Investigation of CO2 capture systems, Lewis acid-base pairs, and oscillating reactions with electronic structure theory and kinetics-based approaches
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Equilibrium is a key theme in chemistry education. Starting in high school and continuing in freshman general chemistry courses, STEM students have to learn the foundations of equilibria. What is the key concept of an equilibrium? How can we describe an equilibrium? The concept of an equilibrium constant K is introduced, and its relation to the Gibbs enthalpy [delta] G [superscript 0] is noted. The equilibrium constant K also is related in a straightforward manner to the forward and backward reaction rate constants. Usually, a few simple applications are discussed, primarily in the area of acid-base chemistry. The topic is revisited in organic chemistry and clarified conceptually with reaction energy diagrams. To study equilibrium as a student is one thing, and to study equilibrium problems as a researcher is quite another. How does one determine equilibrium constants and how does one determine reaction rate constants? What do we know about the accuracy of the experimental quantities reported in the literature? How does one deal with multi-equilibria? How does one account for non-ideal conditions and concentrated solutions? Over the last six years, I have learned how to approach and solve all of these issues. One of the most stunning insights was the realization that even so-called non-linear reactions can in fact be described in some cases by application of complicated systems of equilibrium reactions. The Glaser group very strongly believes that the interplay between experimental and theoretical work is vitally important to really understand a problem. This combination builds a strong focus on quantitative aspects and it often also leads to new insights that might not be attainable from experimentation or modeling alone. The five chapters presented here show that this two-pronged approach is widely applicable to several areas of chemistry. The two main topics of our studies have been carbon dioxide capture from air and reaction mechanisms of oscillating chemical systems. All of the chapters in my dissertation do have a very strong connection between theory and experimentation. I studied both aspects in most cases. Only in one case (Chapter 5) did I not perform the experiments, but even in this case, a very deep engagement with the experimental literature was required to solve a decades-long discrepancy. Chapter 1 is about the study of equilibria between different conformations of substrates and products and an evaluation of their effects on the overall reaction energy. Specifically, we studied the capture of CO2 by small alkylamines. The quality of that discussion was tested directly with the work described in Chapter 2. The work that led to Chapter 2 was an enormous learning experience; it was amazing to see all the pieces of the complicated multi-equilibrium system come together to determine the [delta] G [superscript 0] of the carbamylation of butylamine in aqueous solution. The interest in equilibria actually began with the quest of the non-linear dynamics group to understand oscillating chemical reactions. From the outset, this quest was pursued as an interdisciplinary project between chemistry and mathematics. My work with the dynamics group resulted in Chapters 3 and 4 of the present dissertation. Chapter 3 is a re-evaluation of the video-based kinetic analysis with high temporal resolution and over long timescales. The colorimetric studies revealed unexpected "hysteresis loops" in cerium-catalyzed Belousov-Zhabotinsky oscillating reactions. We studied the reaction progress in RGB space because we wanted to learn under what conditions the video-based analysis would allow for quantitative concentration determinations. The desire to assess the quality of the video-based analysis in RGB space, led to the serendipitous discovery of hysteresis loops. The origins of Chapter 4 had to do with the question as to whether accounting for ionic strength would be essential to obtain accurate simulations of BZ reactions. The goal of my work on phosphate buffers was an evaluation of the usefulness of Debye-Hu��ckel theory to electrolyte solutions with highly-charged ions present in significant concentrations. The phosphate buffer systems are widely in use and outstanding experimental sets of pH values were available to really test the performance of the solution models. Many years of studies of the Lewis acid-base pair F3B[arrow]PH3 illustrate in a beautiful fashion what can go wrong when expertalists interpret their data based on inaccurate theory and when computational chemists do not seek consistency with existing experimental data published in the literature. A careful read of the literature clearly showed early on that experimental and theoretical reports on F3B[arrow]PH3 are entirely inconsistent. It took years to explain what was actually measured, namely the compound F2B-PH2, and to explain why many theoretical reports predicted the wrong dative-bonding geometry.
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