Surface chemistry of molecules of astrophysical interest: theory and simulations
Creators: name: Enrique-Romero, Joan
Publisher: name: Zenodo
Year: 2022
Subjects: Interstellar medium
Surface chemistry
Computational chemistry
Theory, Simulations
Astrochemistry

description: Thesis manuscript on computational chemistry applied to surface astrochemistry problems by Joan ENRIQUE-ROMERO, developed in UNIVERSITÉ GRENOBLE ALPES and UNIVERSITAT AUTÒNOMA DE BARCELONA under the supervision of Cecilia CECCARELLI and Albert RIMOLA. <strong>Abstract:</strong> In this thesis I have investigated some of the critical points towards the formation of iCOMs on interstellar<br> icy dust. In particular I have tackled the problem of the synthesis of iCOMs on the surfaces of interstellar<br> dust grains from a theoretical point of view with quantum chemistry calculations. Such calculations have<br> shown that radical—radical reactions on interstellar ice are (i) can have activation energy barriers mainly<br> due to the radical—surface interaction, (ii) can have competitive channels other than the formation of iCOMs<br> like that of direct hydrogen abstraction, in which one radical takes an H atom from the other and (iii) the<br> occurrence of one channel or the other may entirely depend on their orientation upon encounter. These<br> results have a strong impact in the astrochemistry community since in most cases it is usually assumed that<br> radical–radical reactions are barrierless and that can only produce iCOMs. Another point that we have tackled<br> in this thesis is the importance of binding energies when computing the efficiencies of radical—radical<br> reactions, which strongly depend on the diffusion timescales, which in turn depend on the binding energies<br> and on the diffusion-to-desorption activation energy ratio. We have shown for the formation of acetaldehyde<br> from the coupling of CH3 and HCO radicals the choice of the diffusion-to-desorption activation energy ratio<br> strongly affects the conclusions, and that tunneling effects in direct H-abstraction reactions (in this case<br> HCO + CH3 --&gt; CO + CH4) can be of great importance at low temperatures. The reaction rates related to<br> the activation energies were obtained by means of the Rice-Rampsberger-Kassel-Marcus (RRKM) theory,<br> i.e. the microcanonical counterpart of the classical transition state theory, while the desorption and diffusion<br> rate constants were simulating using Eyring’s equation. Finally, we have also tackled the problem of the<br> fate of the energy after a chemical reaction on top interstellar ices. We have studied how does the energy<br> released by H + CO --&gt; HCO and H + H --&gt; H2 partition in between the product molecule and the surface<br> by means of ab initio molecular dynamics. For the former reaction, the surface was modelled by a proton<br> ordered Ih crystalline ice in order to limit the complexity of the system (in such an ordered surface, the<br> number of binding sites is drastically reduced to a few that periodically repeat). We found that the energy<br> released is very efficiently absorbed and dissipated by the ice structure in about 1 ps, so that the HCO<br> product remains frozen on the ice surface. In the case of H2, we have studied the reaction on crystalline<br> and on three different spots on an amorphous ice model. In all cases the ice structure absorbs about one<br> half of the energy released upon H2 formation, which is still not enough for H2 to remain frozen, so that its<br> fate is probably leave into the gas phase with a certain amount of vibrational excitation (they were found to<br> be vibrationally excited during the first ps). The region where the H2 molecule was formed was observed to<br> remain energized for about 100-200 fs, so that we cannot reject the idea that the energy released by such<br> reactions might be used by other species with low binding energies to be ejected into the gas.