Using NMR to Track Ultrafast Dynamics in Solids

In the liquid state, rapid tumbling naturally averages out many complex magnetic interactions, resulting in sharp peaks. In solids, molecules are locked in place. The resulting dipolar broadening is often three orders of magnitude larger than the signals we want to measure.

Traditionally, we used Magic-Angle Spinning—spinning the sample at 54.7° relative to the magnetic field—to mimic this liquid-state averaging. But traditional MAS rates (10–30 kHz) were too slow to handle “fast” nuclei like protons ($^1$H), which possess a high gyromagnetic ratio and strong couplings. At these slower speeds, the dynamics were simply lost in the noise.

The emergence of rotors with diameters as small as 0.7 mm, capable of spinning at rates beyond 111 kHz and up to 170 kHz, has crossed a critical threshold [1]. According to research published by Lyndon Emsley in Nature Communications, these speeds allow for the first-ever observation of $^1$H-$^1$H J-couplings in solid-state NMR.

Key Benefits of Ultra-Fast MAS:

• Refocused Linewidths: At 160 kHz MAS, coherence lifetimes become long enough to achieve refocused linewidths of less than 15 Hz [1].
• Paramagnetic Sensitivity: Small rotors actually provide superior resolution for paramagnetic materials (like those in EV batteries) because they can handle the massive shift anisotropies that would otherwise shatter the spectra [2].
• Proton Detection: We can now directly detect protons in solids with high resolution, reducing the need for massive sample quantities—a common struggle discussed in our guide on multinuclear NMR spectroscopy of inorganic solids.

To move from “seeing” a signal to “tracking” a motion, researchers use specific relaxation and saturation transfer experiments. These are now being adapted from solution-state protocols to the solid-state environment.

1. CPMG Relaxation Dispersion

The Carr–Purcell–Meiboom–Gill (CPMG) experiment uses a train of 180° pulses to “refocus” magnetization. By varying the frequency of these pulses, researchers can calculate the exchange rate ($k_{ex}$) of molecules transitioning between different states [3]. This is essential for understanding how enzymes move during catalysis, as discussed in our look at using NMR to study enzyme function and dynamics.

2. CEST (Chemical Exchange Saturation Transfer)

CEST is used for slower dynamics (ms to seconds). It works by “labeling” a minor state with a weak radiofrequency field and watching that saturation transfer to the major state [4]. Recent advances in DANTE-CEST allow for multi-site excitation, drastically reducing acquisition times from days to hours [3].

The ability to track these ultrafast motions is not just an academic exercise. It is currently being applied to solve engineering bottlenecks in two major sectors:

Battery Materials and Energy Storage

Mixed-metal oxides used in lithium-ion cathodes are notoriously difficult to study because they are paramagnetic. Ultra-fast MAS has allowed for the site-specific assignment of resonances in complex Fe/Mn/Mg olivine-type materials [2]. This helps scientists track how lithium ions move through the lattice, directly informing our solar cell materials research.

Pharmaceuticals and Plastic Crystals

Researchers have used (1S)-(−)-camphor as a model for “plastic crystals”—solids where molecules undergo rapid isotropic reorientation [1]. By using 170 kHz MAS, they successfully recorded 2D $^1$H-$^1$H J-resolved spectra, identifying through-bond correlations that were previously “invisible” in the solid phase. This is critical for predicting the stability and solubility of polymorphic drug compounds.

• 100+ kHz MAS is the Game-Changer: Scaling MAS speeds beyond 100 kHz (specifically toward 170 kHz) effectively “shuts off” the dipolar couplings that previously made solid-state proton tracking impossible.
• J-Couplings are Now Observable: For the first time, through-bond J-couplings can be measured in solids, allowing for precise structural assignments.
• Resolution Trumps Sample Volume: While ultra-fast MAS uses tiny rotors (0.7 mm), the massive gain in resolution and signal-to-noise ratio per unit of volume justifies the small sample size.
• Versatility in Dynamics: CPMG and CEST are the primary tools for the $\mu$s-ms range, while ultra-fast MAS handles the faster “supra-$\tau_c$” dynamics.

Action Plan for Researchers:

1. Select the Right Rotor: Use 0.7 mm rotors for $^1$H-detected experiments or highly paramagnetic materials to achieve the necessary MAS speeds.
2. Optimize Magnetization Alignment: For off-resonance $R_{1\rho}$ experiments, use adiabatic pulses to ensure robust alignment across a wide range of offsets [3].
3. Choose Your Exchange Regime: Use CEST for slow exchange ($k_{ex} < 500$ s$^{-1}$) and CPMG for fast exchange ($k_{ex}$ up to 5000 s$^{-1}$).
4. Calibrate for Heating: Ultra-fast spinning generates significant heat; always use internal temperature standards (like lead nitrate or methanol) to verify the actual sample temperature.