Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy

Abstract

High-resolution solid-state NMR spectroscopy is a powerful tool for the study of organic and inorganic materials because it can directly probe the symmetry and structure at nuclear sites, the connectivity/bonding of atoms and precisely measure inter-atomic distances. However, NMR spectroscopy is hampered by intrinsically poor sensitivity, consequently, the application of NMR spectroscopy to many solid materials is often infeasible. High-field dynamic nuclear polarization (DNP) has emerged as a technique to routinely enhance the sensitivity of solid-state NMR experiments by one to three orders of magnitude. This perspective gives a general overview of how DNP enables advanced solid-state NMR experiments on a variety of materials. DNP-enhanced solid-state NMR experiments provide unique insights into the molecular structure, which makes it possible to form structure-activity relationships that ultimately assist in the rational design and improvement of materials. Disciplines Materials Chemistry | Physical Chemistry CommentsThis document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in The Journal of Physical Chemistry Letters, copyright © American Chemical Society after peer review. To access the final edited and published work see doi: 10.1021/acs.jpclett.8b01891. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Abstract High-resolution solid-state NMR spectroscopy is a powerful tool for the study of organic and

Recent developments in MAS DNP-NMR of materials

“…Hence, we mainly restrict ourselves to articles reporting on materials with this technique, even if some seminal results about MAS DNP-NMR of small molecules or biomolecules in frozen solutions are also mentioned. The interested reader is referred to other recent review articles on that topic [4,23,24,[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]. We will first describe the transfers of polarization in MAS DNP and the various factors contributing to the global sensitivity of MAS DNP-NMR experiments.…”

“…For these techniques, the choice of the solvent can strongly affect the DNP enhancement. The employed solvent should fulfill the following criteria: (i) it should dissolve the PA; (ii) it should wet the investigated material, (iii) it should be inert towards both the radical and the material and must not dissolve it; (iv) the number of magnetically inequivalent nuclei in the solvent must be low in order to avoid masking the spectrum of the material; (v) the solvent must form a glass at the temperature of DNP experiments; (vi) the solvent molecules should not exhibit sizable motions, such as the rotation of methyl groups, at the desired temperatures; in addition, for the DNP involving protons, (vi) the concentration of protons in the solvent must be less than 20 mol.L −1 [64,129] and (vii) the longitudinal relaxation of the solvent protons should be slow. The crystallization of the solvent must be avoided since it leads to aggregation of the PAs along the grain boundary, which decreases the DNP enhancement [47].…”

“…The optimal concentration of protons depends on the PA, the involved DNP mechanism [161], the MAS frequency [98] and the concentration of protons in the investigated sample [160]. Typically the concentration of protons in the solvent must be on the order of 10 mol.L −1 [64]. For water-soluble PAs, such as AMUPol, [ 2 H8]-glycerol/ 2 H2O/H2O (60/30/10 v/v/v) or [ 2 H6]-DMSO/ 2 H2O/H2O (80/10/10 v/v/v) solvents are typically used.…”

A Strategy for Probing the Evolution of Crystallization Processes by Low-Temperature Solid-State NMR and Dynamic Nuclear Polarization

“…If new transient solid forms are observed within the frozen solution, such NMR techniques have the potential to reveal detailed insights into their structural properties. Furthermore, the low temperatures involved allow dynamic nuclear polarization (DNP) [19][20][21][22][23][24][25][26][27][28][29][30][31][32] to be exploited to enhance the signal intensity in solid-state NMR studies of the frozen crystallization system. The method is demonstrated here in studies of crystallization of glycine [H2C(NH3 + )(CO2…”

A Formulation Protocol with Pyridine to Enable DNP-SENS on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts

“…Of various approaches, dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) is particularly noteworthy because it can increase NMR sensitivity by up to two orders of magnitude (i.e. 660 for protons), decreasing NMR times from years to minutes, [4][5][6][7] while being surface sensitive. It is thus not surprising that this approach has gained importance to characterize the surfaces of materials and has been used to detect reaction intermediates in heterogeneous catalysts.…”