Computational Spectroscopy of Condensed n-alkanes

Abstract

The carbon 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra of alkanes vary with chain length, substitution, and phase change. For short gaseous alkanes, the carbon 1s NEXAFS spectra of alkanes are dominated by Rydberg transitions with distinctive vibronic features. In the case of condensed alkanes, the characteristic C-H feature appears at 287-288 eV. Computational models can effectively reproduce and interpret NEXAFS spectra of simple gaseous alkanes, provided that vibronic transitions are neglected. However, computational methods have been ineffective in reproducing and interpreting the NEXAFS spectra of condensed alkanes. We hypothesize that this shortcoming is due to computational limitations in modeling effect such as structural variations and disorder. An objective of this thesis is to study the effect of structural changes such as chain length on the NEXAFS spectra of n-alkanes. This objective involves computational modelling as well as experimental studies of the spectra of liquid n-alkanes. It should be noted that the NEXAFS spectra of liquid n-alkanes are entirely unexplored. The second objective of this thesis is to study the role of disorder caused by nuclear motion on the NEXAFS spectra of n-alkanes. The effect of nuclear motion refers to the contribution of the range of thermally accessible structures to the average NEXAFS spectrum of a material. In liquids, this effect can give a distribution of molecular structures rather than the single lowest energy structure. These thermally accessible structures include geometry defects such as gauche defects, thermally populated vibrational states, as well as zero-point motion. This thesis will characterize the role of disorder such geometry defects and nuclear motion on the NEXAFS spectra of n-alkanes using a Density Functional Theory approach. As part of this objective, the effect of temperature change on the NEXAFS spectra of n-alkanes will be studied.