Rotation-induced recovery and bleaching in magnetic resonance.

Thurber and Tycko recently described a 'bleaching effect' that occurs in magnetic resonance when solid samples that are doped with paramagnetic agents are subjected to rotation by magic angle spinning (MAS) in a static magnetic field with a rotation period comparable to the longitudinal relaxation time T1e of the electron spins. The bleaching effect has been investigated by Thurber and Tycko in samples spinning at temperatures near 20 K in a field of 9.4 T and by Corzilius et al. near 80 K in a field of 4.9 T. In our experience, the bleaching effect is not very severe at temperatures near 100 K in a field of 9.4 T at spinning frequencies up to 12 kHz. Bleaching can partly cancel DNP enhancements that are normally evaluated by comparing signal intensities with and without microwave irradiation. The signal attenuation due to doping and sample rotation is usually not taken into consideration when defining enhancement factors. In this paper, we describe a novel observation that the rotation of glassy samples doped with lanthanides spinning at frequencies as low as 0.1 kHz can lead to a significant reduction of the spin-lattice relaxation times T1((1)H) of protons. This effect, which bears similarities with the so-called spin refrigerators, may contribute to the success of 'brute force polarization' at sample temperatures in the mK range. The acceleration of longitudinal proton relaxation also allows one to improve the signal-to-noise ratio per unit time.

Insights into the catalytic activity of nitridated fibrous silica (KCC-1) nanocatalysts from (15) N and (29) Si†NMR spectroscopy enhanced by dynamic nuclear polarization.

Fibrous nanosilica (KCC-1) oxynitrides are promising solid-base catalysts. Paradoxically, when their nitrogen content increases, their catalytic activity decreases. This counterintuitive observation is explained here for the first time using (15) N-solid-state NMR spectroscopy enhanced by dynamic nuclear polarization.

Dynamic nuclear polarization enhancement of protons and vanadium-51 in the presence of pH-dependent vanadyl radicals.

We report applications of dynamic nuclear polarization to enhance proton and vanadium-51 polarization of vanadyl sulfate samples doped with TOTAPOL under magic angle spinning conditions. The electron paramagnetic resonance response stemming from the paramagnetic (51)V species was monitored as a function of pH, which can be adjusted to improve the enhancement of the proton polarization. By means of cross-polarization from the proton bath, (51)V spins could be hyperpolarized. Enhancement factors, build-up times, and longitudinal relaxation times T1((1)H) and T1((51)V) were investigated as a function of pH.

Cross polarization from (1)H to quadrupolar (6)Li nuclei for dissolution DNP.

Cross polarization from protons to quadrupolar (6)Li nuclei is combined with dynamic nuclear polarization of protons at 1.2 K and 6.7 T using TEMPOL as a polarizing agent followed by rapid dissolution. Compared to direct (6)Li DNP without cross-polarization, a higher nuclear spin polarization P((6)Li) can be obtained in a shorter time. A double resonance (1)H-(6)Li probe was designed that is equipped for Longitudinally Detected Electron Spin Resonance.

Achievement of high nuclear spin polarization using lanthanides as low-temperature NMR relaxation agents.

Many approaches are now available for achieving high levels of nuclear spin polarization. One of these methods is based on the notion that as the temperature is reduced, the equilibrium nuclear polarization will increase, according to the Boltzmann distribution. The main problem with this approach is the length of time it may take to approach thermal equilibrium at low temperatures, since nuclear relaxation times (characterized by the spin-lattice relaxation time T1) can become very long. Here, we show, by means of relaxation time measurements of frozen solutions, that selected lanthanide ions, in the form of their chelates with DTPA, can act as effective relaxation agents at low temperatures. Differential effects are seen with the different lanthanides that were tested, holmium and dysprosium showing highest relaxivity, while gadolinium is ineffective at temperatures of 20 K and below. These observations are consistent with the known electron-spin relaxation time characteristics of these lanthanides. The maximum relaxivity occurs at around 10 K for Ho-DTPA and 20 K for Dy-DTPA. Moreover, these two agents show only modest relaxivity at room temperature, and can thus be regarded as relaxation switches. We conclude that these agents can speed up solid state NMR experiments by reducing the T1 values of the relevant nuclei, and hence increasing the rate at which data can be acquired. They could also be of value in the context of a simple low-cost method of achieving several-hundred-fold improvements in polarization for experiments in which samples are pre-polarized at low temperatures, then rewarmed and dissolved immediately prior to analysis.

Boosting Dissolution Dynamic Nuclear Polarization by Cross Polarization.

The efficiency of dissolution dynamic nuclear polarization can be boosted by Hartmann-Hahn cross polarization at temperatures near 1.2 K. This enables high throughput of hyperpolarized solutions with substantial gains in buildup times and polarization levels. During dissolution and transport, the (13)C nuclear spin polarization P((13)C) merely decreases from 45 to 40%.

Dynamic Nuclear Polarization and other magnetic ideas at EPFL.

Although nuclear magnetic resonance (NMR) can provide a wealth of information, it often suffers from a lack of sensitivity. Dynamic Nuclear Polarization (DNP) provides a way to increase the polarization and hence the signal intensities in NMR spectra by transferring the favourable electron spin polarization of paramagnetic centres to the surrounding nuclear spins through appropriate microwave irradiation. In our group at EPFL, two complementary DNP techniques are under investigation: the combination of DNP with magic angle spinning at temperatures near 100 K ('MAS-DNP'), and the combination of DNP at 1.2 K with rapid heating followed by the transfer of the sample to a high-resolution magnet ('dissolution DNP'). Recent applications of MAS-DNP to surfaces, as well as new developments of magnetization transfer of (1)H to (13)C at 1.2 K prior to dissolution will illustrate the work performed in our group. A second part of the paper will give an overview of some 'non-enhanced' activities of our laboratory in liquid- and solid-state NMR.

A dedicated spectrometer for dissolution DNP NMR spectroscopy.

Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.