DNP enhanced NMR using a high-power 94 GHz microwave source: a study of the TEMPOL radical in toluene

Abstract

DNP enhanced ¹H NMR at 143 MHz in toluene is investigated using an NMR spectrometer coupled with a modified EPR spectrometer operating at 94 GHz and TEMPOL as the polarisation agent. A 100 W microwave amplifier was incorporated into the output stage of the EPR instrument so that high microwave powers could be delivered to the probe in either CW or pulsed mode. The maximum enhancement for the ring protons increases from ∼−16 for a 5 mM TEMPOL solution to ∼−50 for a 20 mM solution at a microwave power of ∼480 mW. The temperature dependence of the enhancement, the NMR relaxation rates and the ESR spectrum of TEMPOL were also studied in an effort to obtain information on the dynamics of the system.