Electronic characterization of mass-selected acyclic, polycyclic and oxygenated hydrocarbons in neon matrices

Interstellar chemistry embarked with the discovery of CH, CH+, and CN in extraterrestrial clouds. Presently, a large number of molecules have been identified in different galactic environments mostly by millimeter-wave and infrared spectroscopy. Molecular complexity and the spectral features dramatically depend on the particle density and the temperature of the astronomical region where they reside. Hence, spectroscopic analysis of extraterrestrial molecules has a valid mean to probe the physical and chemical condition and history of galactic media.
Life on Earth is carbon based and surprisingly, similar tendency has been found in interstellar molecules. Almost 80 percent of detected species contain carbon as a major constituent. Hence, a better characterization of the molecular universe may elucidate the origin of terrestrial life.
Two long standing riddles in molecular astrophysics are the diffuse interstellar absorption lines in the visible and the broad emission features in the mid-infrared. Carbonaceous systems ranging from small acyclic to polycyclic aromatics are considered to be the carriers of these absorption and emission bands. To recognize individual molecules responsible for these transitions, spectroscopic analysis of astrophysically relevant species in the laboratory is needed; comparison studies between astronomical measurements and laboratory spectra are the way for identification. These exotic molecules may be stable in galactic clouds under very cold and low density conditions but are extremely short-lived in the laboratory framework. Therefore, noncontemporary synthesizing and sensitive characterizing methods are required.
The matrix isolation spectroscopy is considered as an outdated-technique after the discovery of laser-based experiments but in combination with theory, it still serves a pivotal role in characterization of transient species. Exotic organics are synthesized in electrical discharge for the respective precursors. The ions of interest are co-deposited with neon on a cold surface (6 K) after mass-selection. Neutrals are generated in the matrix by UV irradiation.
The acyclic unsaturated organics possess very unique structural flexibility. By mass selective deposition of a particular m/z ratio in solid neon, several isomers have been detected. An advantage of the matrix isolation technique is that all possible electronic transitions of trapped species in the experimental measurement range can be recorded at once. Rare gases provide an environment in which the guest-host interactions slightly perturb the experimental band positions as compared to the gas-phase. Still matrix isolated spectrum is a good starting point for high resolution study and thence astrophysical findings.
Moderately intense absorptions are observed both in the visible and UV for C7Hn+/0 and C5Hn+/0, and charged oxygen containing polycarbon chains H2C6O+, HC7O+, and C4O2+ in neon matrices. The structural assignments of the electronic systems have been made on the basis of calculated ground-state stabilities with DFT and MP2 level of theory and computed excitation energies with TD DFT, SAC-CI, and CASPT2 methods. However, some of these ions and radicals have strong possibility to be the carriers of diffuse interstellar bands. The neutral oxygenated hydrocarbons are excluded as the carriers because no transition was detected after irradiation of the matrix. Theory explains that they possess strong transition in the deep UV. In addition, protonated PAHs and their oxygen containing analogs, which are credited for unidentified emission features, have been studied. Strong optical transitions suggest that they could be carriers of diffuse interstellar bands as well. A key species in combustion chemistry likely responsible for PAHs formation via mass-growth processes, fulvenallenyl radical, has been electronically characterized.
A part of this dissertation is devoted to physical organic chemistry. Reaction intermediates are too short-lived to probe. Nevertheless, identification of these species helps to infer a probable synthetic mechanism. Vibrationally resolved electronic spectra of fluorenylium, phenalenylium and fluorenyl radical have been measured in a neon matrix.
This electronic transition database of transient molecules created in the thesis can be used for their further gas-phase analysis and in situ detection in reaction or combustion systems.