Studies of Molecules and Molecular Complexes Using Microwave Spectroscopy

Abstract

The work presented in this thesis covers a variety of small molecules and complexes, which can roughly be grouped into two categories: molecules with different conformations and molecules/complexes potentially relevant to atmosphere aerosol nucleation. Chapter 1 presents work on the syn- and anti- conformers of thioacetic acid, where we discovered that the methyl group internal rotation barrier differed between the two forms by a nearly a factor of five. With the collaboration of Yirong Mo (Western Michigan University) and Huaiyu Zhang (Hebei Normal University), we aimed to explain this unusually large difference. In regard to the second subject, atmospheric aerosols have become of great interest in recent years. Aerosols are ubiquitous in the atmosphere and can affect the quality of life in several ways, such as negatively affecting human health, decreasing visibility and influencing climate. While the mechanism(s) by which particles nucleate and grow in the atmosphere is not yet fully understood, homogenous nucleation is one possible route for particle formation. Homogeneous nucleation is a process by which individual gas molecules aggregate together to first form complexes, then a stable cluster, and finally an aerosol particle through continued spontaneous growth. Sulfuric acid has long been known to be a key component in particle formation. It is formed in the atmosphere by the oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3), followed by hydrolysis. Sulfuric acid is highly hygroscopic, however, binary models involving sulfuric acid and water do not accurately predict new particle formation rates. Therefore, other constituents must be involved and scientists have started to incorporate common atmospherically relevant organic species, such as amines, carboxylic acids, and oxidative products of hydrocarbons into their models. However, while much has been learned about aerosols and their formation over the past few decades, a full understanding of the nucleation pathways, new particle formation rates, and aerosol composition at various stages, is still unclear. Recent theoretical research incorporating carboxylic acids in the early stages of nucleation, which showed that formic acid (HCOOH) catalyzes the hydration of SO3, converting SO3 to sulfuric acid, caught our attention. Furthermore, calculations showed that the activation barrier of this formic acid catalyzed reaction was not only lowered, but essentially zero. Finally, the authors proposed that this alternative pathway for generating atmospheric sulfuric acid could be competitive with current proposed mechanisms. When our group set out to investigate related complexes such H2SO4 – HCOOH and SO3 – HCOOH, a new project transpired, as an entirely new molecule was discovered, FSA (formic sulfuric anhydride). While this new class of molecules, carboxylic sulfuric anhydrides, is relatively unknown in the chemical literature, they could have great importance in the mechanisms for formation of atmospheric aerosols. Chapter 2 presents work on s-cis- and s-trans¬-acrylic sulfuric anhydride (s-cis-AcrSA and s-trans-AcrSA), which are formed from trans- and cis-acrylic acid, respectively, and provides experimental evidence that a variety of carboxylic acids can react with SO3 to generate their corresponding carboxylic sulfuric anhydrides Chapter 3 illustrates our work on the acetic sulfuric anhydride – water complex. Our aim was to hydrate a carboxylic sulfuric anhydride in order to understand its interaction with water, a first step in understanding anhydride hydrolysis, which would result in the generation of the sulfuric acid – acetic acid complex. Chapter 4 contains a study of propiolic sulfuric anhydride (PSA) and reviews all the carboxylic sulfuric anhydrides characterized by our lab to date, both experimentally and theoretically. Comparisons among their structures and energetics are made and detailed statistical thermodynamic calculations are carried out to estimate their equilibrium constants and concentrations over a range of atmospherically relevant temperatures. Comparisons between their concentrations and the concentrations of other atmospherically relevant species are also highlighted.