Ab initio quantum mechanical study of carbon monoxide adsorption on surface models of interstellar grains

The aim of this thesis was the study of the energetic and spectroscopic features of the adsorption process of carbon monoxide (CO) on surface models of interstellar dust grains by means of computational ab initio techniques. Dusts grains are among the most important components of the so called molecular clouds (MCs), regions of the Interstellar Medium (ISM) characterized by the presence of molecular species such as hydrogen (H2), carbon monoxide, water, ammonia and many others; if star formation is occurring, molecular clouds are sometimes referred as ¿stellar nursery¿. The physical conditions (number density and temperature) within a molecular cloud are such that molecules can form; however, this can only occur thanks to the presence of solid dust grains, which act as surface catalysts. Two kinds of dust grains have been observed through spectroscopy techniques, silicon-based (most commonly olivines and pyroxenes) and carbon-based. Forsterite (magnesium silicate) is maybe, among olivines, the most important phase while carbon monoxide is, after H2, the most diffuse molecular species within such regions. For these reasons, a detailed study of the interaction between forsterite and CO can improve our knowledge about the formation of more complex molecules, such as glycine, which have also been observed in MCs and that play a fundamental role in exogenous theories about the origin of life on Earth. All the quantum mechanical simulations of this work have been performed within an ab initio DFT-B3LYP-D* level of theory using the CRYSTAL14 code with a 2D-periodic approach. The starting point for this work was a recent paper of Bruno et al. (J. Phys. Chem. C, 2014, 118, 2498−2506), where the most appropriate basis sets for forsterite have been developed; from this work, seven optimized crystalline surface models, representing the most natural occurring forsterite surfaces, have been considered. They are, respectively, the (010), (120), (101), (001), (111), (021) and (110) surfaces. Firstly, to assess the relative stability of the free surfaces, the surface energies were computed; then, the search for possible adsorption sites for CO leaded to 37 different cases spread throughout the seven models. The average binding energies, corrected for dispersion and BSSE, and the CO vibrational frequencies were computed. Moreover the simulated IR spectra were compared with the experimental one, revealing the computed key structures contributing most to the latter. In cold molecular clouds, silicates cores are partially covered by an icy mantle mainly constituted by amorphous water ice, thus, the second part of this work deals with the CO adsorption on water ice models. Firstly, the (010) surface of a crystalline proton-ordered ice was selected, and then, the binding energies and the CO frequencies were computed. In the second step, some different periodic models resembling the amorphous ices of the mantle have been built. The CO adsorption features were computed and compared with those obtained for the crystalline case. Finally, a mixed system composed of (010) forsterite surface, water and CO was studied in order to simulate covered grains. This could represent the starting point for future works dealing with the interaction of carbon monoxide and some other molecules with ice-covered forsterite models.