Matrix Isolation and Gas-Phase Kinetics of Astrochemically Relevant Species

Abstract

Following the first detection of an interstellar molecule in 1937, there have been over 300 detected chemical species as of the writing of this dissertation. Interstellar molecules typically exist in a range of conditions including very low-temperature environments, making their existence unexpected and their chemistry exotic. The formation and evolution of such molecules can be strikingly different than here on Earth. This dissertation work reproduced the reaction conditions of astrophysical environments for laboratory experiments. Two classes of chemicals were studied to gain a more complete understanding of the potential energy surfaces of astrophysically relevant molecules. First, the formation of transient species was studied using a custom-built matrix-isolation spectrometer (detailed in Chapter 2). Second, the destruction of astrochemically relevant molecules via ultraviolet (UV) radiation was studied using multiplexed photoionization mass spectrometry. In the first portion of this dissertation, spanning Chapters 3−5, we study (a) the noncovalent interactions that lead to the formation of weakly bound complexes, (b) the structure of transient intermediates, and (c) fundamental effects of different matrix environments. In Chapter 3, we demonstrate the utility of this instrument by isolating and characterizing the weakly bound complexes between hydrogen cyanide (HCN) and methyl chloride (CH3Cl) using FTIR spectroscopy and quantum chemistry calculations. The study ultimately led to a hypothesis that the formation of weakly bound complexes with CH3Cl could catalyze formation of the isomers of prebiotic molecules. Isomerism became the focus of subsequent studies using this instrumentation, as an emphasis was placed on the importance of considering how host/guest interactions may perturb gas-phase isomer ratios during matrix deposition. Chapter 4 demonstrates the change in conformer abundance of methyl nitrite (CH3ONO) in relation to the gas-phase ratio as a result of depositing with different low-temperature matrices, an important finding in the continued development of matrix-isolation techniques. Chapter 5 continues this investigation, expanding to investigate how different matrices influence the photodynamics of CH3ONO upon UV irradiation. These chapters reiterate the need for a deeper understanding of not only the chemical systems, but the methods used to study them as well. The second portion—Chapters 6 and 7—investigates the fate of a molecule important both on Earth and in space: methanol (CH3OH). The products formed upon UV excitation of CH3OH have not been well-constrained previously. In a collaborative project with Sandia National Laboratories (SNL) and Lawrence Berkeley National Laboratory (LBNL), we carried out UV photodissociation studies on CH3OH at the Advanced Light Source (ALS) synchrotron, identifying and quantifying the photodissociation products via Multiplexed Photoionization Mass Spectrometry. Chapter 6 provides direct observation of the formation mechanism and subsequent reactivity under gas-phase reaction conditions of hydroxymethylene (HCOH)—an elusive singlet carbene—which was previously unattainable due to the transient nature of the molecule. Additionally, the results in Chapter 7 inform scientists of the destruction processes possible for this important astrochemical in regions of space with high ultraviolet radiation fields, as well as quantitatively assign branching ratios for all of the major photodissociation channels of CH3OH for the first time. Finally, Chapter 8 details future work that will utilize both instruments to completely characterize the potential energy surface of possible formation routes to polycyclic aromatic hydrocarbon and other unique transient species.