Methodology development of high sensitivity pulsed EPR and DNP

Abstract

Dynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei and is a technique that has been widely used to improve nuclear magnetic resonance (NMR) sensitivity. This thesis presents the implementation of a DNP extension to a home-built high power pulsed electron paramagnetic resonance (EPR) spectrometer, HiPER, integrated with an arbitrary waveform generator (AWG), operating at 94 GHz. It describes the use of high-peak power inverting chirped pulses to increase the polarisation gradient of dipolar-coupled electrons, where static cross-effect DNP enhancements of 340 are achieved within 3 seconds at 65 K with a mixture of 4-amino TEMPO and DNP juice. Several other nitroxide radical polarising agents are also investigated and systematic study into the DNP temperature dependence for different polarising agents reveals the impact of spectral diffusion and the molecular structure of polarising agents.

In addition, for the first time at 94 GHz, a comparative study is made of different coherent pulsed solid-effect DNP schemes including XiX, TPPM, and the adiabatic solid-effect. An ENDOR upgrade to the DNP/EPR spectrometer is also described, and preliminary room-temperature ¹H ENDOR and low-temperature ¹⁹F ENDOR experiments presented, where a combined approach involving shaped pulse excitation and data processing at an intermediate frequency in down-conversion for DC suppression yields a threefold enhancement in SNR. Finally, a brief summary to ongoing projects on magnet shimming, transition metal DNP and DNP characterisation for lithium dendrites is provided.