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

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

Conformations and CH/π Interactions in Aliphatic Alkynes and Alkenes

The carbon 1s photoelectron spectra of a series of aliphatic alkynes and alkenes that have the possibility of possessing two or more conformers have been recorded with high resolution. The two conformers of 2-hexyne and 4-methyl-1-pentyne, anti and gauche, have been identified and quantified from an analysis of their carbon 1s photoelectron spectra, yielding 30 ± 5% and 70 ± 6% anti conformers, respectively. In the case of 1-hexyne, the photoelectron spectrum is shown to provide partial information on the distribution of conformers. Central to these analyses is a pronounced ability of the C1s photoemission process to distinguish between conformers that display weak γ-CH/π hydrogen bonding and those that do not. For the corresponding alkene analogs, similar analyses of their C1s photoelectron spectra do not lead to conclusive information on the conformational equilibria, mainly because of significantly smaller chemical shifts and higher number of conformers compared with the alkynes

Conformations and CH/π Interactions in Aliphatic Alkynes and Alkenes

The carbon 1s photoelectron spectra of a series of aliphatic alkynes and alkenes that have the possibility of possessing two or more conformers have been recorded with high resolution. The two conformers of 2-hexyne and 4-methyl-1-pentyne, anti and gauche, have been identified and quantified from an analysis of their carbon 1s photoelectron spectra, yielding 30 ± 5% and 70 ± 6% anti conformers, respectively. In the case of 1-hexyne, the photoelectron spectrum is shown to provide partial information on the distribution of conformers. Central to these analyses is a pronounced ability of the C1s photoemission process to distinguish between conformers that display weak γ-CH/π hydrogen bonding and those that do not. For the corresponding alkene analogs, similar analyses of their C1s photoelectron spectra do not lead to conclusive information on the conformational equilibria, mainly because of significantly smaller chemical shifts and higher number of conformers compared with the alkynes

Chemical Reactivity of Alkenes and Alkynes As Seen from Activation Energies, Enthalpies of Protonation, and Carbon 1s Ionization Energies

Electrophilic addition to multiple carbon–carbon bonds has been investigated for a series of twelve aliphatic and aromatic alkenes and the corresponding alkynes. For all molecules, enthalpies of protonation and activation energies for HCl addition across the multiple bonds have been calculated. Considering the protonation process as a cationic limiting case of electrophilic addition, the sets of protonation enthalpies and gas-phase activation energies allow for direct comparison between double- and triple-bond reactivities in both ionic and dipolar electrophilic reactions. The results from these model reactions show that the alkenes have similar or slightly lower enthalpies of protonation, but have consistently lower activation energies than do the alkynes. These findings are compared with results from high resolution carbon 1s photoelectron spectra measured in the gas phase, where the contribution from carbons of the unsaturated bonds are identified. Linear correlations are found for both protonation and activation energies as functions of carbon 1s energies. However, there are deviations from the lines that reflect differences between the three processes. Finally, substituent effects for alkenes and alkynes are compared using both activation and carbon 1s ionization energies