The process of homogeneous nucleation from solution involves an incipient stage preceding spontaneous crystal growth, in which an embryo-like cluster is formed in a supersaturated homogeneous phase. Elucidating the physicochemical processes that govern nucleation (e.g. solute speciation, solute-solvent interactions/solvation, solute-solute 'self'-association, as well as charge density and bond length variations in the solute molecules) is often referred to as one of the remaining grand challenges in the physical sciences. Laboratory analytical techniques such as IR, Raman, NMR spectroscopy are in principle sensitive to the relevant molecular level changes in solution, but the type of information they provide is often only indirectly related to molecular structure. This dissertation shows that core-level X-ray spectroscopies (near-edge X-ray absorption fine-structure, NEXAFS, and resonant inelastic X-ray scattering, RIXS) are emerging as versatile techniques that provide more direct insight into the electronic and geometrical structure of molecular species in solution and in the solid state. Another idea explored in this dissertation is that laboratory UV-Vis spectroscopy provides complementary information to NEXAFS and RIXS on the occupied and unoccupied states in the valence region of the solutes. The dissertation reports these studies through a series of work packages, as follows.First, a study of a series of crystalline saccharides was carried out to examine the hypothesis that even subtle changes in intramolecular bond lengths should manifest through detectable shifts in the shape resonances in the NEXAFS spectra even for the solid state of relatively complex organic molecules. Detection of C-O bond length variations in the saccharides, including non-crystalline solids without long-range order, demonstrates the applicability of the 'bond length with a ruler' approach and thereby extends its use for gas-phase and adsorbed molecule NEXAFS to the organic solid state.Second, it is shown in two chapters how pH changes and the nature of the solvent induce chemical state and local electronic structure variations in solute molecules, including the formation of cationic and anionic species. The high sensitivity to bond length changes in dissolved molecular species is shown for para-aminobenzoic acid (PABA) through comparison with reference NEXAFS data for the solid state. This technique thus provides an avenue to computational predictions of solute properties and solute-solvent interactions.Finally, combining NEXAFS and RIXS PABA in solution with UV-Vis spectra demonstrates that UV absorption peak shifts associated with changes in the electronic structure triggered by pH variation, are compatible with the electronic structure changes observed by NEXAFS/RIXS. Moreover, a shift of the main UV absorption band to shorter wavelengths as a function of solvent polarity is noticed, which further contributes to an understanding of solute-solvent interactions in solution.
|Date of Award||31 Dec 2016|
- The University of Manchester
|Supervisor||Roger Davey (Supervisor) & Sven Schroeder (Supervisor)|
- para-aminobenzoic acid