The Effect of Configuration and Functionalisation on Interstellar PAH Reactivity

Research output: Book/anthology/dissertation/reportPh.D. thesis

In spite of the low densities and harsh conditions of the interstellar medium (ISM), 204 molecules have been discovered to date, indicating the presence of rich chemistry quite different from the chemistry we are familiar with here on earth. In this thesis, the contribution of polycyclic aromatic hydrocarbons (PAHs), which are ubiquitous in the ISM accounting for 10 - 20% of the cosmic carbon, to interstellar chemistry is explored. The interaction of neutral pentacene, 6, 13 - pentacenequinone (PQ), and corannulene with the most abundant atom in the Universe, hydrogen (H), was studied using temperature programmed desorption (TPD) coupled with mass spectrometry. Products with high degrees of superhydrogenation were seen. The cross-section for the reaction, PAH + H --> HPAH, was obtained from the TPD data. For all three PAHs, the cross-section was found to be higher than that for coronene, studied previously, thereby providing evidence for the enhanced reactivity of zigzag edges (pentacene), oxygen functional groups (PQ) and 5-membered rings (corannulene). The comparison of the cross-section and superhydrogenated product distribution also sheds light on the effect of the size, shape and functional groups on PAH reactivity towards H atoms. The mass distribution of superhydrogenated products shows a higher yield of certain superhydrogenated (HnPAH) products indicating the presence of barriers towards further H addition to these products. Theoretical calculations are underway to understand the exceptional stability of these products. Infrared multiple-photon dissociation (IRMPD) spectroscopy was used to obtain the IR spectra of the functionalised PAHs - 6, 13 - dihydropentacene, PQ, 5, 7, 12, 14 - pentacenetetrone (PT) and hexadecahydropyrene in protonated and in cationic form for the first time. The IRPMD technique allowed us to study the fragmentation pattern of the PAH ions when irradiated with IR photons which gives an understanding of the formation of small molecules from functionalised PAH ions by photodissociation or heating, in general, which could constitute a top-down approach towards formation of small molecules in the ISM. While 2H/H2 loss is seen to be the dominant pathway for protonated pentacene, for protonated PQ and PT, the main fragmentation pathway involves the loss of oxygen containing molecules. Theoretical calculations to determine the structure of the fragments from their IR spectra, molecular mass and structure of the parent PAH ion are in progress.
Original languageEnglish
PublisherAarhus Universitet
Number of pages213
Publication statusPublished - 2020

Note re. dissertation

PhD defence: 30.11.2020

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