Microwave spectroscopy of sulfur-bearing molecular species of astrophysical interest

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Sun, Wenhao
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Microwave spectroscopy, which measures rotational transitions in the centimeter-wave region, is a robust technique to study the fundamental chemical and physical properties of gaseous molecules, such as the geometry and the electronic structure. This thesis presents a selection of studies on several compounds of great astrophysical interest including phenyl isocyanate (PhNCO), phenyl isothiocyanate (PhNCS), ethynyl isothiocyanate (HCCNCS) and its longer chain form HCCCCNCS, cyanogen isothiocyanate (NCNCS) and its longer chain form NCCCNCS. The experiments were carried out with two Fourier transform microwave (FTMW) spectrometers: the broadband chirped pulse type, which has the capability of simultaneously probing many molecules together with a bandwidth up to 6 GHz; the narrowband cavity-based type, which focuses on a frequency window of 1 MHz each time with high resolution and sensitivity. Unlike PhNCO and PhNCS which are commercially available, the other four chemical species are not likely to be synthesized on a laboratory benchtop and were thus prepared by employing a dc electrical discharge. The transient products in the discharge source were probed by the spectrometers and were unambiguously identified by their rotational transitions out of a number of discharge dependent species including both closed-shell compounds and open-shell radicals. Furthermore, in order to better understand the chemical reactivities and kinetics in complex discharge plasmas, a thiazole discharge was investigated on the basis of the identified products in the rich spectrum. Possible decomposition pathways of the products from unimolecular dissociation and isomerization reactions were proposed and modeled using quantum-chemistry calculations. Collectively, these studies not only provided fundamental insights for a series of potential interstellar species but also allowed better understanding of the chemistry of the electrical discharge technique.
Microwave spectroscopy, Astrophysics, S-bearing molecules
Sun, W.; Silva, W. G. D. P.; van Wijngaarden, J. Rotational Spectra and Structures of Phenyl Isocyanate and Phenyl Isothiocyanate. J. Phys. Chem. A 2019, 123 (12), 2351-2360.
Sun, W.; Davis, R. L.; Thorwirth, S.; Harding, M. E.; van Wijngaarden, J. A Highly Flexible Molecule: The Peculiar Case of Ethynyl Isothiocyanate HCCNCS. J. Chem. Phys. 2018, 149 (10), 104304.
Sun, W.; van Wijngaarden, J. Isothiocyanato-Containing Carbon Chains: The Laboratory Detection of HCCCCNCS and NCCCNCS via Rotational Spectroscopy. J. Phys. Chem. A 2018, 122 (38), 7659-7665.