Intermolecular forces play an important role in determining the properties of a substance. It is therefore highly desirable to be able to characterize the atomic-level structural detail of surfaces in order to know how they will behave in particular conditions. Characterization of solid materials thus allows researchers to understand how the materials function and enables the development of materials specifically designed to address the needs particular research or application requirements.
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Nuclear magnetic resonance (NMR) spectroscopy is a particularly desirable tool for such analyses since it does not damage the sample and can be readily used to image solids. It provides detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
Although there are many NMR imaging technologies now available to facilitate such surface characterisation, the system to be studied may be too small or have too low a natural abundance of nuclei to obtain high resolution images.
The importance of elucidating the structure of blended materials, host-guest pairs, or cooperative catalysts has motivated the development of new solid-state NMR (SSNMR) technologies that can image complex materials with the desired sensitivity and resolution to provide unique insight.
Solid-state nuclear magnetic resonance
SSNMR relies on the presence of 13C, which can limit the sensitivity and impede thorough characterization if these nuclei are not naturally abundant1. Sometimes this can be overcome by using a larger samples size, or a longer scan duration, but this is not always feasible or desirable. Modification to NMR imaging techniques have become a more popular solution.
Two-dimensional (2D) 1H-1H correlation NMR spectra can be used to increase the sensitivity of imaging of simple molecular systems2. However, the low resolution and narrow chemical shift range necessitate the use of ultrafast magic angle spinning (MAS) and/or 1H homonuclear decoupling, which precludes its use in the study of complex materials.
A wider chemical shift range can be achieved using heteronuclear 1H-X spectroscopy, another standard two-dimensional NMR technique. The low efficiency of long-range polarization transfers and dipolar truncation effects, mean that the resolution is generally not sufficient to unequivocally detect all species. In addition, heteronuclei typically possess a lower gyromagnetic ratio that further challenges the already low sensitivity of SSNMR. Consequently, isotopic enrichment was typically required in homonuclear correlation experiments between heteronuclei usually. Such an approach, however, is expensive, commonly complicated, and leads to unwanted dipolar truncation effects.
Most recently, the sensitivity of SSNMR has been significantly increased by a technique known as dynamic nuclear polarization (DNP).
Dynamic nuclear polarization SSNMR
In DNP, NMR signals are enhanced by using highly polarized electrons to improve the nuclear spin polarization3. Nuclear spins are polarized in the solid state and then rapidly dissolved using a superheated solvent to yield a “hyperpolarized” solution. Spin polarization from the electrons is transferred to the nuclei of the sample to be analysed giving an NMR signal several thousand-fold greater than at thermal equilibrium.
Recent advances in dynamic nuclear polarization (DNP) SSNMR, which include the development of gyrotrons, low-temperature MAS probes, and biradical polarizing agents, have provided unprecedented signal enhancements and created opportunities for homonuclear double-quantumsingle-quantum (DQ/SQ) correlation spectroscopy between unreceptive and rare nuclei, such as 13C and 29Si in natural-abundance solids. The technique has been used in a wide range of applications with great success4,5. The use of DNP enables the acquisition of high quality homonuclear 13C-13C correlation spectra in a practical experimental time.
The sensitivity of DNP has been further increased by enhancing polarization transfer using a range of proton-driven spin diffusion techniques. The spatial range of such techniques is limited by the weak dipolar coupling between heteronuclei, which is further exacerbated by the low natural abundance of nuclei.
An adaptation of the technique that uses two cross-polarization transfers has been used to obtain long-range 13C-13C correlations and provided useful structural information in isotopically-labelled polymers and biological materials6.
Homonuclear CHHC solid-state nuclear magnetic resonance experiment
Most recently the spatial proximity between different moieties on the surface of a catalyst has been determined for the first time without using isotope enrichment7. This was achieved using DNP-enhanced SSNMR. The technique was shown to be effective for the study of long-range correlations in a variety of materials with high resolution.
The DNP-enhanced measurements were performed using a Bruker Biospin AVANCE III 400 DNP NMR spectrometer, equipped with a 264 GHz gyrotron and a low-temperature MAS probe.
DNP enabled intermolecular 13C-13C homonuclear correlations at natural abundance to be observed in complex samples with improved sensitivity. Furthermore, the latest study succeeded for the first time to detect intermolecular correlations on a catalyst surface7.
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- Kobayashi T, et al. Angew. Chem. Int. Ed. 2013;52:14108‑14111.
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- Kobayashi T, et al. Phys. Chem. Chem. Phys. 2017;19:1781‑1789.
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- Kobayashi T, et al. J. Phys. Chem. C 2017;121(44):24687‑24691.
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