Theoretical Insights on the $$\mathrm{S(}^{1}\mathrm{D)}~+~{\textrm{H}_{2}\textrm{O}}$$ Reaction and Implications on the Chemistry at the Surface of Ice in Extraterrestrial Environments

With the aim to characterize the possible role of in the chemistry of interstellar or cometary ice, we have carried out a theoretical characterization of the $$\mathrm{S(}^{1}\mathrm{D)}~+~{\textrm{H}_{2}\textrm{O}}$$ reaction in the gas-phase and in the presence of a cluster of four water molecules. Remarkably, the presence of the 4-water-molecules cluster drastically changes the reaction mechanism since the $$\text {SO}~ +~\textrm{H}_{2}$$ channel, which is the only open channel in the gas-phase reaction, cannot occur because of the hindrance caused by the H-bonds between the involved reaction intermediates and the water molecules of the cluster. At the same time, a global reduction of the energy content with respect to the reactants makes the H-displacement channels leading to HSO and HOS thermoneutral, while they are endothermic for the isolated system. Overall, we predict that the ice matrix will stabilize the reactive intermediates HSOH and $$\text {H}_{2}\text {SO}$$ . Further work is necessary to simulate the ice environment better and confirm these preliminary results.