Química de superfícies de molècules d'interès astroquímic: teoria i simulacions
- Autores: Joan Enrique Romero
- Directores de la Tesis: Maria Cecilia Ceccarelli (dir. tes.), Albert Rimola Gibert (codir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2021
- Idioma: catalán
- Tribunal Calificador de la Tesis: Pierre Beck (presid.), Herma Cuppen (secret.), Charlotte Vastel (voc.), Alexander Tieelns (voc.), Mariona Sodupe Roure (voc.)
- Programa de doctorado: Programa de Doctorado en Química por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
In this thesis I have investigated some of the critical points towards the formation of iCOMs on interstellar icy dust. In particular I have tackled the problem of the synthesis of iCOMs on the surfaces of interstellar dust grains from a theoretical point of view with quantum chemistry calculations. Such calculations have shown that radical—radical reactions on interstellar ice are (i) can have activation energy barriers mainly due to the radical—surface interaction, (ii) can have competitive channels other than the formation of iCOMs like that of direct hydrogen abstraction, in which one radical takes an H atom from the other and (iii) the occurrence of one channel or the other may entirely depend on their orientation upon encounter. These results have a strong impact in the astrochemistry community since in most cases it is usually assumed that radical–radical reactions are barrierless and that can only produce iCOMs. Another point that we have tackled in this thesis is the importance of binding energies when computing the efficiencies of radical—radical reactions, which strongly depend on the diffusion timescales, which in turn depend on the binding energies and on the diffusion-to-desorption activation energy ratio. We have shown for the formation of acetaldehyde from the coupling of CH3 and HCO radicals the choice of the diffusion-to-desorption activation energy ratio strongly affects the conclusions, and that tunneling effects in direct H-abstraction reactions (in this case HCO + CH3 --> CO + CH4) can be of great importance at low temperatures. The reaction rates related to the activation energies were obtained by means of the Rice-Rampsberger-Kassel-Marcus (RRKM) theory, i.e.
the microcanonical counterpart of the classical transition state theory, while the desorption and diffusion rate constants were simulating using Eyring’s equation. Finally, we have also tackled the problem of the fate of the energy after a chemical reaction on top interstellar ices. We have studied how does the energy released by H + CO --> HCO and H + H --> H2 partition in between the product molecule and the surface by means of ab initio molecular dynamics. For the former reaction, the surface was modelled by a proton ordered Ih crystalline ice in order to limit the complexity of the system (in such an ordered surface, the number of binding sites is drastically reduced to a few that periodically repeat). We found that the energy released is very efficiently absorbed and dissipated by the ice structure in about 1 ps, so that the HCO product remains frozen on the ice surface. In the case of H2, we have studied the reaction on crystalline and on three different spots on an amorphous ice model. In all cases the ice structure absorbs about one half of the energy released upon H2 formation, which is still not enough for H2 to remain frozen, so that its fate is probably leave into the gas phase with a certain amount of vibrational excitation (they were found to be vibrationally excited during the first ps). The region where the H2 molecule was formed was observed to remain energized for about 100-200 fs, so that we cannot reject the idea that the energy released by such reactions might be used by other species with low binding energies to be ejected into the gas.
