Published January 10, 2022 | Version v1
Thesis Open

Surface chemistry of molecules of astrophysical interest: theory and simulations

  • 1. Universitat Autonoma de Barcelona

Contributors

  • 1. Université Grenoble Alpes
  • 2. Universtiat Autònoma de Barcelona

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.

Abstract:

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.

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Additional details

Funding

QUANTUMGRAIN – Quantum Chemistry on Interstellar Grains 865657
European Commission
DOC – The Dawn of Organic Chemistry 741002
European Commission