Dynamic nuclear polarization and electron spin resonance in paramagnetic solids at high field

Abstract

Recent advances in high resolution solid state nuclear magnetic resonance spectroscopy have permitted the detailed structural study of large biological systems. The feasibility of many of these experiments is limited by the inherently low sensitivity of solid state NMR, and the implementation of complex multi-dimensional homo- and hetero-nuclear recoupling pulse sequences has been restricted to small model compounds. The primary focus of this thesis is the description of investigations of dynamic nuclear polarization (DNP) at high magnetic field as a means of improving the signal to noise in solid state NMR spectroscopy. DNP transfers the large polarization of unpaired electron spins to nuclei in a process involving sample irradiation at or near the electronic Larmor frequency. Large signal enhancements have been achieved in a nitroxide doped frozen aqueous solution at 5T. The relevant relaxation times governing the transfer process have been measured via the application of various high frequency pulsed electron paramagnetic resonance (EPR) techniques, and a model incorporating cross-relaxation is used to explain the results. It is shown that the electronic and nuclear spin dynamics are consistent with the thermal mixing mechanism of polarization transfer. The high frequency (139.5 GHz, 5T) EPR spectrometer used to perform these experiments is described in detail and several other advancements in the application of high power, high frequency microwave technology to magnetic resonance are also discussed.