TY - JOUR

T1 - Determination of the temperature dependence of the dynamic nuclear polarisation enhancement of water protons at 3.4 Tesla

AU - Kryukov, Eugeny V.

AU - Pike, Kevin J.

AU - Tam, Thomas K. Y.

AU - Newton, Mark E.

AU - Smith, Mark E.

AU - Dupree, Ray

PY - 2011

Y1 - 2011

N2 - 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, ε0, 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 ε0(T) = −2 ± 2 − (1.35 ± 0.02)T for 6 °C ≤ T ≤ 100 °C. The value determined for the coupling factor, 0.055 ± 0.003 at 25 °C, 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 (|ε0| > 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 40 °C 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.

AB - 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, ε0, 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 ε0(T) = −2 ± 2 − (1.35 ± 0.02)T for 6 °C ≤ T ≤ 100 °C. The value determined for the coupling factor, 0.055 ± 0.003 at 25 °C, 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 (|ε0| > 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 40 °C 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.

U2 - 10.1039/C0CP02188A

DO - 10.1039/C0CP02188A

M3 - Journal article

VL - 13

SP - 4372

EP - 4380

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9084

IS - 10

ER -