UC Davis

Astrochemistry relies heavily on rotational and vibrational spectroscopy to study the chemical kinetics and dynamics of reactions in space and to determine the exact structure of astrochemically relevant molecules. Recent advances in spectroscopic tools, along with technological breakthroughs from powerful radio telescopes around the world, have enabled a deeper understanding of the complex chemistry of space. Even though interstellar space appears to be dark and empty, it contains a wide variety of molecular species; however, the mechanisms involved in producing these species remain unclear.

There have been approximately 270 unique molecular detections in the interstellar medium (ISM) and approximately 20% are radicals. It has been suggested that radical-neutral reactions play an important role in the formation of molecules in the ISM. These types of reactions are sometimes associated with gas-phase barrierless reaction pathways, which are suited for low temperature regions of the ISM (e.g., cold molecular clouds like TMC-1). Several polycyclic aromatic hydrocarbons and their nitrogen substituted derivatives (PAH/PANH) have been detected in chondritic meteorites and their synthesis may have origins in the low temperature regions of space via radical-mediated gas-grain reactions before accretion to the meteorite parent body. Based on the molecule’s non-terrestrial isotopic ratios, they are thought to have extraterrestrial origins, yet their formation mechanism is unknown. Our understanding of how complex chemical reactions operate in space is improving, and the likelihood of radicals being involved is high, but it still remains unclear how these reactions are carried out.

To improve chemical models of astronomical environments, experimental data, including accurate rate constants, branching ratios, and structural information, is essential. It is generally possible to determine the input parameters for these astrochemical models by using experimental rotational/vibrational spectroscopic methods in conjunction with theory. Chapter 2 of this dissertation describes the coupled-cluster method, along with a detailed discussion of the calculations necessary for rotational and vibrational spectroscopy. Chapter 3 applies this theoretical method to the pyridyl radicals, which are of interest due to their structural similarity to prebiotic molecules and their possible role as precursors to PANHs.

High-level computational results also assist experimental studies. Two instruments capable of measuring microwave spectra of radicals were custom-built and tested: a segmented chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer and a Ka-band CP-FTMW spectrometer. An overview of the instrumentation theory is presented in Chapter 4, and the optimizations/preliminary results for both instruments are presented in Chapters 5 and 6, respectively. It is expected that future developments will include the measurement of more radicals, thus broadening the chemical applications of these instruments.