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Development of a dual-mode microwave-EPR cavity for studies of paramagnetic systems

Magri, Giuseppina 2023. Development of a dual-mode microwave-EPR cavity for studies of paramagnetic systems. PhD Thesis, Cardiff University.
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There are many challenges when studying the mechanisms and speciation of inter-mediates formed during chemical reaction pathways. Often within many of these systems radical species are formed, making EPR spectroscopy the ideal analytical tool. However, studying such systems in-situ can be complicated by the short-lived nature of some paramagnetic species, with the study of such systems under non-equilibrium conditions near impossible. For example, when studying excited states, or to alter the product distribution and proportion of unstable species, this requires the development of new measurement techniques or hardware capabilities. For the first time, this Thesis will detail the development a dual-mode EPR cavity which has been designed and developed for simultaneous dielectric heating and EPR detection. The utilisation of the incredibly efficient heating capabilities of microwaves to generate volumetric heating and temperature jumps (T-jumps) has initially been tested, to better understand the proposed role of microwaves in enhancing the rate of reactions and chemical transformations. Specifically, the ca. 9.5 GHz TM110 mode of the EPR cavity will monitor the EPR response to the dielectric heating effects produced from a second lower frequency, ca. 6.1 GHz TM010. The design and proof of concept of three iterations of dual-mode cavities has been extensively examined, alongside alterations of specific elements within their design and the corresponding experimental implications. Notably, the movement of the 6.1 GHz heating port 135° in the second- and third-generation with respect to the 9.5 GHz port has been monitored. Without the use of a low pass filter, the signal-to-noise ratio (SNR) of the resulting EPR spectra is destroyed, due to interference between the two modes, which was not possible when the heating port was located 90° to the EPR port in the first-generation cavity. Proof that dielectric heating was successful was monitored by alterations in rotational diffusion tensors involving a spin-labelled micelle, and rotational correlation times of a small transition metal complex from room temperature to elevated temperatures. Methods of accurately determining sample temperature during dielectric heating have been examined, with potential limitations to the various techniques identified. Specifically, the temperature readings using an external thermal imaging camera severely fluctuate when altering its location and positioning. An alternative gauge of temperature was used in the form of a fiberoptic probe, which was inserted into the sample tube. Using this approach, a sample temperature gradient was identified, highlighting the importance of accurate probe placement when determining sample temperature, however this was proved to be the iii best approach to determine the temperature of the sample. The capabilities of such dual-mode cavities has been explored by comparing the thermal decomposition of 2,2’-Azobis(2-methylpropionitrile) (AIBN) in a toluene solution, under conventional and dielectric heating methods. The increased efficiency of dielectric heating was confirmed through monitoring heating rates and corresponding radical products produced. Key considerations before drawing conclusions regarding dielectric heating have been identified. Explicitly, understanding the chemical kinetics of the reaction compared to recorded heating rates prior to analysing product distribution through EPR detection when using dielectric heating. Finally, a preliminary test on a solution-based cobalt spin-crossover (SCO) complex has been conducted, observing the changes in the low-/high-spin (LS/HS) distribution upon altering the pKa-H of an axially bound pyridyl ligand. It was observed that overall, the higher the pKa-H of the pyridyl substrate, the lower the LS/HS energy gap of the overall cobalt complex, confirmed by Density Functional Theory (DFT) calculations, alongside a more intense HS EPR signal. This represent an initial investigation into ligand mediated SCO, with the aim to study such systems in the future using the dual-mode cavity, monitoring any alterations in the SCO or spin-distribution event upon dielectric heating.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 20 June 2023
Last Modified: 20 Jun 2024 01:30

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