Determinants for optimal enhancement in Ex situ DNP experiments

Dynamic nuclear polarization (DNP) is a potent method to enhance the sensitivity of nuclear magnetic resonance (NMR) signals. In a recent implementation of DNP, samples are polarized at low temperature in a glassy state followed by dissolution with hot solvent and transfer to an NMR magnet (J.H. Ardenkjaer-Larsen, B. Fridlund, A. Gram, G. Hansson, L. Hansson, M.H. Lerche, R. Servin, M. Thaning, K. Golman: Proc. Natl. Acad. Sci. USA 100, 10158, 2003). Although enormous polarizations in the order of tens of thousands were reported, such enhancements were only obtained for very few molecules. Here we have polarized a range of small molecules in order to identify determinants of polarizability. The key factors include functional groups which allow for a good contact to the radical and substances with long longitudinal relaxation times T ₁. We also observe that methyl groups play a particularly interesting role in ¹³C-DNP-NMR spectra taken at low temperature.