Exploratory DFT calculations on some H2Cn isomers for astrochemical applications

Our knowledge of chemistry is limited by our narrow terrestrial perspective. Unexpected and unique types of chemistry take place in the universe and, aside from spectroscopy, computational quantum chemistry is essential to explain and understand their secrets. After the discovery of the radiotelescope, the interest in astrochemistry grew rapidly, and, to date, about 250 molecules have been discovered in the interstellar medium or the circumstellar shells. The identification of interstellar molecules is crucial to the assignment of the diffuse interstellar bands, “a set of ubiquitous absorption features” that can be observed in the optical region of the stellar spectra. The aim of this thesis is the computational study of the H-deficient hydrocarbon CnH2 isomers, a class of interstellar molecules that is believed to play a key role in the formation of fullerenes and polycyclic aromatic hydrocarbons. Some CnH2 molecules have been detected in space and/or in the laboratory, and recently many new structures have been proposed as possible isomers or intermediates. Finding a cheaper alternative to the commonly employed highly correlated computational methods has been the main interest of this work, since it is essential for studying simultaneously a large number of isomers as well as structures with an elevated number of atoms. After a brief explorative density functional theory (DFT) benchmark, several new C11H2, C13H2, and C5H2 structures, generated by a stochastic procedure, have been theoretically characterized. The performance of DLPNO-CCSD(T) method in the prediction of the relative stabilities has been assessed for the well-known C5H2 and C7H2 series. Finally, the so-called “C=C multiple bonds out of plane (OPB) frequency issue”, relative to the correct prediction of the harmonic and anharmonic vibrational modes, has been also addressed.