Quantum-chemical approaches for gas-phase astrochemistry: from molecular structures to reactivity

This dissertation focuses on two aspects of astrochemistry: (i) investigation of reaction pathways in interstellar conditions for molecules observed in the interstellar medium (ISM) employing state-of-the-art chemical calculations and (ii) computational characterization of the rotational spectra for new molecules suggested from the studies at point (i). The aim of the work collected in the thesis is to explain the presence of detected molecules via gas-phase formation routes feasible in the ISM and to provide new data to facilitate laboratory and astronomical detection. The first part of the thesis summarizes the current census of detected molecules and puts into context the studies carried out during the PhD. The second chapter describes the computational background, and the methodologies employed to carry out the rotational spectroscopic characterization as well as the steps required to derive gas-phase reaction pathways. The results reported in the third chapter focus first on the computational spectroscopic characterization of polycyclic aromatic hydrocarbons PAHs and polycyclic aromatic nitrogen heterocycles PANH(s). Next, the focus shifts to gas-phase reactivity. Each reaction is presented by illustrating first its reaction mechanism, with the most relevant pathways to the ISM being highlighted. Then, the kinetic analysis is discussed, suggesting the feasibility of the processes and the favored products. The focus here is mainly on gas-phase mechanisms involving as one of the reactants methanimine or thioformaldehyde, and a radical, like CN or OH.