Solid-state nuclear magnetic resonance

Solid-state nuclear magnetic resonance (ssNMR) is a spectroscopy technique used to characterize atomic-level structure and dynamics in solid materials.

These interactions directly affect the lines shapes of experimental ssNMR spectra which can be seen in powder and dipolar patterns.

ssNMR is often combined with magic angle spinning (MAS) to remove anisotropic interactions and improve the sensitivity of the technique.

However, in media with no or little mobility (e.g. crystalline powders, glasses, large membrane vesicles, molecular aggregates), anisotropic local fields or interactions have substantial influence on the behaviour of nuclear spins, which results in the line broadening of the NMR spectra.

Chemical shielding is a local property of each nuclear site in a molecule or compound, and is proportional to the applied external magnetic field.

These induced currents create local magnetic fields that lead to characteristic changes in resonance frequency.

[6][7] A similar method to MQMAS is satellite transition magic angle spinning (STMAS) NMR developed in 2000.

In presence of a chemical shift anisotropy interaction, each orientation with respect to the magnetic field gives a different resonance frequency.

Fitting of the pattern in a static ssNMR experiment gives information about the shielding tensor, which are often described by the isotropic chemical shift

[9][10] This improved resolution results from manipulating a sample's spin interactions with the applied magnetic field.

To achieve the complete averaging of these interactions, the sample needs to be spun at a rate that is at least higher than the largest anisotropy.

[9][10] Spinning a powder sample at a slower rate than the largest component of the chemical shift anisotropy results in an incomplete averaging of the interaction, and produces a set of spinning sidebands in addition to the isotropic line, centred at the isotropic chemical shift.

Cross-polarization (CP) if a fundamental (Radiofrequency) RF pulse sequence and a building-block in many solid-state NMR.

It is typically used to enhance the signal of a dilute nuclei with a low gyromagnetic ratio (e.g. 13C, 15N) by magnetization transfer from an abundant nuclei with a high gyromagnetic ratio (e.g. 1H), or as a spectral editing method to get through space information (e.g. directed 15N→13C CP in protein spectroscopy).

The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetisation transfer.

Spin interactions can be removed (decoupled) to increase the resolution of NMR spectra during the detection, or to extend the lifetime of the nuclear magnetization.

Ultra fast MAS (from 60 kHz up to above 111 kHz) is an efficient way to average all dipolar interactions, including 1H–1H homonuclear dipolar interactions, which extends the resolution of 1H spectra and enables the usage of pulse sequences used in solution state NMR.

The reintroduction of such dipolar coupling reduces the intensity of the NMR signal compared to a reference spectrum where no dephasing pulse is used.

Fast MAS and reduction of dipolar interactions by deuteration have made proton ssNMR as versatile as in solution.

This includes spectral dispersion in multi-dimensional experiments[27] and structurally valuable restraints and parameters important for studying material dynamics.

[29] Magic angle spinning dynamic nuclear polarization (MAS-DNP) is a technique that increases the sensitivity of NMR experiments by several orders of magnitude.

[35] Beta-detected nuclear magnetic resonance (β-NMR) is specialized technique that has working principles similar to muon spin spectroscopy.

[36] It is used in domains such as chemistry, materials science, condensed matter physics, and biology as a powerful probe.

[38] What makes β-NMR different than conventional NMR is firstly, where and when the spin polarization of the nuclei occurs and secondly, how the signal is produced.

[36][39] To conduct a β-NMR experiment, optical pumping is performed on a radioactive beam of particles, such as 8Li and 31Mg, to polarize their nuclear spin to nearly one-hundred percent.

[36][40] The isotopes are subsequently implanted into a sample in vacuum in the dilute-limit to eliminate homonuclear probe interactions.

[36][40] ssNMR spectroscopy serves as an effective analytical tool in biological, organic, and inorganic chemistry due to its close resemblance to liquid-state spectra while providing additional insights into anisotropic interactions.

[41] It is used to characterize chemical composition, structure, local motions, kinetics, and thermodynamics, with the special ability to assign the observed behavior to specific sites in a molecule.

[46] ssNMR is used to study biomaterials such as bone,[47][48] teeth,[49][50] hair,[51] silk,[52] wood,[53] as well as viruses,[54][55] plants,[56][57] cells,[58][59] and collected biopsies.

ssNMR has been successfully used to study metal organic frameworks,[64] solid-state batteries, [65] surfaces of nanoporous materials,[66] and polymers.

Solid-state 900 MHz (21.1 T [ 1 ] ) NMR spectrometer at the Canadian National Ultrahigh-field NMR Facility for Solids
Bruker MAS rotors. From left to right: 1.3 mm (up to 67 kHz), 2.5 mm (up to 35 kHz), 3.2 mm (up to 24 kHz), 4 mm (up to 15 kHz), 7 mm (up to 7 kHz)
Vectors important for dipolar coupling between nuclear spins I 1 and I 2 . θ is the angle between the vector connecting I 1 and I 2 , and the magnetic field B.
Simulations of the shape of different powder patterns for different asymmetry and chemical shift anisotropy parameters.
Dipolar powder pattern (Pake pattern)
Simulation of an increasing MAS rate on the 13 C solid-state NMR spectrum of 13 C-Glycine at 9.4 T (400 MHz 1 H frequency). MAS introduces a set of spinning sidebands separated from the isotropic frequency by a multiple of the spinning rate.
Cross-polarization pulse sequence
The CP pulse sequence. The sequence starts with a 90º pulse on the abundant channel (typically H). Then CP contact pulses matching the Hartmann-Hahn condition are applied to transfer the magnetisation from H to X. Finally, the free induction decay (FID) of the X nuclei is detected, typically with 1 H decoupling.
Rotational Echo DOuble Resonance (REDOR) pulse sequence. The first excitation step (90º pulse or CP step) puts the magnetization in the transverse plane. Then two trains of 180º pulses synchronized with the rotor half-period are applied on the Y channel to reintroduce X-Y heteronuclear dipolar interactions. The trains of pulses are interrupted by a 180º pulse on the X channel that allows the refocussing of the X magnetization for the X-detection (spin echo). The delay between the 90º pulse and the beginning of the acquisition is referred to as the "rephrasing time".