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

There are many challenges in studying the mechanisms and speciation of reaction
intermediates in catalytic systems, particularly under in situ conditions. Quite often the
lifetime and concentrations of these species are below the detection limits of the
interrogating spectroscopic technique. This is particularly true for paramagnetic
reaction intermediates. To address these challenges, we are developing a unique dualmode
reactor-resonator for Electron Paramagnetic Resonance (EPR) spectroscopy,
utilising the incredibly efficient heating capabilities of microwaves (MWs) to generate
volumetric and rapid sample heating (a temperature jump, or TJ, capability), to better
understand the proposed role of MWs in enhancing the rate of reactions and chemical
transformations. The TE102 mode of a conventional EPR cavity (the resonator) will
monitor the EPR signals whilst a lower frequency TE101 mode will be simultaneously
accessed via an external CW or pulsed MW source to induce the sample heating (the
reactor). The rapid heating offers multiple benefits, not only to accelerate a reaction, but
also to alter the product distribution and proportion of unstable species and thereby
enable TJ-relaxation measurements to be performed using EPR spectroscopy.
In this experimental project, we will specifically employ the dual mode resonator to
initially study three key application areas. Firstly, the resonator will be utilised to
investigate the kinetics of exchange reactions in organic radicals, and particularly
explore 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 coupling
reactions. In some cases, a proposed MW enhancement of reaction products has been
proposed, and we will seek to investigate the fundamental origins of these proposed
accelerations. Finally, we will also examine the reactive species involved in oxidation
catalysis employing cobalt bearing complexes. These complexes are relevant not only to
homogeneous catalysis, but also as model systems for studies of spin-cross over
compounds, which can be manipulated using the variable in situ heating capabilities of
the resonator.
Understanding the chemistry of reactions shifted far from equilibrium is an important
aspiration in the chemical and physical sciences. Although many T-jump perturbation
techniques are available, the benefits of this in situ rapid heating EPR will provide a new
capability to the UK. Whilst a major focal point of this project is the testing of the dualmode
reactor-resonator, applied to problems in reaction kinetics and catalysis, it must
be recognised that the fundamental knowledge of how MWs interact with materials
remains poorly understood, despite the growing use of MW-radiation in synthetic
chemical transformations. It must also be stated that catalysis is an important research
area within the EPSRC portfolio. Therefore, the research in this project will serve to
underpin both of these major challenges and research areas.