HIGH-FIELD DNP / EPR LABORATORY
FAQ
Why Nuclear Magnetic Resonance Spectroscopy?
Nuclear Magnetic Resonance (NMR) is one of the most powerful, informative, versatile methods of structural and functional characterization of complex systems, with applications ranging from pure science to medicine and engineering.
NMR can report even minute changes in the local environment of a nucleus. For simple molecules this is sufficient to fully reconstruct their structure.
For more complex molecules, multidimensional experiments come to help. These experiments allow to obtain the connectivity between different nuclei. By selecting an appropriate experiment, it is possible to deduce which nuclei are adjacent to which. The molecule Lewis structure can be reconstructed using an NMR experiment providing through-bond connectivity, while the three-dimensional structure can be reconstructed using one providing through-space connectivity.
So NMR is already perfect, what is there left to do?
An inherent caveat of NMR is the limited sensitivity. NMR involves applying nuclei to an external magnetic field. The nuclei may align in several ways, according to their spin states, with respect to the field (figure), and there is a population difference between different spin states, called polarization. The NMR signal intensity is directly proportional to the polarization, which in turn is very small.
Conventional NMR deals with this problem by simultaneously measuring many molecules. This results in "enough" spins for the measurement. This makes NMR inapplicable where it is impossible to pack a lot of sample in the NMR sample tube. For example, surface effects are very hard to study with NMR because surface sites are only a small fraction of the sample and there are "not enough" of them for conventional NMR measurement.
What is DNP?
Dynamic Nuclear Polarization (DNP) is a method to boost the signal in Nuclear Magnetic Resonance experiments by several orders of magnitude.
Is DNP a big deal?
Yes it is. DNP allows for a signal enhancement up to several hundred fold. A factor of 100 in the signal translates to a factor of 10000 in time. So a week-long experiment can now be completed in a minute. Previously unthinkable experiments that would require signal averaging of several years now become feasible. DNP is revolutionizing the field of NMR.
How does DNP work?
Electron spins have a 660 larger magnetic moment compared to proton nuclear spins. This translates to a larger polarization.
Fortunately, the electron and nuclear spins are quantum mechanically coupled to each other. This allows a polarization transfer between them, yielding a nuclear hyperpolarization providing a much stronger NMR signal.
The underlying mechanisms are still incompletely understood.
To learn more about DNP you are encouraged to look at these publications by our colleagues:
Lee, D.; Hediger, S.; De Paëpe, G. Is Solid-State NMR Enhanced by Dynamic Nuclear Polarization? Solid State Nuclear Magnetic Resonance 2015, 66–67, 6–20. https://doi.org/10.1016/j.ssnmr.2015.01.003.
Rossini, A. J.; Zagdoun, A.; Lelli, M.; Lesage, A.; Copéret, C.; Emsley, L. Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy. Acc. Chem. Res. 2013, 46 (9), 1942–1951. https://doi.org/10.1021/ar300322x.
Maly, T.; Debelouchina, G. T.; Bajaj, V. S.; Hu, K.-N.; Joo, C.-G.; Mak–Jurkauskas, M. L.; Sirigiri, J. R.; van der Wel, P. C. A.; Herzfeld, J.; Temkin, R. J.; Griffin, R. G. Dynamic Nuclear Polarization at High Magnetic Fields. The Journal of Chemical Physics 2008, 128 (5), 052211. https://doi.org/10.1063/1.2833582.
Lilly Thankamony, A. S.; Wittmann, J. J.; Kaushik, M.; Corzilius, B. Dynamic Nuclear Polarization for Sensitivity Enhancement in Modern Solid-State NMR. Progress in Nuclear Magnetic Resonance Spectroscopy 2017, 102 (Supplement C), 120–195. https://doi.org/10.1016/j.pnmrs.2017.06.002.
What does it take to make DNP work?
Only unpaired electrons, which share their orbital with no other electron, are "DNP-active". Typical examples are organic radicals and transition metals.
The DNP sample consists of: (i) The source of electron spin (radical), (ii) the species in whose NMR spectrum we are interested, and (iii) the solvent/matrix.
As one must acquire an NMR spectrum, the DNP instrument contains a complete NMR spectrometer: a magnet and an NMR console which contains the transmitter and receiver in the MHz frequency range. Next, one needs the ability to manipulate the electron spins, requiring a hundreds of GHz frequency source. Most DNP experiments work best at low temperatures, so a cryostat is needed. For more details about DNP instrumentation, the reader is referred to the following publications:
Siaw, T. A.; Leavesley, A.; Lund, A.; Kaminker, I.; Han, S. A Versatile and Modular Quasi Optics-Based 200 GHz Dual Dynamic Nuclear Polarization and Electron Paramagnetic Resonance Instrument. Journal of Magnetic Resonance 2016, 264, 131–153. https://doi.org/10.1016/j.jmr.2015.12.012.
Leavesley, A.; Kaminker, I.; Han, S. Versatile Dynamic Nuclear Polarization Hardware with Integrated Electron Paramagnetic Resonance Capabilities. In eMagRes; John Wiley & Sons, Ltd, 2018; pp 133–154. https://doi.org/10.1002/9780470034590.emrstm1564.
More details on our instrument are available here.