Development of a Dual-Mode Microwave-EPR Cavity for Studies of Paramagnetic Systems

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There are many challenges in studying the mechanisms and speciation of reactionintermediates in catalytic systems, particularly under in situ conditions. Quite often thelifetime and concentrations of these species are below the detection limits of theinterrogating spectroscopic technique. This is particularly true for paramagneticreaction intermediates. To address these challenges, we are developing a unique dualmodereactor-resonator for Electron Paramagnetic Resonance (EPR) spectroscopy,utilising the incredibly efficient heating capabilities of microwaves (MWs) to generatevolumetric and rapid sample heating (a temperature jump, or TJ, capability), to betterunderstand the proposed role of MWs in enhancing the rate of reactions and chemicaltransformations. The TE102 mode of a conventional EPR cavity (the resonator) willmonitor the EPR signals whilst a lower frequency TE101 mode will be simultaneouslyaccessed via an external CW or pulsed MW source to induce the sample heating (thereactor). The rapid heating offers multiple benefits, not only to accelerate a reaction, butalso to alter the product distribution and proportion of unstable species and therebyenable TJ-relaxation measurements to be performed using EPR spectroscopy.In this experimental project, we will specifically employ the dual mode resonator toinitially study three key application areas. Firstly, the resonator will be utilised toinvestigate the kinetics of exchange reactions in organic radicals, and particularlyexplore the role and behaviour of the solvent in these electron transfer events. Secondly,we will study a series of low valent transition metal ions for MW assisted cross couplingreactions. In some cases, a proposed MW enhancement of reaction products has beenproposed, and we will seek to investigate the fundamental origins of these proposedaccelerations. Finally, we will also examine the reactive species involved in oxidationcatalysis employing cobalt bearing complexes. These complexes are relevant not only tohomogeneous catalysis, but also as model systems for studies of spin-cross overcompounds, which can be manipulated using the variable in situ heating capabilities ofthe resonator.Understanding the chemistry of reactions shifted far from equilibrium is an importantaspiration in the chemical and physical sciences. Although many T-jump perturbationtechniques are available, the benefits of this in situ rapid heating EPR will provide a newcapability to the UK. Whilst a major focal point of this project is the testing of the dualmodereactor-resonator, applied to problems in reaction kinetics and catalysis, it mustbe recognised that the fundamental knowledge of how MWs interact with materialsremains poorly understood, despite the growing use of MW-radiation in syntheticchemical transformations. It must also be stated that catalysis is an important researcharea within the EPSRC portfolio. Therefore, the research in this project will serve tounderpin both of these major challenges and research areas.