Formation Routes of Interstellar Metal Oxides: A Computational Chemistry Approach

We have investigated possible formation routes of interstellar oxides in gas phase via the reactions of the atoms of several metals (Mg, Ca, Al, Fe) with the OH radical and the reaction of atomic Ti with the HO2 radical in a first explorative computational study based on DFT calculations. The aim is to understand if investing more effort in a higher-level theoretical characterization of these reactions can help to solve open issues on the formation of metal oxides in circumstellar envelopes (CSE) of O-rich AGB stars. The reactions of Ca and Fe with OH are blandly endothermic and can only occur at high temperature or if vibrationally excited OH radicals are involved. The reaction Mg + OH is too endothermic to be significant in the CSE chemistry, while the Al + OH reaction is exothermic and barrierless. Therefore, it is expected to be an efficient AlO formation route, also in other low temperature media. The reaction Ti + HO2 → TiO2 + H is strongly exothermic and barrierless. It is, therefore, very efficient and able to establish a direct connection between TiO2 and Ti atoms, provided that HO2 has a non-negligible abundance, in the circumstellar envelopes of AGB stars. A competitive channel leads to the TiO + OH products. From a computational perspective, the present computational approach, based on DFT, nicely reproduces the structures of the investigated species and provides a general view of the reaction mechanism. However, more accurate methods are needed to derive the energy of intermediates and transition states to be used in the calculations of the reaction rate coefficients.