Chasing Astronomical Complex Organic Molecules: The Role of (Vibro-)Rotational Spectroscopy

Astrochemistry is a multidisciplinary field that embraces complex topics on different space-time scales, delving into the study of molecular speciesin the domain that is typically associated with astrophysics, i.e., the observable cosmos. As a matter of fact, the determination of the chemicalcomposition of the universe is crucial for understanding its evolution: molecules are excellent diagnostic probes for the derivation of physicalcharacteristics of astronomical objects. The existence of molecules in the interstellar medium has been established more than 80 years ago, and, sincethen, a huge number of molecular species has been detected. Indeed, the astronomical census now counts about 300 molecules, and ca. 90% of themhave been identified using radio astronomy techniques, which collect the cosmic emission of radiation in the microwave and radio domains. Inparticular, the recording of broadband large-scale surveys captures the rotational transitions of all the polar species present in the astronomicalobject under study. These features are exceptionally molecule-specific; moreover, thanks to the low energies involved, excited rotational states arepopulated even at extremely low temperatures. For these reasons, radio astronomical observations allow univocal identifications even from coldquiescent interstellar regions. On the other hand, the requirement for accuracy on the frequency of rotational transitions is very demanding,and it is achievable only resorting to experimental studies. Given these premises, this thesis is directed to the collection of necessary experimentalspectroscopic data on eight different proved or potential interstellar species (namely allenylacetylene, cyanovinylacetylene, phenylmethanimine,2- and 3-furonitrile, 3-aminoisoxazole, and iso- and n-propylamine), in order to guide their detection in the interstellar medium in the millimeter-and submillimeter-wave frequency ranges. To support and complement the experiment, accurate computations exploitingcomposite schemes rooted in coupled-cluster theory and density functional theory have been carried out for all species, thus providing precise structural information as well as reliable spectroscopic parameters and, in some cases, the energetic description of the system. For all the investigated species, the outcomes of both theoretical and experimental studies provide the fundamental and robust basis required for guiding the astronomical searches in the interstellar medium.