Microwave spectroscopy of sulfur-bearing molecular species of astrophysical interest by Wenhao Sun A thesis submitted to the Faculty of Graduate Studies of the University of Manitoba in partial fulfillment of the requirements of the degree of Doctor of Philosophy Department of Chemistry University of Manitoba Winnipeg, Canada Copyright © 2019 by Wenhao Sun Abstract 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 I fundamental insights for a series of potential interstellar species but also allowed better understanding of the chemistry of the electrical discharge technique. II Acknowledgement I would like to express my deep and sincere gratitude to: Dr. Jennifer van Wijngaarden, for being a so supportive and patient supervisor in the past five years of study. It is your continuous guidance and great enthusiasm that make it possible for me to complete my Ph.D program and become a qualified researcher. My dear advisory committee members: Dr. Hélène Perreault, Dr. Georg Schreckenbach, and Dr. Juliette Mammei, thank you all for your great support and assistance during my PhD. program. My research collaborators: Weslley G. D. P. Silva, Dr. Rebecca L. Davis, Dr. Sven Thorwirth from Universität zu Köln, and Dr. Michael E. Harding from Karlsruher Institut für Technologie (KIT), for many useful discussions on rotational spectroscopy and computational calculations. Jorge Dourado, for organic synthesis of a commercially unavailable gas-phase compound. The present and past members in the van Wijngaarden group: Omar Mahassneh, Susanna Stephens, Issiah Byen Lozada, Aimee Bell, Olamide Paul Sogeke, and Joseph Stitsky, for being so amazing and supportive. III My parents, Baixing Sun and Mingxiang Wang, for your unconditional understanding and support in the progress of my growing up. Finally, I am also grateful for the financial support provided through a University of Manitoba Graduate Fellowship and the GETs program from the Faculty of Graduate Studies. IV Table of Contents Abstract…… .................................................................................................................. I Acknowledgement ....................................................................................................... III Table of Contents .......................................................................................................... V List of Figures .............................................................................................................. IX List of Tables ............................................................................................................ XVI Chapter 1. Introduction 1.1 Overview of astrochemistry ............................................................................ 1 1.2 Astrochemistry of sulfur.................................................................................. 8 1.3 Polycyclic aromatic hydrocarbons (PAHs) ................................................... 10 References ............................................................................................................... 14 Chapter 2. Fourier transform microwave spectroscopy 2.1 Introduction ................................................................................................... 24 2.2 Microwave spectroscopy ............................................................................... 24 2.2.1 Introduction ............................................................................................ 24 2.2.2 Linear tops ............................................................................................. 25 2.2.3 Symmetric tops ...................................................................................... 28 2.2.4 Asymmetric tops .................................................................................... 30 2.2.5 Nuclear quadrupole interaction .............................................................. 32 2.2.6 l-type doubling ....................................................................................... 35 2.3 Microwave spectroscopy instrumentation ..................................................... 36 2.3.1 Introduction ............................................................................................ 36 2.3.2 Balle-Flygare FTMW spectrometer ....................................................... 37 2.3.3 Chirped pulse FTMW spectrometer....................................................... 41 2.4 Analysis of the spectra .................................................................................. 45 V References ............................................................................................................... 51 Chapter 3. Rotational spectra and structures of phenyl isocyanate (PhNCO) and phenyl isothiocyanate (PhNCS) 3.1 Introduction ................................................................................................... 54 3.2 Experimental methods ................................................................................... 56 3.3 Computational details .................................................................................... 58 3.4 Spectral analysis ............................................................................................ 60 3.5 Structural determination ................................................................................ 64 3.6 Discussion ..................................................................................................... 65 3.7 Summary ....................................................................................................... 71 References ............................................................................................................... 72 Chapter 4. Investigation of the rich chemistry of the dc electrical discharge of 1,2-ethanedithiol 4.1 Introduction ................................................................................................... 79 4.2 Experimental and theoretical considerations................................................. 82 4.3 The identified discharge dependent species .................................................. 86 4.3.1 Thiols ..................................................................................................... 87 4.3.2 Thioketene chains .................................................................................. 89 4.3.3 Thials...................................................................................................... 92 4.4 Chemical considerations from the dc electrical discharge of 1,2-ethanedithiol ....................................................................................................................... 95 4.4.1 Empirical reaction pathways .................................................................. 95 4.4.2 Prediction of the rotational constants ..................................................... 96 4.4.3 Formation of the thioketene/thiol or thioketene/thial isomer pairs in the electrical discharge source .................................................................................. 98 VI 4.5 Summary ..................................................................................................... 101 References ............................................................................................................. 102 Chapter 5. Rotational spectra of ethynyl isothiocyanate (HCCNCS) and cyanogen isothiocyanate (NCNCS) 5.1 Introduction ................................................................................................. 111 5.2 Experimental methods ................................................................................. 113 5.3 Computational details .................................................................................. 114 5.4 Results and discussion ................................................................................