Using Microwave Pulses to Expand the Scope of DNP Enhanced Solid-State NMR Spectroscopy

Dynamic nuclear polarization has revolutionized the field of solid-state NMR by providing the sensitivity enhancement of orders of magnitude. Currently, most DNP experiments are performed using continuous-wave (CW) microwave irradiation near EPR frequency of the polarizing agents using dedicated gyrotrons. The CW-DNP mechanisms, i.e. solid-effect (SE), cross-effect (CE) and thermal mixing (TM) become less efficient at higher magnetic fields due to inefficient state mixing caused by large Zeeman splitting. Therefore, the signal enhancements achieved at high magnetic fields are well below the theoretical maximum of 658.
 
An alternative approach, which is recently gaining foothold, will be discussed. In this approach, microwave pulses with varying phase, length, amplitude and frequency are employed to obtain field independent electron-nuclear polarization transfer by fulfilling the required matching condition. Such methods require microwave irradiation with low duty cycle, which minimized the problem of sample heating that is prominent is CW-DNP experiments. Moreover, the paramagnetic effects, i.e. NMR line-broadening, signal quenching and relaxation enhancement, caused by the presence of a paramagnetic center can also be reversed using microwave pulses with frequency switching capabilities. I will show experimental evidences of reversal of paramagnetic effects at a magnetic field of 6.9 T and 4K temperature. 
 
At the end, I will talk about my future research plans focusing at developments and applications of the magnetic resonance techniques (NMR and DNP) to study materials and biological samples.