Eugney V. Kryukov, Kevin J. Pike, Thomas K. Y. Tam, Mark E. Newton, Mark E. Smith and Ray Durpee
http://dx.doi.org/10.1039/c0cp02188a
It is shown that the temperature dependence of the DNP enhancement of the NMR signal from water protons at 3.4 T using TEMPOL as a polarising agent can be obtained provided that the nuclear relaxation, T1I, is sufficiently fast and the resolution sufficient to measure the 1H NMR shift. For high radical concentrations (~100 mM) the leakage factor is approximately 1 and,provided sufficient microwave power is available, the saturation factor is also approximately 1. In this situation the DNP enhancement is solely a product of the ratio of the electron and nuclear gyromagnetic ratios and the coupling factor enabling the latter to be directly determined. Although the use of high microwave power levels needed to ensure saturation causes rapid heating of the sample, this does not prevent maximum DNP enhancements, e0, being obtained since T1I is very much less than the characteristic heating time at these concentrations. It is necessary, however, to know the temperature variation of T1I to allow accurate modelling of the behaviour. The DNP enhancement is found to vary linearly with temperature with e0(T) = -2 +/- 2 - (1.35 +/- 0.02)T for 6ºC < T < 100ºC. The value determined for the coupling factor, 0.055 +/- 0.003 at 25ºC, agrees very well with the molecular dynamics simulations of Sezer et al. (Phys. Chem. Chem. Phys., 2009, 11, 6626) who calculated 0.0534, however the experimental values increase much more rapidly with increasing temperature than predicted by these simulations. Large DNP enhancements (e0 > 100) are reported at high temperatures but it is also shown that significant enhancements (e.g. ~40) can be achieved whilst maintaining the sample temperature at 40ºC by adjusting the microwave power and irradiation time. In addition, short polarisation times enable rapid data acquisition which permits further enhancement of the signal, such that useful liquid state DNP-NMR experiments could be carried out on very small samples.