Cross-term Splittings Due to the Orientational Inequivalence of Proton Magnetic Shielding Tensors: Do Water Molecules Trapped in Crystals Hop or Tunnel?

Frozen water NMR lineshape analysis enables absolute polarization quantification

Typical magnetic resonance experiments are routinely limited by weak signal responses. In some cases, the low intrinsic sensitivity can be alleviated by the implementation of hyperpolarization technologies. Dissolution-dynamic nuclear polarization offers a means of hyperpolarizing small molecules. Hyperpolarized water is employed in several dynamic nuclear polarization studies, and hence accurate and rapid quantification of the ¹H polarization level is of utmost importance. The solid-state nuclear magnetic resonance spectrum of water acquired under dissolution-dynamic nuclear polarization conditions has revealed lineshapes which become asymmetric at high levels of ¹H polarization, which is an interesting fundamental problem in itself, but also complicates data interpretation and can prevent correct estimations of polarization levels achieved. In previous studies, attempts to simulate the ¹H spectral lineshape of water as a function of the ¹H polarization led to significant disagreement with the experimental results. Here we propose and demonstrate that such simulations, and therefore polarization quantification, can be implemented accurately, in particular by taking into account the detector dead time during ¹H signal acquisition that can lead to severe spectral distortions. Based on these findings, we employed an echo-based radiofrequency pulse sequence to achieve distortion-free ¹H spectra of hyperpolarized water, and adequate simulations of these echo-based spectra were implemented to extract the absolute ¹H polarization level from the hyperpolarized water signal only, thus alleviating the need for lengthy and insensitive measurements of thermal equilibrium signals.

Natural abundance oxygen-17 solid-state NMR of metal organic frameworks enhanced by dynamic nuclear polarization

The 17O resonances of zirconium-oxo clusters that can be found in porous Zr carboxylate metal-organic frameworks (MOFs) have been investigated by magic-angle spinning (MAS) NMR spectroscopy enhanced by dynamic nuclear polarization (DNP). High-resolution 17O spectra at 0.037% natural abundance could be obtained in 48 hours, thanks to DNP enhancement of the 1H polarization by factors ε(1H) = Swith/Swithout = 28, followed by 1H → 17O cross-polarization, allowing a saving in experimental time by a factor of ca. 800. The distinct 17O sites from the oxo-clusters can be resolved at 18.8 T. Their assignment is supported by density functional theory (DFT) calculations of chemical shifts and quadrupolar parameters. Protonation of 17O sites seems to be leading to large characteristic shifts. Hence, natural abundance 17O NMR spectra of diamagnetic MOFs can thus be used to probe and characterize the local environment of different 17O sites on an atomic scale.

Spin Thermometry: A Straightforward Measure of Millikelvin Deuterium Spin Temperatures Achieved by Dynamic Nuclear Polarization

Dynamic nuclear polarization of samples at low temperatures, typically between 1.2 and 4.2 K, allows one to achieve spin temperatures of as low as 2 mK so that for many nuclear isotopes the high-temperature approximation is violated for the nuclear Zeeman interaction. This leads to characteristic asymmetries in powder spectra. We show that the line shapes due to the quadrupolar couplings of deuterium spins present in virtually all solvents used for such experiments (DNP juice) allow the quick yet accurate determination of the deuterium spin temperature or, equivalently, the deuterium polarization. The observation of quadrupolar echoes excited by small flip-angle pulses allows one to monitor the build-up and decay of the positive or negative deuterium polarization.

Orientation-Dependent Proton Relaxation of Water Molecules Trapped in Solids: Crystallites with Long-Lived Magnetization

The longitudinal spin-lattice relaxation properties of water molecules trapped in a static powdered polycrystalline sample of barium chlorate monohydrate are investigated by means of solid-state ¹H NMR spectroscopy. Different portions of the inhomogeneous Pake pattern that are associated with crystallites at different orientations with respect to the external magnetic field show either a mono- or a biexponential recovery. At high field (9.4 T), the chemical shift anisotropy is the main interaction that is responsible for the inhomogeneity of the relaxation rates. A theoretical description of rapid two-site hopping about the H-O-H bisector in the framework of Liouville space agrees very well with the experimental evidence. Numerical simulations predict a distribution of monoexponential time constants associated with individual single-crystal orientations. Overlapping signals give rise to biexponential recovery. This is confirmed experimentally by ¹H NMR spectra of static single crystals.