis to solvated cationic molecular clusters, we find that the hemibonded motif in the [CH3S∴CH3SH]+ cluster is disrupted in the DC-r2SCAN simulation, in contrast to r2SCAN that preserves the (three-electron-two-center)-bonded motif. Similarly, for the [SH∴SH2]+ cluster, r2SCAN restores the hemibonded motif through spin leakage, while DC-r2SCAN predicts a weaker hemibond formation influenced by solvent-solute interactions. Our findings demonstrate the potential of DC-r2SCAN combined with the auxiliary density matrix method to improve electronic structure calculations, providing insights into the properties of solvated cationic molecular clusters. This work contributes to the advancement of self-interaction corrected electronic structure theory and offers a computational framework for modeling condensed phase systems with intricate correlation effects.

Reversal of the relative stability of the isomeric radicals HSO and HOS upon hydration and their reactions with ozone

The radical HSO is an oxidation product of pollutants such as H(2)S and CH(3)SH in Earth's atmosphere. For the first time, the interaction of HSO and its tautomer HOS with single water molecules to yield the hydrates HSO.nH(2)O and HOS.nH(2)O was studied for n = 1-3, applying the high-level G3X(MP2) theory. A large number of structures corresponding to local minima on the potential energy surfaces has been identified. While gaseous HSO is more stable than HOS, the enthalpy diffference between HSO.nH(2)O and HOS.nH(2)O decreases with increasing degree of hydration and becomes practically zero for n = 3. Thus, in aqueous solution as well as in fog and rain droplets, HOS is expected to compete with HSO. The barrier for the tautomerization of HSO to HOS is dramatically lowered by the presence of water molecules since a cyclic transition state allows a concerted proton shift within the system of neighboring hydrogen bonds. The corresponding activation enthalpy of only 73.5 kJ mol(-1) predicted for the transformation of HSO.2H(2)O into HOS.2H(2)O may be compared to the 202 kJ mol(-1) reported for the tautomerization of the unhydrated gaseous HSO/HOS molecules. The impact of water of hydration on the fundamental vibrational modes of HSO and HOS has also been studied. Furthermore, HOS is predicted to dimerize at low temperatures to give two van der Waals molecules with singlet (symmetry C(2)) or triplet configuration (symmetry C(2h)), the latter being more stable than the singlet isomer. The disproportionation of 2HSO to H(2)S and SO(2) is predicted to be exothermic by -263.5 kJ mol(-1). The reaction of HSO with ozone to HSO(2) and O(2) is also strongly exothermic by -274.0 kJ mol(-1) and seems to proceed without any barrier. HOS forms a 1:1 van der Waals complex with O(3); the redox reaction of its two components is calculated as exothermic by -410.9 kJ mol(-1) and results in a rather stable adduct between HOSO and O(2) with the structure of a peroxo isomer of HOSO(3). This unprecedented hydrogen peroxosulfite radical might open a novel route to atmospheric sulfate without the intermediate formation of SO(2) and SO(3).