Unexpected high DNP enhancements of more than 10 have been achieved in liquid water samples at room temperature and magnetic fields of 9.2 T (corresponding to 400 MHz (1)H NMR frequency and 260 GHz EPR frequency). The liquid samples were polarized in situ using a double-resonance structure, which allows simultaneous excitation of NMR and EPR transitions and achieves significant DNP enhancements at very low incident microwave power of only 45 mW. These results demonstrate the first important step toward the application of DNP to high-resolution NMR, increasing the sensitivity on biomolecules with small sample volumes and at physiologically low concentrations.

A series of 12 asphaltene samples extracted from heavy oils and the oxidized bitumen of different origin has been studied with high-frequency W-band (94 GHz) pulsed electron paramagnetic resonance (EPR) spectroscopy. Transverse (T 2e ) and longitudinal (T 1e ) relaxation times of the free radical (FR) and the vanadyl porphyrin (VO 2+ ) were measured for each sample. A significant contribution of the spectral diffusion to T 2e has been revealed and ascribed to the dipole−dipole interaction between the FR and VO 2+ . This indicates that the distance between the FR and VO 2+ does not exceed a few nanometers, which means, in turn, that VO 2+ can participate in construction of the asphaltene aggregates via the intermolecular interactions.

The structural properties of crude oils and asphaltenes, especially related to the dynamics of their aggregation, have been investigated by different experimental and theoretical methods during the last few decades. However, there are only a few works devoted to study the dynamics of asphaltenes in a native hydrocarbon environment. In this report, we illustrate a possibility to use electron paramagnetic resonance (EPR) spectra of vanadyl porphyrins in asphaltenes for the qualitative and quantitative analysis of their rotational mobility in the crude oil samples. On the basis of the simulation of the EPR spectra, a simple semi-empirical parameter sensitive to the transition between motional regimes is proposed. This mobility parameter can be potentially useful for the prediction and analysis of the thermal influence on heavy oil reservoirs during hydrocarbon production. It is found that the rotational correlation time of the complexes in heavy oil samples changes discontinuously with the temperature. The observed jump could be attributed to a disaggregation of supramolecular complexes of asphaltenes in the close vicinity of a phase transition.

Dynamic nuclear polarization (DNP) at high magnetic fields (9.2 T, 400 MHz (1)H NMR frequency) requires high microwave power sources to achieve saturation of the EPR transitions. Here we describe the first high-field liquid-state DNP results using a high-power gyrotron microwave source (20 W at 260 GHz). A DNP enhancement of -29 on water protons was obtained for an aqueous solution of Fremy's Salt; in comparison the previous highest value was -10 using a solid-state microwave power source (maximum power 45 mW). The increased enhancements are partly due to larger microwave saturation and elevated sample temperature. These experimentally observed DNP enhancements, which by far exceed the predicted values extrapolated from low-field DNP experiments, demonstrate experimentally that DNP is possible in the liquid state also at high magnetic fields.