Thesis of N.Osseiran

 Thesis of N.Osseiran - laboratory Phlam Abstract : Biogenic volatile organic compounds (BVOCs), and especially monoterpenes (C10H16), are molecules naturally occurring in the atmosphere, which have been linked to the formation of secondary organic aerosol (SOA). They can alter the physical and chemical properties in the atmosphere, have negative effects on human health and contribute to climate change. There exists a strong relationship between the structure of a molecular system and the inter- and intramolecular interactions present on the molecular scale.
Hence, having in-depth information about the gas phase structure and internal dynamics of these molecules, or their molecular complexes, is important to better understand their reaction pathways and complexation patterns.
The synergic combination of quantum chemical calculations and Fourier transform microwave (FTMW) spectroscopy has been shown to be a reliable approach to examine the conformational landscape, structure and internal dynamics of several types of molecules of atmospheric interest, their oxidation products and their complexes. In the framework of this thesis, we have applied this theoretical-experimental approach to characterize the complexes of two monoterpenoids: fenchol (C10H18O) and fenchone (C10H16O) with another atmospheric contaminant: the H2S molecule. The gas phase stable conformations were identified in the pure rotational spectrum with the support
of ab initio and DFT calculations. A comparative analysis of the observed complexes with their water analogues confirmed the presence of weaker hydrogen bonds. On top of that, we observed a large amplitude motion, that was qualitatively described. The stabilizing non-covalent interactions of the two complexes were also evaluated.
In a similar manner, and within the same general context, we also characterized the conformational landscape and methyl internal rotation in the case of limona ketone (C9H14O), which is a biogenic volatile organic compounds (BVOC) originating from the oxidation limonene. The experimental barrier height of the methyl torsion showed some deviation from the calculated values, which pushed toward a more thorough examination, that revealed the presence of an intermolecular interaction.
The second part of this thesis was dedicated to the construction and evaluation of a broadband chirped pulse FTMW spectrometer, operating in the range 6-18 GHz. A detailed technical description of the spectrometer is given herein. Moreover, the preliminary tests performed to evaluate the performance of the spectrometer are reported. Keywords : rotational spectroscopy,quantum chemical calculations,microsolvation,large amplitude motions,chirped-pulse Fourier transform microwave spectrometer