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

Abstract

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, T(1I), is sufficiently fast and the resolution sufficient to measure the (1)H 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, ε(0), being obtained since T(1I) is very much less than the characteristic heating time at these concentrations. It is necessary, however, to know the temperature variation of T(1I) to allow accurate modelling of the behaviour. The DNP enhancement is found to vary linearly with temperature with ε(0)(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 (|ε(0)| > 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.

Comparison of Overhauser DNP at 0.34 and 3.4 T with Frémy’s Salt

“…However, these enhancements are considerably lower than the recently reported values of up to −170 using TEMPOL at 3.4 T [12, 13]. In both reports from the literature, these high enhancements were attributed to elevated sample temperatures during microwave irradiation exceeding 100 °C.…”

“…Moreover, high DNP enhancements of water proton signals up to two orders of magnitudes have been recently reported not only at low microwave frequencies (9 GHz, [7–9]) but also at intermediate (94 GHz, [12, 13]) and high (260 GHz, [14]) microwave frequencies. The enhancements at 94 GHz/3.4 T in [12, 13] were much higher than in our in initial reports [6, 7]. Irradiation times needed to reach maximum DNP effects have mainly been optimized experimentally, and the build-up of dynamic nuclear polarization has been interpreted with the nuclear relaxation time T 1n and its temperature dependence [12, 13].…”

“…The enhancements at 94 GHz/3.4 T in [12, 13] were much higher than in our in initial reports [6, 7]. Irradiation times needed to reach maximum DNP effects have mainly been optimized experimentally, and the build-up of dynamic nuclear polarization has been interpreted with the nuclear relaxation time T 1n and its temperature dependence [12, 13]. In order to rationalize the observed frequency dependence of liquid DNP enhancements up to medium frequency/fields and particularly compare the maximal enhancements reported so far at 94 GHz in our [7] and in other groups [12, 13], we have measured the DNP signals as a function of the irradiation time and compared with independent T 1n measurements [8, 15].…”

Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

“…In Osaka University, Fujiwara's group succeeded in 600 MHz DNP NMR using a 394.5 GHz Gyrotron FU CW II as collaborative work with our FIR FU [4,5]. In the University of Warwick, Dupree's group is developing 285 MHz DNP NMR using a 187 GHz Gyrotron FU CW VII [6,7]. In every case, the enhancement factor is typically several tens to several hundreds.…”

Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy’s Salt Measured in a Magnetic Field of 9.2 T

“…Equation (2) describes the major effect of temperature on the DNP enhancement: with increasing temperature, both D S and D R will increase, therefore shortening s t and increasing the DNP enhancement e. Additionally, the temperature dependence of the relaxation rates of the electron and nuclear spin might change the saturation factor s and leakage factor f. In this work, we measured the DNP enhancements of water protons with Fremy's Salt as radicals for temperatures between 15 and 65°C at a magnetic field of 9.2 T. So far, such temperature measurements were done and discussed only up to magnetic fields of B = 3.4 T [14,15]. The high enhancements obtained are compared with predictions based on classical Overhauser theory for liquids, taking translational correlation times extracted from low-field DNP and NMRD experiments [16][17][18][19][20], and calculations of the coupling factor from MD simulations [21,22].…”