Soft X-ray near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy of liquids, an emerging synchrotron radiation technique, has been applied to characterise the local environment of imidazole in aqueous solution to elucidate the structural nature of the well known self-association effects in this system. Atomic core level spectroscopies such as NEXAFS are extremely sensitive to short range structure, including bond lengths and angles as well as coordination numbers, in condensed matter and in molecular systems.N K-edge NEXAFS data were successfully acquired for aqueous imidazole solutions were studied from low concentrations (10−2 mol L−1) to saturation (> 8 mol L−1). Previous UV/Vis studies had established that imidazole undergoes significant self-association already at concentration well below 1 mol L−1 while energy-dispersive X-ray diffraction studies indicated the formation of water mediated hydrogen-bonded self-associated structures of imidazole.NEXAFS revealed that in the concentration range from 0.50 (~100 H2O molecules per imidazole molecule) to the saturation concentration of 8.20 mol L−1 (~7 H2O molecules per imidazole molecule) no significant variation in the local coordination around imidazole molecules takes place. It appears that progressive self-association at higher concentration is not associated with local structural changes.Using ab initio calculations of N K-edge spectra for complex structure models that explicitly included the evaluation of imidazole gas phase data, the imidazole crystal structure as well as imidazole-imidazole and imidazole-solvent interactions in solution, it was possible to relate the observed imidazole spectra to structure models. Systematic calculations evaluating gas phase monomers and oligomers of pure imidazole, the solid state of imidazole, stacked self-associated imidazole clusters in solution, chained structures of self-associated clusters in solution, and explicitly including the water solvation shell were carried out. Comparative measurements in with chloroform solutions shed some light on the nature of the self-associates present at the lower concentrations investigated in this work.Examining also synergistic interactions between imidazole and water the view was obtained that increasing the concentration of imidazole solutions leads to the assembly of self-associated clusters of hydrated imidazole monomers, which are held together by secondary interactions that are weaker than the hydrogen bonding in the hydrate shell. This result demonstrates the suitability of soft X-ray spectroscopy measurements for the investigation of local structure in solutions of molecular organic solutes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:574316 |
| Date | January 2013 |
| Creators | Thomason, Matthew James |
| Contributors | Schroeder, Sven |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/soft-xray-spectroscopy-of-molecular-species-in-solution-studies-of-imidazole-and-imidazolewater-systems(ae99d759-ca04-4e9a-b585-7a91b8aa2929).html |
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