Theoretical investigation on reactions involving electronically-excited atoms occurring in the gas phase of extraterrestrial environments and at the surface of interstellar icy grains

Over the last decade, there has been an important advance in the comprehension of the formation and destruction routes of the most common interstellar chemical species. However, a significant lack of knowledge concerning the chemistry of other minor species is still present, often due to the limited amount of computational and experimental data. In this contribution, the possible role of the electronically-excited sulphur S(1D) and oxygen O(1D)  in the chemistry of interstellar or cometary ice has been explored through a theoretical characterization of the S(1D) reaction with H2O and CH3OH, and the O(1D) reaction with CH3SH, in the gas-phase and in the presence of a cluster of four water molecules. All the stationary points of the investigated potential energy surface (PES) were optimized at the density functional (DFT) level of theory, using the B3LYP functional and the correlation-consistent valence-polarized basis set aug-cc-pVTZ basis set, augmented with a tight d function on sulphur atoms. The presence of the 4-water-molecules cluster drastically changes the reaction mechanisms since the SO + H2 channel, which is the only open channel in the gas-phase S(1D) + H2O reaction, cannot occur due to the hindrance caused by the H-bonds between the involved reaction intermediates and the water molecules of the cluster. As regards the S(1D) + CH3OH and O(1D) + CH3SH, the only H-displacement channels together with SH- and OH-group ejection were found to be occurring, while all the other dissociation routes, which are thermally and kinetically active in the gas phase, were found to be hindered due to the presence of the water cluster. In addition, a global reduction of the energy content with respect to the reactants makes most of the H-displacement channels allowed from a thermodynamic standpoint, whereas in some cases they are endothermic, thus unfavored, for the isolated system. Overall, the ice matrix has been predicted to stabilize the reactive intermediates HSOH, H2SO, and CH2OHSH. Therefore, we started to investigate the effect of adding more water molecules by including an 18-water-molecule cluster in the case of the S(1D) + H2O system finding an enzyme-like effect of the cluster. In fact, the reaction pathways are found to be consistently favored because of the substantial reduction of the energy barriers connecting the minima of the potential energy surface. Additional work is necessary to simulate the ice environment better and confirm these preliminary results.

Figure 1 illustrates PES arising from the S(1D) + H2O reaction in the gas phase. The bold lines of Figure 1 highlight the exothermic pathway leading to the single set of reaction products SO + H2. 

 

Figure 2 illustrates the  PES calculated at the same level of theory arising from the S(¹D) + H₂O reaction considering a 4-membered cluster of H₂O interacting
with the reaction intermediates, reactants, and products.

 

 

Figure 2 illustrates the  PES calculated at the wB97XD functional level arising from the S(¹D) + H₂O reaction considering a 18-membered cluster of H₂O interacting with the reaction intermediates, evidencing the decrease of the minimum energy path between each intermediate of the reaction.