Fluid flow dynamics in MAS systems

High resolution solid-state NMR on the desktop

High-resolution low-field nuclear magnetic resonance (NMR) spectroscopy has found wide application for characterization of liquid compounds because of the low maintenance cost of modern permanent magnets. Solid-state NMR so far is limited to low-resolution measurements of static powders, because of the limited space available in this type of magnet. Magic-angle sample spinning and low-magnetic fields are an attractive combination to achieve high spectral resolution especially for paramagnetic solids. Here we show that magic angle spinning modules can be miniaturized using 3D printing techniques so that high-resolution solid-state NMR in permanent magnets becomes possible. The suggested conical rotor design was developed using finite element calculations and provides sample spinning frequencies higher than 20 kHz. The setup was tested on various diamagnetic and paramagnetic compounds including paramagnetic battery materials. The only comparable experiments in low-cost magnets known so far, had been done in the early times of magic angle spinning using electromagnets at much lower sample spinning frequency. Our results demonstrate that high-resolution low-field magic-angle-spinning NMR does not require expensive superconducting magnets and that high-resolution solid-state NMR spectra of paramagnetic compounds are feasible. Generally, this could introduce low-field solid-state NMR for abundant nuclei standard as a routine analytical tool.

Diamond rotors

The resolution of magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra remains bounded by the spinning frequency, which is limited by the material strength of MAS rotors. Since diamond is capable of withstanding 1.5-2.5x greater MAS frequencies, compared to state-of-the art zirconia, we fabricated rotors from single crystal diamond. When combined with bearings optimized for spinning with helium gas, diamond rotors could achieve the highest MAS frequencies to date. Furthermore, the excellent microwave transmission properties and thermal conductivity of diamond could improve sensitivity enhancements in dynamic nuclear polarization (DNP) experiments. The fabrication protocol we report involves novel laser micromachining and produced rotors that presently spin at ωr/2π = 111.000 ± 0.004 kHz, with stable spinning up to 124 kHz, using N2 gas as the driving fluid. We present the first proton-detected 13C/15N MAS spectra recorded using diamond rotors, a critical step towards studying currently inaccessible ex-vivo protein samples with MAS NMR. Previously, the high aspect ratio of MAS rotors (∼10:1) precluded fabrication of MAS rotors from diamond.

One-Shot Resin 3D-Printed Stators for Low-Cost Fabrication of Magic-Angle Spinning NMR Probeheads

Additive manufacturing such as three-dimensional (3D)-printing has revolutionized the fast and low-cost fabrication of otherwise expensive NMR parts. High-resolution solid-state NMR spectroscopy demands rotating the sample at a specific angle (54.74°) inside a pneumatic turbine, which must be designed to achieve stable and high spinning speeds without mechanical friction. Moreover, instability of the sample rotation often leads to crashes, resulting in costly repairs. Producing these intricate parts requires traditional machining, which is time-consuming, costly, and relies on specialized labor. Herein, we show that 3D-printing can be used to fabricate the sample holder housing (stator) in one shot, while the radiofrequency (RF) solenoid was constructed using conventional materials available in electronics stores. The 3D-printed stator, equipped with a homemade RF coil, showed remarkable spinning stability, yielding high-quality NMR data. At a cost below 5 €, the 3D-printed stator represents a cost reduction of over 99% compared to repaired commercial stators, illustrating the potential of 3D-printing for mass-producing affordable magic-angle spinning stators.

In-house fabrication of 1.3 to 7 mm MAS drive caps using desktop 3D printers

The 3D-printing technology has emerged as a well-developed method to produce parts with considerably low cost and yet with high precision (<100 μm). Recent literature has shown that the 3D-printing technology can be exploited to fabricate a magic-angle spinning (MAS) system in solid-state nuclear magnetic resonance (NMR) spectroscopy. In particular, it was demonstrated that advanced industry-grade 3D printers could fabricate 3.2 mm MAS drive caps with intricate features, and the caps were shown to spin > 20 kHz. Here, we show that not only lab-affordable benchtop 3D printers can produce 3.2 mm drive caps with a similar quality as the commercialized version, but also smaller 2.5 mm and 1.3 mm MAS drive caps-despite a slight compromise in performance. All in-house fabricated drive caps (1.3 to 7 mm) can be consistently reproduced (>90 %) and achieve excellent spinning performances. In summary, the > 3.2 mm systems have similar performances as the commercial systems, while the 2.5- and 1.3-mm caps can spin up to 26 kHz ± 2 Hz, and 46 kHz ± 1 Hz, respectively. The low-cost and fast in-house fabrication of MAS drive caps allows easy prototyping of new MAS drive cap models and, possibly, new NMR applications. For instance, we have fabricated a 4 mm drive cap with a center hole that could allow better light penetration or sample insertion during MAS. Besides, an added groove design on the drive cap allows an airtight seal suitable for probing air- or moisture-sensitive materials. Moreover, the 3D-printed cap was shown to be robust for low-temperature MAS experiments at ∼ 100 K, making it suitable for DNP experiments.

Disentangling the Effect of Pressure and Mixing on a Mechanochemical Bromination Reaction by Solid-State NMR Spectroscopy

Mechanical forces, including compressive stresses, have a significant impact on chemical reactions. Besides the preparative opportunities, mechanochemical conditions benefit from the absence of any organic solvent, the possibility of a significant synthetic acceleration and unique reaction pathways. Together with an accurate characterization of ball-milling products, the development of a deeper mechanistic understanding of the occurring transformations at a molecular level is critical for fully grasping the potential of organic mechanosynthesis. We herein studied a bromination of a cyclic sulfoximine in a mixer mill and used solid-state nuclear magnetic resonance (NMR) spectroscopy for structural characterization of the reaction products. Magic-angle spinning (MAS) was applied for elucidating the product mixtures taken from the milling jar without introducing any further post-processing on the sample. Ex situ ¹³ C-detected NMR spectra of ball-milling products showed the formation of a crystalline solid phase with the regioselective bromination of the S-aryl group of the heterocycle in position 4. Completion is reached in less than 30 minutes as deduced from the NMR spectra. The bromination can also be achieved by magnetic stirring, but then, a longer reaction time is required. Mixing the solid educts in the NMR rotor allows to get in situ insights into the reaction and enables the detection of a reaction intermediate. The pressure alone induced in the rotor by MAS is not sufficient to lead to full conversion and the reaction occurs on slower time scales than in the ball mill, which is crucial for analysing mixtures taken from the milling jar by solid-state NMR. Our data suggest that on top of centrifugal forces, an efficient mixing of the starting materials is required for reaching a complete reaction.

Highly stable magic angle spinning spherical rotors

The use of spherical rotors for magic angle spinning offers a number of advantages, including improved sample exchange, efficient microwave coupling for dynamic nuclear polarization nuclear magnetic resonance (NMR) experiments, and, most significantly, high frequency and stable spinning with minimal risk of rotor crash. Here we demonstrate the simple retrofitting of a commercial NMR probe with MAS spheres for solid-state NMR. We analyze a series of turbine groove geometries to investigate the importance of the rotor surface for spinning performance. Of note, rotors lacking any surface modification spin rapidly and stably even without feedback control. The high stability of a spherical rotor about the magic angle is shown to be dependent on its inertia tensor rather than the presence of turbine grooves.

Thermo-mechanical analysis of a probe for electron paramagnetic resonance spectroscopy operating at cryogenic temperatures

In this article, we present the thermo-mechanical analysis of an electron paramagnetic resonance (EPR) probe operating at cryogenic temperatures using finite element analysis. Thermo-mechanical analysis plays a key role in the mechanical design evaluation process as EPR probes are often subjected to large stresses under such extreme conditions. For simplification, we assume thermal conduction to be the dominant mode of heat transfer over convection and radiation. The simulation model consists of a cryostat-probe assembly with appropriate thermal and structural boundary conditions. The predicted temperature distributions from the steady-state thermal analysis is then used for the stress analysis of the EPR probe. The stress analysis indicated that stresses in the EPR probe are below the ultimate strengths of each component, and thus safe for running EPR experiments. Furthermore, the simulation results were confirmed experimentally, and we found that the predicted heat losses for the EPR probe assembly and the sample holder are in excellent agreement with the experimental measurements.

Magic angle spinning NMR below 6 K with a computational fluid dynamics analysis of fluid flow and temperature gradients

We report magic angle spinning (MAS) up to 8.5 kHz with a sample temperature below 6 K using liquid helium as a variable temperature fluid. Cross polarization ¹³C NMR spectra exhibit exquisite sensitivity with a single transient. Remarkably, ¹H saturation recovery experiments show a ¹H T₁ of 21 s with MAS below 6 K in the presence of trityl radicals in a glassy matrix. Leveraging the thermal spin polarization available at 4.2 K versus 298 K should result in 71 times higher signal intensity. Taking the ¹H longitudinal relaxation into account, signal averaging times are therefore predicted to be expedited by a factor of >500. Computer assisted design (CAD) and finite element analysis were employed in both the design and diagnostic stages of this cryogenic MAS technology development. Computational fluid dynamics (CFD) models describing temperature gradients and fluid flow are presented. The CFD models bearing and drive gas maintained at 100 K, while a colder helium variable temperature fluid stream cools the center of a zirconia rotor. Results from the CFD were used to optimize the helium exhaust path and determine the sample temperature. This novel cryogenic experimental platform will be integrated with pulsed dynamic nuclear polarization and electron decoupling to interrogate biomolecular structure within intact human cells.

A generalized theoretical framework for the description of spin decoupling in solid-state MAS NMR: Offset effect on decoupling performance

We present a generalized theoretical framework that allows the approximate but rapid analysis of residual couplings of arbitrary decoupling sequences in solid-state NMR under magic-angle spinning conditions. It is a generalization of the tri-modal Floquet analysis of TPPM decoupling [Scholz et al., J. Chem. Phys. 130, 114510 (2009)] where three characteristic frequencies are used to describe the pulse sequence. Such an approach can be used to describe arbitrary periodic decoupling sequences that differ only in the magnitude of the Fourier coefficients of the interaction-frame transformation. It allows a ∼100 times faster calculation of second-order residual couplings as a function of pulse sequence parameters than full spin-dynamics simulations. By comparing the theoretical calculations with full numerical simulations, we show the potential of the new approach to examine the performance of decoupling sequences. We exemplify the usefulness of this framework by analyzing the performance of commonly used high-power decoupling sequences and low-power decoupling sequences such as amplitude-modulated XiX (AM-XiX) and its super-cycled variant SC-AM-XiX. In addition, the effect of chemical-shift offset is examined for both high- and low-power decoupling sequences. The results show that the cross-terms between the dipolar couplings are the main contributions to the line broadening when offset is present. We also show that the SC-AM-XIX shows a better offset compensation.