Electron-driven spin diffusion supports crossing the diffusion barrier in MAS DNP.

Dynamic nuclear polarization (DNP) can be applied to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude due to microwave-driven transfer of spin polarization from unpaired electrons to nuclei. While the underlying quantum mechanical aspects are sufficiently well understood on a microscopic level, the exact description of the large-scale spin dynamics, usually involving hundreds to thousands of nuclear spins per electron, is still lacking consensus. Generally, it is assumed that nuclear hyperpolarization can only be observed on nuclei which do not experience strong influence of the unpaired electrons and thus being significantly removed from the paramagnetic polarizing agents. At the same time, sufficiently strong hyperfine interaction is required for DNP transfer. Therefore, efficient nuclear spin diffusion from the strongly-interacting nuclei to the NMR-observable bulk is considered to be essential for efficient nuclear hyperpolarization. Based on experimental results obtained on the endohedral fullerene N@C60 as a polarizing agent sparsely diluted in C60, we discuss the effect of the spin-diffusion barrier. We introduce electron-driven spin diffusion (EDSD) as a novel mechanism for nuclear polarization transfer in the proximity of an electron spin which is particularly relevant under magic-angle spinning (MAS) DNP conditions.

Frequency-agile gyrotron for electron decoupling and pulsed dynamic nuclear polarization.

We describe a frequency-agile gyrotron which can generate frequency-chirped microwave pulses. An arbitrary waveform generator (AWG) within the NMR spectrometer controls the microwave frequency, enabling synchronized pulsed control of both electron and nuclear spins. We demonstrate that the acceleration of emitted electrons, and thus the microwave frequency, can be quickly changed by varying the anode voltage. This strategy results in much faster frequency response than can be achieved by changing the potential of the electron emitter, and does not require a custom triode electron gun. The gyrotron frequency can be swept with a rate of 20 MHz/µs over a 670 MHz bandwidth in a static magnetic field. We have already implemented time-domain electron decoupling with dynamic nuclear polarization (DNP) magic angle spinning (MAS) with this device. In this contribution, we show frequency-swept DNP enhancement profiles recorded without changing the NMR magnet or probe. The profile of endofullerenes exhibits a DNP profile with a <10 MHz linewidth, indicating that the device also has sufficient frequency stability, and therefore phase stability, to implement pulsed DNP mechanisms such as the frequency-swept solid effect. We describe schematics of the mechanical and vacuum construction of the device which includes a novel flanged sapphire window assembly. Finally, we discuss how commercially available continuous-wave gyrotrons can potentially be converted into similar frequency-agile high-power microwave sources.

Probing Interactions of N-Donor Molecules with Open Metal Sites within Paramagnetic Cr-MIL-101: A Solid-State NMR Spectroscopic and Density Functional Theory Study.

Understanding host-guest interactions is one of the key requirements for adjusting properties in metal-organic frameworks (MOFs). In particular, systems with coordinatively unsaturated Lewis acidic metal sites feature highly selective adsorption processes. This is attributed to strong interactions with Lewis basic guest molecules. Here we show that a combination of 13C MAS NMR spectroscopy with state-of-the-art density functional theory (DFT) calculations allows one to unravel the interactions of water, 2-aminopyridine, 3-aminopyridine, and diethylamine with the open metal sites in Cr-MIL-101. The 13C MAS NMR spectra, obtained with ultrafast magic-angle spinning, are well resolved, with resonances distributed over 1000 ppm. They present a clear signature for each guest at the open metal sites. Based on competition experiments this leads to the following binding preference: water < diethylamine ≈ 2-aminopyridine < 3-aminopyridine. Assignments were done by exploiting distance sum relations derived from spin-lattice relaxation data and 13C{1H} REDOR spectral editing. The experimental data were used to validate NMR shifts computed for the Cr-MIL-101 derivatives, which contain Cr3O clusters with magnetically coupled metal centers. While both approaches provide an unequivocal assignment and the arrangement of the guests at the open metal sites, the NMR data offer additional information about the guest and framework dynamics. We expect that our strategy has the potential for probing the binding situation of adsorbate mixtures at the open metal sites of MOFs in general and thus accesses the microscopic interaction mechanisms for this important material class, which is essential for deriving structure-property relationships.

Transient effects in π-pulse sequences in MAS solid-state NMR.

Dipolar recoupling techniques that use isolated rotor-synchronized π pulses are commonly used in solid-state NMR spectroscopy to gain insight into the structure of biological molecules. These sequences excel through their simplicity, stability towards radio-frequency (rf) inhomogeneity, and low rf requirements. For a theoretical understanding of such sequences, we present a Floquet treatment based on an interaction-frame transformation including the chemical-shift offset dependence. This approach is applied to the homonuclear dipolar-recoupling sequence Radio-Frequency Driven Recoupling (RFDR) and the heteronuclear recoupling sequence Rotational Echo Double Resonance (REDOR). Based on the Floquet approach, we show the influence of effective fields caused by pulse transients and discuss the advantages of pulse-transient compensation. We demonstrate experimentally that the transfer efficiency for homonuclear recoupling can be doubled in some cases in model compounds as well as in simple peptides if pulse-transient compensation is applied to the π pulses. Additionally, we discuss the influence of various phase cycles on the recoupling efficiency in order to reduce the magnitude of effective fields. Based on the findings from RFDR, we are able to explain why the REDOR sequence does not suffer in the recoupling efficiency despite the presence of effective fields.

Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR.

The field of dynamic nuclear polarization has undergone tremendous developments and diversification since its inception more than 6 decades ago. In this review we provide an in-depth overview of the relevant topics involved in DNP-enhanced MAS NMR spectroscopy. This includes the theoretical description of DNP mechanisms as well as of the polarization transfer pathways that can lead to a uniform or selective spreading of polarization between nuclear spins. Furthermore, we cover historical and state-of-the art aspects of dedicated instrumentation, polarizing agents, and optimization techniques for efficient MAS DNP. Finally, we present an extensive overview on applications in the fields of structural biology and materials science, which underlines that MAS DNP has moved far beyond the proof-of-concept stage and has become an important tool for research in these fields.

Optimizing symmetry-based recoupling sequences in solid-state NMR by pulse-transient compensation and asynchronous implementation.

Pulse imperfections like pulse transients and radio-frequency field maladjustment or inhomogeneity are the main sources of performance degradation and limited reproducibility in solid-state nuclear magnetic resonance experiments. We quantitatively analyze the influence of such imperfections on the performance of symmetry-based pulse sequences and describe how they can be compensated. Based on a triple-mode Floquet analysis, we develop a theoretical description of symmetry-based dipolar recoupling sequences, in particular, R26411, calculating first- and second-order effective Hamiltonians using real pulse shapes. We discuss the various origins of effective fields, namely, pulse transients, deviation from the ideal flip angle, and fictitious fields, and develop strategies to counteract them for the restoration of full transfer efficiency. We compare experimental applications of transient-compensated pulses and an asynchronous implementation of the sequence to a supercycle, SR26, which is known to be efficient in compensating higher-order error terms. We are able to show the superiority of R26 compared to the supercycle, SR26, given the ability to reduce experimental error on the pulse sequence by pulse-transient compensation and a complete theoretical understanding of the sequence.

Heteronuclear Cross-Relaxation under Solid-State Dynamic Nuclear Polarization.

We report on the spontaneous polarization transfer from dynamically hyperpolarized 1H to 13C during magic-angle spinning dynamic nuclear polarization (DNP) at temperatures around 100 K. The transfer is mediated by 1H-13C cross-relaxation within methyl groups due to reorientation dynamics, and results in an inverted 13C NMR signal of enhanced amplitude. Further spreading of transferred polarization can then occur via 13C-13C spin-diffusion. The resulting process is equal to the nuclear Overhauser effect (NOE) where typically continuous saturation of 1H by radio frequency irradiation is employed. Here, hyperpolarization by irradiation with microwaves in the presence of typical bis-nitroxide polarizing agents is utilized for steady-state displacement of 1H polarization from thermal equilibrium and perpetual spin-lattice relaxation. An effective 13C enhancement factor of up to -15 has been measured. Presence of Gd(III) furthermore amplifies the effect likely by accelerated relaxation of 1H. We provide experimental evidence for the proposed mechanism and show that DNP-induced cross-relaxation is a robust feature within proteins and single amino acids and discuss potential applications.

Accelerating proton spin diffusion in perdeuterated proteins at 100 kHz MAS.

Fast magic-angle spinning (>60 kHz) has many advantages but makes spin-diffusion-type proton-proton long-range polarization transfer inefficient and highly dependent on chemical-shift offset. Using 100%-HN-[2H,13C,15N]-ubiquitin as a model substance, we quantify the influence of the chemical-shift difference on the spin diffusion between proton spins and compare two experiments which lead to an improved chemical-shift compensation of the transfer: rotating-frame spin diffusion and a new experiment, reverse amplitude-modulated MIRROR. Both approaches enable broadband spin diffusion, but the application of the first variant is limited due to fast spin relaxation in the rotating frame. The reverse MIRROR experiment, in contrast, is a promising candidate for the determination of structurally relevant distance restraints. The applied tailored rf-irradiation schemes allow full control over the range of recoupled chemical shifts and efficiently drive spin diffusion. Here, the relevant relaxation time is the larger longitudinal relaxation time, which leads to a higher signal-to-noise ratio in the spectra.

Quantification and compensation of the influence of pulse transients on symmetry-based recoupling sequences.

Deviations of amplitude and phase of radio-frequency pulses from the desired values, can have a severe impact on the performance of multiple-pulse sequences in NMR spectroscopy. A particular problem are pulse transients that appear every time there is a discontinuity in amplitude or phase. Based on a Floquet description using pulses with arbitrarily shaped amplitudes and phases we present a systematic study of the influence of pulse transients on symmetry-based pulse sequences in solid-state NMR under magic-angle spinning. This treatment explains the dependence of the experimentally observed transfer efficiency on the details of experimental setups. In addition, three approaches are compared which have the aim to re-establish highly efficient recoupling. We demonstrate that the application of transient-compensated pulses as basic elements of symmetry-based sequences leads to a significantly improved robustness of the experiments with respect to variations in the experimental setup.

Compensating Pulse Imperfections in Solid-State NMR Spectroscopy: A Key to Better Reproducibility and Performance.

The power and versatility of NMR spectroscopy is strongly related to the ability to manipulate NMR interactions by the application of radio-frequency (rf) pulse sequences. Unfortunately, the rf fields seen by the spins differ from the ones programmed by the experimentalist. Pulse transients, i.e., deviations of the amplitude and phase of the rf fields from the desired values, can have a severe impact on the performance of pulse sequences and can lead to inconsistent results. Here, we demonstrate how transient-compensated pulses can greatly improve the efficiency and reproducibility of NMR experiments. The implementation is based on a measurement of the characteristics of the resonance circuit and does not rely on an experimental optimization of the NMR signal. We show how the pulse sequence has to be modified to use it with transient-compensated pulses. The efficiency and reproducibility of the transient-compensated sequence is greatly superior to the original POST-C7 sequence.

Probing self-assembled 1,3,5-benzenetrisamides in isotactic polypropylene by 13C DQ solid-state NMR spectroscopy.

Using (13)C double quantum solid-state NMR spectroscopy, we were able to observe nuclei of a supramolecular BTA based additive on the nanoscale in a matrix of i-PP at a concentration of only 0.09 wt%. These nuclei exhibit the analogous structural features as the crystalline phase of the neat additive.