Chemical Properties of Alkenes and Alkynes from Carbon 1s Photoelectron Spectroscopy and Theory

Abstract

The field of electron spectroscopy has evolved extensively the last couple of decades. On one hand, the technology at the synchrotron radiation facilities and of electron analyzers has improved, providing experimental data with more information about the sample. On the other, new and powerful computational resources have made it possible to analyze and increase our understanding of the experimental data. With these tools at hand, we are now in position to study molecular properties such as electronegativity, acidity, reactivity, and conformational isomerism. X-ray photoelectron spectroscopy (XPS) is the preferred technique to explore inner-shell ionization energies. In the present work, carbon 1s photoelectron spectra of a series of alkenes and alkynes have been measured and analyzed. As the molecular size of the alkene or alkyne increases, the complexity of the spectrum increases correspondingly. In the most difficult cases, results from the spectral analyses often are neither credible nor reproducible. One way to avoid this situation, is to calculate shifts in carbon 1s ionization energy with high accuracy and use them as constraints in the spectral analysis. In this thesis, shifts have been calculated using a number of ab initio and density functional theory (DFT) methods. To get an overview of the most promising methods, theoretical shifts were compared with the corresponding experimental values. Some of the larger systems in this thesis may possess two or more geometries obtained by rotation about carbon–carbon bonds. Such stable geometries are called conformers, and are an important and fundamental property of molecules. In the present work, XPS analyses are performed on a subset of alkenes and alkynes with the ability of possessing two or more conformers. It is shown that some of the conformers give rise to unique carbon 1s photoelectron spectra, and these are identified and used to determine the relative amount and stability of the different conformers. Carbon 1s ionization energies of hydrocarbons depend on the ability of a carbon atom to accept a positive charge, and there are other chemical properties that also depend on this ability. This work investigates the relationship between carbon 1s ionization energies and chemical reactivity in electrophilic addition reactions for twelve pairs of alkenes and alkynes. The relative chemical reactivity of carbon-carbon double and triple bonds in proton addition reactions has been a recurrent question for decades, and this thesis facilitates a direct comparison of the reactivity of the two classes of compounds as seen from C1s spectroscopy as well as activation energies and enthalpies of protonation.