Materials Characterization by Dynamic Nuclear Polarization

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Perspective

Materials Characterization by Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy Aaron J Rossini J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01891 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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The Journal of Physical Chemistry Letters

Materials Characterization by Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy Aaron J. Rossini1,2* 1

Iowa State University, Department of Chemistry, Ames, IA, USA, 50011

2

US DOE Ames Laboratory, Ames, Iowa, USA, 50011

AUTHOR INFORMATION Corresponding Author *e-mail: [email protected], phone: 515-294-8952.

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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 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-enhanced solid-state NMR spectroscopy can be applied to a variety of inorganic and organic 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.

TOC GRAPHIC

R R

R O

N O O

R R

R

O

O N O

R R

DNP


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High-resolution solid-state NMR spectroscopy is a powerful tool for the study of organic and inorganic materials and biomolecules. Solid-state NMR spectroscopy can directly probe the symmetry and structure at nuclear sites, the connectivity/bonding of atoms and precisely measure inter-atomic distances. However, solid-state NMR spectroscopy suffers from intrinsically poor sensitivity due to the small Boltzmann polarization of the nuclear spin states, unfavorable relaxation time constants, low concentrations of nuclear spins and signal broadening from a variety of different mechanisms. Furthermore, it is often challenging or prohibitively expensive to isotopically label many materials. Consequently, solid-state NMR experiments on materials are often restricted to basic 1D measurements or are infeasible. Poor sensitivity is the main barrier that prevents the application of solid-state NMR spectroscopy for characterization of many materials. Primarily due to the efforts of Griffin and co-workers, high-field dynamic nuclear polarization (DNP) has emerged in the past 20 years as a technique to routinely enhance the sensitivity of solid-state NMR experiments by one to three orders of magnitude.1 NMR sensitivity is enhanced in a DNP experiment by transferring the high spin polarization from unpaired electrons to NMR-active nuclei (usually 1H). In modern high-field DNP experiments, polarization transfer from electrons to nuclei is achieved by saturating the electron paramagnetic resonance (EPR) of specially designed exogenous radical polarizing agents (PA, Figure 1) at cryogenic sample temperatures of ca. 100 K. Gyrotron microwave sources provide the high power (> 10 W), continuous-wave (CW) microwaves (ca. 130 to 600 GHz) that are required to efficiently saturate the electron spins and obtain large DNP signal enhancements.1 Cryogenic sample temperatures increase electron and nuclear relaxation times, facilitating more complete saturation of the electron spins and accumulation of more nuclear spin polarization. DNP can



theoretically provide sensitivity enhancements of 658 in NMR experiments where 1H spins are initially excited. 1H DNP enhancements and absolute sensitivity gains on the order of ca. 50 – 200 are now routinely obtained with optimal PA and commercial DNP instrumentation. Prior to 2010, high-field DNP was primarily aimed at enhancing the sensitivity of biomolecular solid-state NMR experiments.1 In a seminal work, Lesage, Emsley, Copéret and co-workers demonstrated that DNP-enhanced solid-state NMR spectroscopy could be used to enhance the NMR signals of molecules immobilized on silica, in an approach dubbed DNP surface-enhanced NMR spectroscopy (DNP SENS, see below).2 The availability of commercial DNP instrumentation,3 optimization of PA and sample preparation protocols, and the work of Lesage2 has stimulated development of DNP-enhanced solid-state NMR for the characterization of a range of inorganic and organic materials. This perspective gives a general overview of how DNP is applied to materials systems and examples where it has been used to obtain detailed structural information about inorganic and organic materials. Challenges and opportunities for further development and application of DNP for materials characterization are highlighted. The interested reader is referred to more comprehensive review articles about DNP-enhanced solidstate NMR spectroscopy1,4,5 and several other perspective articles in this issue that describe the development of DNP instrumentation, methods and their applications in imaging and characterization of biomolecules. A key figure of merit in DNP solid-state NMR experiments is the DNP sensitivity enhancement (ε) which is derived from transferring polarization from electron spins to nuclear spins. ε is usually determined by measuring the gain in signal intensity with and without microwave irradiation. Τhe gain in sensitivity achieved with DNP is approximated by ε, although, several additional factors including depolarization, paramagnetic broadening,


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relaxation time constants, dilution, etc., must be considered to accurately determine the absolute gains in sensitivity provided by DNP as compared to conventional solid-state NMR experiments.6,7 In addition to the properties of the sample, the magnitude of ε is strongly influenced by the specific setup of the DNP system hardware. For example, the magnetic field strength, the sample temperature, rotor diameter, gyrotron power, pulsed vs. CW microwave output, etc., all greatly influence ε. These issues are addressed in detail elsewhere1,5 and briefly discussed in the conclusions section. The parameters related to preparation of the sample that have the largest impact on ε and absolute sensitivity gains are described below.



Figure 1. Exogenous radical polarizing agents (PA) used for high-field DNP-enhanced solidstate NMR experiments. (A) TOTAPOL, one of the first biradical PA. (B) TEKPol and (C) AMUPol routinely yield εH > 200 at 9.4 T with organic solvents and aqueous solutions, respectively. (D) TEMTriPol–1, a hybrid trityl-nitroxide biradical that has yielded εH > 65 at 18.8 T.


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The PA provides the unpaired electrons that are the source of spin polarization in the DNP experiment (Figure 1) and has a large impact on ε and absolute sensitivity. The choice of PA is determined by the magnetic field used for experiments and whether 1H nuclei or lower-γ nuclei will be polarized. In an indirect DNP experiment, the 1H nuclei nearby to the PA are polarized by DNP from the electron spins and 1H spin diffusion then distributes magnetization throughout the sample. Then, solid-state NMR methods such as cross-polarization magic angle spinning (CPMAS) are used to transfer the DNP-enhanced 1H polarization to lower-γ spins (e.g., electron→1H→1H spin diffusion→1H-13C CPMAS). In a direct DNP experiment, the nuclei nearby to the PA are directly polarized by DNP, possibly followed by homonuclear spin diffusion to more remote nuclei, then the NMR signal is measured with a direct excitation experiment (e.g., electron→13C→13C spin diffusion→13C direct excitation). Indirect DNP is more commonly used to enhance solid-state NMR experiments because CPMAS can efficiently transfer 1H polarization to other spin-1/2 nuclei, highly optimized PA for 1H DNP are available and rapid 1H spin diffusion typically causes the 1H magnetization to build up within a few seconds. Griffin and co-workers introduced the concept of tailored exogenous PA when they developed nitroxide biradicals designed to provide substantial cross effect (CE) indirect DNP enhancements at high fields (Figure 1).8 Subsequently, a number of research groups further refined and optimized biradical PA for CE DNP of 1H nuclei and indirect DNP experiments. These modifications focused on imposing a fixed orthogonal orientation of the two nitroxide moieties to better fulfill the CE matching condition.9 Replacing methyl groups adjacent to the nitroxide group with cyclohexyl or phenyl groups increases electronic relaxation times, facilitating more complete saturation of the EPR.10 Today, the nitroxide biradicals AMUPol11



and TEKPol10 are typically the most efficient PA for 9.4 T/263 GHz DNP experiments, routinely providing εH up to 250 for 1H (Figure 1). AMUPol and TEKPol are highly optimized for indirect DNP experiments at 9.4 T, however, the achievable εH rapidly drops off at higher magnetic fields, e.g., on model samples AMUPol yields εH on the order of 150 and 30 are obtained at 14.1 T and 18.8 T, respectively.12,13 Alternative PAs are required to open up DNP at high magnetic fields > 14 T and take advantage of the superior sensitivity and resolution provided by high magnetic fields. Recently, hybrid biradicals featuring a carbon-centered radical with an isotropic EPR spectrum and a tethered nitroxide radical have been shown to provide high εH up to 65 at 18.8 T (Figure 1).12 These radicals perform better at high-field because the narrow isotropic EPR of the carbon-centered radical is more completely saturated and strong magnetic exchange (J) interactions increase coupling between the two electron spins.12,13 The hybrid radicals also have minimal depolarization, meaning that the measured εH are more indicative of the absolute gain in sensitivity.13 Optimized hybrid biradicals will eventually allow DNP enhancements above 100 to be routinely obtained and open up DNP at high fields above 14 T. Direct DNP is not as commonly used as indirect DNP, likely because the build-up times in direct DNP experiments are typically longer than proton build-up times because spin diffusion is not very efficient at transporting magnetization amongst lower-γ spins. However, many materials are free of protons or feature nuclear sites distant from proximate 1H spins and direct DNP may be the only option. Additionally, direct DNP could be preferred for quadrupolar nuclei because it is often challenging to efficiently transfer 1H magnetization to quadrupolar spins. Maly et al. obtained 2H direct CE DNP enhancements of ca. 700 on per-deuterated amino acids dissolved in water/glycerol solutions at 5 T with trityl radical as the PA.14 Michaelis et al.


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obtained

17

O direct CE DNP enhancements of ca. 100 on

17

O labeled water at 5 T with trityl

radical as the PA.15 They also showed that mixtures of BDPA and trityl monoradicals were efficient PA for CE direct DNP of spins with Larmor frequencies below that of

13

C.16 They

obtained 13C direct CE DNP enhancements of 620 at a magnetic field of 5 T, which corresponds to ca. 25% of the theoretical maximum 13C direct DNP enhancement of 2620. The BDPA-trityl mixture works well because the BDPA resonance is nearly isotropic and has long electronic relaxation times that allows it to be fully saturated and the EPR frequency of both radicals are approximately separated by the 13C Larmor frequency, fulfilling the CE match condition.16 More recently Buntkowsky and co-workers demonstrated a trityl-nitroxide biradical PA that provided 13

C direct CE DNP enhancements of ca. 40 at 9.4 T.17 Blanc and co-workers obtained 17O direct

CE DNP enhancements of ca. 40 with the bTbK PA for direct 17O NMR experiments on porous MgO nanocrystals.18 Interestingly, they observed that both core and surface oxygen sites were highly enhanced,18 which suggests that direct DNP can proceed over nm length scales. This observation is consistent with previous direct

29

Si DNP experiments on mesoporous silica that

showed polarization of sub-surface silicon sites.19 These studies have shown that direct DNP is feasible on a range of nuclei and materials. There is likely significant space for the further optimization and refinement of PA for CE direct DNP, given the limited number of prior studies. In addition to the choice of PA, there are several other aspects of sample preparation that strongly influence the achievable ε. The solvent/matrix used to dissolve the PA and dissolve/suspend/impregnate the analyte has a large influence on ε. DNP experiments on biomolecules or water-compatible materials are performed with deuterated glycerol-water or DMSO-water mixtures and water-soluble PA like AMUPol or TOTAPOL.1 The glycerol or DMSO act as cryoprotectants that prevent crystallization of solvent and keep the radical and



analyte homogeneously mixed. However, cryoprotectants are typically not required for DNP experiments on porous materials because crystallization is hindered for solvent trapped in micropores. Deuteration generally increases εH by prolonging the nuclear T1 of the matrix and focusing the electron polarization into fewer 1H spins. A 1H concentration of ca. 10 M (e.g., 60/30/10 glycerol-d8/D2O/H2O) usually provides maximum εH. For water-incompatible materials, halogenated alkanes such as 1,1,2,2-tetrachloroethane (TCE) are typically the best solvents with TEKPol as the PA.10,20 Halogenated solvents have low vapor pressures, high melting points and intrinsically low concentrations of 1H spins, which often eliminates the need to obtain deuterated solvent (e.g., for TCE the natural 1H concentration is ca. 20 M).10,20 Deuterated orthoterphenyl has also been identified as a promising matrix for very high-field DNP and potentially allows DNP experiments to be performed with sample temperatures above 200 K.21 High temperature DNP could obviate the need for cryogen handling and simplify experimental setups, making DNP more accessible. However, DNP enhancements and absolute sensitivity are reduced at higher sample temperatures. An under-appreciated factor that can strongly influence the magnitude of ε is the dielectric properties of the material under study.22 Kubicki et al. showed that by adding materials such as sapphire and KBr that have high real dielectric constants (refractive indices) and low imaginary dielectric constants (loss tangents) the microwave field inside the rotor can be enhanced, leading to εH above 500 at 9.4 T.22 Unfortunately, the addition of dielectric material displaces active sample volume and often does not provide a significant net gain in sensitivity.22 Some materials intrinsically have favorable intrinsic optical characteristics that lead to high ε.22 On the other hand, unfavorable optical microwave properties may also explain why ε are lower than expected or negligible in some materials.


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The availability of commercial DNP instrumentation and optimized PA has made it possible to apply DNP-enhanced solid-state NMR spectroscopy for the characterization of a variety of interesting materials. The reactivity and activity of heterogeneous catalysts are determined by the structure of active surface sites. However, the standard analytical techniques used for characterization of surface sites such as infrared spectroscopy, X-ray adsorption spectroscopy, X-ray photo-electron spectroscopy, etc., provide only partial insight into the molecular structure of surface sites. Solidstate NMR is an ideal technique for the characterization of surface sites because the technique does not require long-range order and NMR observables like quadrupolar, dipolar and scalar couplings and chemical shifts provide direct insight into symmetry, bonding and inter-nuclear distances. Surface-selective 1H→13C and 1H→

29

Si cross-polarization magic angle spinning

(CPMAS) solid-state NMR experiments were first performed on functionalized silica.23 Subsequently, surface-selective 1H→17O and 1H→27Al static CP NMR experiments were also performed on silica and alumina, respectively.24,25 CP experiments on these inorganic oxide materials are surface-selective because 1H nuclei are usually absent from the bulk and only surface nuclei will be proximate to 1H spins. Surface-selective NMR experiments are normally very challenging because the atoms constituting the surface usually make up an extremely small fraction of most materials. The high sensitivity gains provided by DNP enables the dilute surface sites to be observed by solid-state NMR spectroscopy.



Figure 2. (A) Schematic representation of DNP SENS experiments on an oxide material such as SiO2, Al2O3, etc. Impregnation coats the surface of the material with a PA solution, bringing the PA and solvent molecules into close proximity with the surface. Microwave irradiation causes polarization transfer directly to nuclei at the surface (direct DNP, red dashed line) or to 1H spins of the solvent (indirect DNP, blue dashed line). 1H spin diffusion transports the polarization to surface bound ligands/molecules/adsorbates (orange sphere) or to surface hydroxyl groups. Cross-polarization (CP) or other methods (solid red lines) can then be used to transfer the DNP-enhanced 1H polarization to surface nuclei or nuclei within the surface molecules. S = solvent molecule, M = surface molecule, E = element such as Si, Al, Ti, etc. (B) Pulse sequence representation of a DNP-enhanced 1H→13C CPMAS solid-state NMR experiment. (C) 1H→13C CPMAS solid-state NMR spectra of phenol functionalized mesoporous silica obtained with and without microwave irradiation to drive DNP. (B) and (C) reprinted with permission from Lesage et al. Copyright 2010 American Chemical Society (ref. 2).


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In the DNP SENS approach, impregnation wetness is used to coat the surface of materials with PA solution (Figure 2).2,6 Impregnation brings the PA molecules into close proximity with NMR-active nuclei on the material surface, allowing them to be highly polarized by DNP.2,6 DNP SENS was originally demonstrated on functionalized mesoporous silica that are relevant to heterogeneous catalysis.2,6 With DNP it is typically possible to obtain natural isotopic abundance 13

C,

15

N and

29

Si solid-state NMR spectra in a few minutes from organic and organometallic

molecules grafted onto the silica surface.2,6

13

C and

molecular structure of grafted organic fragments.

29

15

N solid-state NMR spectra confirm the

Si solid-state NMR is useful to confirm

molecules are covalently grafted to the support and obtain information about the conformation of molecules on the surface.26,27 Notably, Emsley and co-workers recently showed that by combining DNP-enhanced 2D HETCOR experiments and REDOR-based dipolar coupling measurements it was possible to determine the three-dimensional structure of molecules grafted onto silica surfaces (Figure 3A).27 DNP-enhanced 2D 29Si-29Si correlation experiments were used to observe the bonding/connectivity of silicon atoms at the silica surface (Figure 3B)28 and probe the spatial distribution of organic fragments on functionalized silica surfaces.29



Figure 3. Examples of solid-state NMR experiments on heterogeneous catalyst materials enabled by DNP. (A) Determination of the three-dimensional structure of molecules grafted onto silica with DNP-enhanced REDOR NMR experiments. Reprinted with permission from Berruyer et al. Copyright 2017 American Chemical Society (ref. 26). (B) Natural isotopic abundance 2D 29Si29 Si double quantum-single quantum homonuclear correlation spectrum of functionalized silica nanoparticles showing the connectivity of the different surface silicon atoms. Reprinted with permission from Lee et al. Copyright 2014 American Chemical Society (ref. 27). (C) Natural isotopic abundance 2D 29Si-27Al INEPT-HETCOR spectrum of alumina grafted onto silica (Al/SiO2) showing the silica-alumina interface primarily consists of tetrahedral aluminum sites linked to tetrahedral silicon sites (adapted from ref. 30). (D) Natural isotopic abundance 1H{17O} proton detected local field (PDLF) experiments to measure O-H bond lengths of surface hydroxyl groups on mesoporous silica and silica grafted onto alumina. Reprinted with permission from Perras et al. Copyright 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim (ref. 34).


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DNP SENS has also been applied to enhance the sensitivity of solid-state NMR experiments with half-integer quadrupolar nuclei (I = 3/2, 5/2, 7/2 or 9/2) such as 27Al and 17O. It is challenging to obtain the NMR spectra of quadrupolar nuclei because they give rise to broadened NMR signals and only the spins residing in the central transition m = +1/2 spin state are usually observable. DNP SENS was used to accelerate surface selective 1D and 2D

27

Al

solid-state NMR experiments on alumina30,31 and amorphous alumina/silica materials that have a myriad number of applications in heterogeneous catalysis.32 The surface-selective 1D 1H→27Al CPMAS of alumina/silica confirm the presence of Lewis acidic 5-coordinate aluminum sites on the surface. Scalar

29

Si→27Al INEPT-HETCOR 2D NMR spectra illustrated that the silica-

alumina interface primarily consists of tetrahedral aluminum sites linked to tetrahedral silicon sites.32 Recently, DNP was also used to enable

13

C{27Al} RESPDOR distance measurements

which showed that methionine coordinates to the alumina surface.33 Another difficulty that arises when working with half-integer quadrupolar nuclei is that CPMAS efficiency is highly dependent upon the quadrupolar parameters, making its performance highly variable across different sites and materials. Perras et al. showed that the PRESTO pulse sequence is a more efficient and robust method for transferring DNP-enhanced 1

H polarization to quadrupolar nuclei.34 Using PRESTO they demonstrated acquisition of natural

isotopic abundance surface-selective

17

O solid-state NMR spectra of catalyst materials such as

silica and alumina.35,36 DNP-enhanced measurements of 1H-17O inter-nuclear distances showed that they directly correlate to the Bronsted acidity of the solid acids.36 Buntkowsky and coworkers used direct DNP to enhance the

51

V solid-state NMR spectra of mixed vanadium-

molybdenum-tungsten oxide catalysts and obtained

V

of ca. 50 for surface sites.37 Their

experiments suggested that direct DNP preferentially enhanced the surface vanadium sites.37



Grey and co-workers showed that for direct

17

O NMR experiments on

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17

O-labelled CeO2 the

surface layers showed faster polarization build-up and higher signal enhancement.38 Colloidal semiconductor nanoparticles (NP) have many potential applications as materials for displays, lighting, photovoltaics, catalysis and bio-sensing.39,40 NP have a very large surface area to volume ratio which allows their chemical and photophysical properties to be controlled and manipulated by altering the surface structure.39,40 Therefore, characterization of the surface structure of semiconductor NP is of great fundamental and practical importance. Kovalenko and co-workers applied DNP SENS to obtain 1H→119Sn CPMAS NMR spectra of core/shell Sn/SnOx colloidal NP with diameters of ca. 10 nm.41 The samples were prepared for DNP by simply adding PA into the NP solution, then rapidly freezing the solution inside of the pre-cooled DNP spectrometer. The surface-selective 119Sn solid-state NMR spectra showed that the surface of the nanoparticles consisted exclusively of SnO2.41 Combining the NMR data with Mossbauer spectroscopy and TEM images suggested that the nanoparticles had a Sn/SnO/SnO2 core/shell/shell structure. A more general approach to apply DNP SENS to colloidal semiconductor NP with diameters on the order of 1 to 5 nm was also demonstrated.42 Low ε were obtained when DNP experiments were attempted on the frozen NP solutions because during freezing the colloidal NP precipitate and segregate from the PA (Figure 4A). To address this problem, the NP and PA solution were impregnated into porous silica to trap the NP and PA together in the pores and prevent phase segregation (Figure 4C).42 High indirect DNP enhancements were achieved for archetypal semiconductor NP such as InP, CdSe and CdTe enabling the acquisition of surfaceselective

31

P,

77

Se,

113

Cd and

125

Te solid-state NMR spectra.42,43 DNP-enhanced 1H→113Cd

CPMAS NMR spectra of CdSe NP showed NMR signals from both core/sub-surface and ligand


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bound surface Cd sites. A 2D 13C{111Cd} HMQC experiment confirmed that the surface site Cd NMR signals arise from sites bound to

13

C labeled oleic acid ligands, consistent with the

expected Cd-rich stoichiometry and a cadmium carboxylate surface termination of the CdSe NP (Figure 4). DNP was also used to obtain 2D

113

Cd phase adjusted spinning sideband (PASS)

correlation spectra of CdSe and CdS nanomaterials that could be used to identify core and surface cadmium sites on the basis of their isotropic and anisotropic chemical shifts.43 Emsley and co-workers showed that aqueous acrylamide gels could also be used to suspend NP and prevent phase segregation of the PA.44 The acrylamide gels provided high ε with AMUPol as the PA, allowing 113Cd CPMAS NMR spectra of CdTe NP to be rapidly obtained.44 These examples demonstrate that DNP-enhanced solid-state NMR should be a powerful tool to obtain a deeper fundamental insight into the structure and chemistry of semiconductor NP surfaces. This will enable the detection of different ligand binding surface sites and possibly even surface defects that strongly influence the photophysical properties of the NP.



Figure 4. DNP SENS experiments on colloidal semiconductor NP. (A) The PA is added directly to the NP solution. Upon freezing, the NP precipitate, while the PA remains dissolved in solution. Phase segregation results in a low DNP enhancements. (B) 1H→31P CPMAS spectra of the frozen InP NP solution obtained with and without DNP-enhancement illustrate that a low ε is obtained. (C) The NP and PA solution is impregnated into mesoporous silica, trapping the PA and NP together in the pores. (D) A substantial ε of 56 is obtained for InP NP trapped in the silica pores with the PA. The DNP-enhanced 31P CPMAS spectrum shows signal from oxidized surface sites (POx) and core/sub-surface sites (InPcore). (E) DNP-enhanced 1H→113Cd CP/CPMG and 2D 13C{111Cd} D-HMQC spectrum of CdSe NP in mesoporous silica. Only the surface Cd sites are visible in the HMQC spectrum. Adapted from ref. 39.


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In order for DNP to succeed it is normally necessary to bring the PA within a few nanometers of the target nuclei. However, many organic materials, such as pharmaceuticals, polymers, and bulk inorganic solids exist as micro-particulate solids with negligible porosity and low surface area and the PA will necessarily be restricted to the surface of the particles. Likewise, many biradical PA are too large to diffuse into microporous materials such as zeolites and metal-organic frameworks (MOF). It would seem challenging or impossible to apply DNP to non-porous organic solids or microporous materials. This problem can be solved by recognizing that homonuclear 1H spin diffusion spontaneously transports polarization over length scales of ca. 100 nm in several seconds.45 Griffin and co-workers obtained high DNP enhancements for needle-like polypeptide nanocrystals with widths of ca. 100 – 200 nm that were suspended in TOTAPOL glycerol-water solutions.45 In these experiments, the PA are located at the surface of the crystals and DNP-enhanced 1H polarization is relayed from the surface into the interior of the crystals by 1H spin diffusion (Figure 5). Subsequently, relayed DNP was shown to be a general method to polarize microcrystalline organic solids, including pharmaceuticals.46-48 Alternatively, DNP NMR was performed on amorphous solid dispersions (ASD) of pharmaceuticals and polymers by directly incorporating the PA during synthesis.49 Relayed DNP has enabled a number of challenging solid-state NMR experiments on organic solids including natural isotopic abundance 2D double-quantum 1

13

C-13C correlation (Figure 5C),46 2D

13

C-15N HETCOR50 and

H-15N distance measurements.51 These advanced solid-state NMR experiments provide a wealth

of structural information that can potentially be incorporated into NMR crystallography structure determination protocols. Relayed DNP has also been applied to inorganic materials such as MOFs,52,53 surfactantfilled silica,54 organometallic molecules grafted onto MCM-41,55 metal-substituted zeolites,56-58



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cementitious materials,59 metal hydroxides18 and hydroxyapatite.60 Very recently, Björgvinsdóttir et al. demonstrated that DNP relayed by homonuclear spin diffusion between moderately abundant, slowly relaxing, low-γ nuclei such as

29

Si,

119

Sn and

113

Cd could be used to polarize

nuclei within the core of bulk inorganic materials.61 In the experiments on tin-substituted zeolite beta (Sn-β)

the 1H nuclei of solvent

molecules or water in the pores provide a network of coupled 1H spins that can transport DNPenhanced 1H magnetization into the zeolite (Figure 5B).56,57 DNP enabled acquisition of natural isotopic abundance 119Sn solid-state NMR spectra of Sn-β catalysts with Sn loading down to 0.5 wt.%.56,57 DNP-enhanced

29

Si and

119

Sn solid-state NMR spectra of Sn-β catalysts prepared by

different synthetic procedures illustrate that the sample preparation greatly affects the distribution of tin-sites within the zeolite framework (Figure 5D).58 These measurements enable the catalytic activity of different Sn-β catalysts to be correlated to differences in molecular structure.58


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Figure 5. A schematic illustration of relayed DNP experiments on (A) insoluble microparticulate organic solids such as pharmaceuticals and (B) microporous solids such as zeolites or MCM-41. (C) DNPenhanced 2D 13C-13C DQ-SQ INADEQUATE correlation spectrum of microcrystalline glucose obtained with natural isotopic 13C abundance and a total experiment time of 16 hours (adapted from ref. 42). (D) DNP-enhanced isotropic 1H→119Sn CPMAS solid-state NMR spectra of tin substituted zeolite-β (Sn-β) with 1.0 wt% Sn loading. Vertical dashed lines highlight differences in isotropic chemical shifts. The Sn-



β were prepared by different synthetic procedures that change the metal site distribution in the zeolite (adapted from ref. 53).


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In summary, the examples discussed here highlight how DNP addresses the fundamental problem of poor NMR sensitivity, enabling the application of solid-state NMR spectroscopy to materials systems that were previously considered challenging to study or completely inaccessible. The advanced solid-state NMR experiments enabled by DNP provide unique insights into the molecular structure, which makes it possible to form structure-activity relationships that will ultimately assist in the rational design and improvement of materials. Spurred by the increasing availability of commercial instrumentation, optimized sample preparation methods and improved PA, DNP-enhanced solid-state NMR spectroscopy is becoming an indispensable technique for the structural characterization of materials. Looking ahead, the development of improved hardware, DNP methodology and better PA has the potential to further increase absolute sensitivity by orders of magnitude. First, improved PA make DNP at very high magnetic fields feasible.12,13 The improved sensitivity and resolution provided by higher magnetic fields is crucial for DNP NMR experiments with halfinteger quadrupolar nuclei such as

17

O.62 Second, several groups are developing helium cooled

DNP systems.63-66 Reducing the sample temperature to less than 30 K can potentially provide further absolute sensitivity gains of one to two orders of magnitude by increasing both DNP enhancements and thermal nuclear polarization.63-66 Third, the development of coherent, pulsed microwave sources for time-domain high-field DNP could revolutionize the field in much the same way that pulsed methods revolutionized NMR spectroscopy. Pulsed DNP will improve polarization transfer efficiency and make it possible to use simple monoradical PA.67,68 It may also be possible to eliminate deleterious paramagnetic broadening effects with electron decoupling.69



Biography Aaron J. Rossini completed his PhD (2005 – 2010) at the University of Windsor under the supervision of Prof. Robert W. Schurko. His PhD work centered on the development and application of exotic NMR active nuclei for solid-state NMR studies of inorganic solids. He was a Marie-Curie post-doctoral fellow (2011-2014) with Prof. Lyndon Emsley and Dr. Anne Lesage at the Centre de Resonance Magnetique Nucleaire à Très Haut Champs (CRMN Lyon). His post-doctoral research primarily focused on developing DNP-enhanced solid-state NMR spectroscopy for the improved characterization of heterogeneous catalysts, materials and pharmaceuticals. In 2014 he moved to École Polytechnique Fédérale de Lausanne (EPFL) in Lausanne, Switzerland to continue working with Prof. Emsley. In August 2015, he joined the Department of Chemistry at Iowa State University as an Assistant Professor. His current research centers on the development and application of fast MAS and DNP solid-state NMR spectroscopy for characterization of inorganic materials, catalysts and pharmaceuticals.

Acknowledgments This material was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The Ames Laboratory is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358. AJR thanks his past and current co-workers, collaborators and advisors whom he has worked with on DNP including Prof. Lyndon Emsley, Dr. Anne Lesage, Prof. Christophe Copéret, Prof. Paul Tordo, Prof. Olivier Ouari, Prof. Moreno Lelli, Prof. Maksym Kovalenko, Prof. Ive Hermans, Prof. Matthew Conley, Prof. Robert Schurko, Dr. Marek Pruski, Dr. Fréderic Perras, Dr. Takeshi Kobayashi and all members of his research group. References (1) Maly, T.; Debelouchina, G. T.; Bajaj, V. S.; Hu, K. N.; Joo, C. G.; Mak-Jurkauskas, M. L.; Sirigiri, J. R.; van der Wel, P. C. A.; Herzfeld, J.; Temkin, R. J.; et al. Dynamic Nuclear Polarization at High Magnetic Fields. J. Chem. Phys. 2008, 128, 052211. (2) Lesage, A.; Lelli, M.; Gajan, D.; Caporini, M. A.; Vitzthum, V.; Mieville, P.; Alauzun, J.; Roussey, A.; Thieuleux, C.; Mehdi, A.; et al. Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization. J. Am. Chem. Soc. 2010, 132, 15459-15461. (3) Rosay, M.; Tometich, L.; Pawsey, S.; Bader, R.; Schauwecker, R.; Blank, M.; Borchard, P. M.; Cauffman, S. R.; Felch, K. L.; Weber, R. T.; et al. Solid-State Dynamic Nuclear Polarization at 263 GHz: Spectrometer Design and Experimental Results. Phys. Chem. Chem. Phys. 2010, 12, 5850-5860. (4) Su, Y.; Andreas, L.; Griffin, R. G. Magic Angle Spinning NMR of Proteins: High-Frequency Dynamic Nuclear Polarization and 1H Detection. Annu. Rev. Biochem 2015, 84, 465-497. (5) Lilly Thankamony, A. S.; Wittmann, J. J.; Kaushik, M.; Corzilius, B. Dynamic Nuclear Polarization for Sensitivity Enhancement in Modern Solid-State NMR. Prog. Nucl. Magn. Reson. Spectrosc. 2017, 102-103, 120-195. (6) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Lesage, A.; Copéret, C.; Emsley, L. Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy. Acc. Chem. Res. 2013, 46, 1942-1951.


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(26) Zagdoun, A.; Casano, G.; Ouari, O.; Lapadula, G.; Rossini, A. J.; Lelli, M.; Baffert, M.; Gajan, D.; Veyre, L.; Maas, W. E.; et al. A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials. J. Am. Chem. Soc. 2012, 134, 2284-2291. (27) Berruyer, P.; Lelli, M.; Conley, M. P.; Silverio, D. L.; Widdifield, C. M.; Siddiqi, G.; Gajan, D.; Lesage, A.; Coperet, C.; Emsley, L. Three-Dimensional Structure Determination of Surface Sites. J. Am. Chem. Soc. 2017, 139, 849-855. (28) Lee, D.; Monin, G.; Duong, N. T.; Lopez, I. Z.; Bardet, M.; Mareau, V.; Gonon, L.; De Paepe, G. Untangling the Condensation Network of Organosiloxanes on Nanoparticles Using 2D Si-29-Si-29 SolidState NMR Enhanced by Dynamic Nuclear Polarization. J. Am. Chem. Soc. 2014, 136, 13781-13788. (29) Kobayashi, T.; Singappuli-Arachchige, D.; Wang, Z.; Slowing, I. I.; Pruski, M. Spatial Distribution of Organic Functional Groups Supported on Mesoporous Silica Nanoparticles: A Study by Conventional and DNP-Enhanced 29Si Solid-State NMR. Phys. Chem. Chem. Phys. 2017, 19, 1781-1789. (30) Vitzthum, V.; Mieville, P.; Carnevale, D.; Caporini, M. A.; Gajan, D.; Copéret, C.; Lelli, M.; Zagdoun, A.; Rossini, A. J.; Lesage, A.; et al. Dynamic Nuclear Polarization of Quadrupolar Nuclei Using Cross Polarization from Protons: Surface-Enhanced Aluminium-27 NMR. Chem. Commun. 2012, 48, 1988-1990. (31) Lee, D.; Takahashi, H.; Thankamony, A. S. L.; Dacquin, J. P.; Bardet, M.; Lafon, O.; De Paëpe, G. Enhanced Solid-State NMR Correlation Spectroscopy of Quadrupolar Nuclei Using Dynamic Nuclear Polarization. J. Am. Chem. Soc. 2012, 134, 18491–18494. (32) Valla, M.; Rossini, A. J.; Caillot, M.; Chizallet, C.; Raybaud, P.; Digne, M.; Chaumonnot, A.; Lesage, A.; Emsley, L.; van Bokhoven, J. A.; et al. Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations. J. Am. Chem. Soc. 2015, 137, 10710-10719. (33) Perras, F. A.; Johnson, R. L.; Wang, L. L.; Schwartz, T. J.; Kobayashi, T.; Dumesic, J. A.; Shanks, B. H.; Johnson, D. D.; Pruski, M. Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by DNP-Enhanced Double-Resonance NMR Spectroscopy. J. Am. Chem. Soc. 2017, 139, 2702–2709. (34) Perras, F. A.; Kobayashi, T.; Pruski, M. Presto Polarization Transfer to Quadrupolar Nuclei: Implications for Dynamic Nuclear Polarization. Phys. Chem. Chem. Phys. 2015, 17, 22616-22622. (35) Perras, F. A.; Kobayashi, T.; Pruski, M. Natural Abundance 17O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy. J. Am. Chem. Soc. 2015, 137, 8336–8339. (36) Perras Frédéric, A.; Wang, Z.; Naik, P.; Slowing Igor, I.; Pruski, M. Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts. Angew. Chem. Int. Ed. 2017, 56, 9165-9169. (37) Thankamony, A. S. L.; Knoche, S.; Bothe, S.; Drochner, A.; Jagtap, A. P.; Sigurdsson, S. T.; Vogel, H.; Etzold, B. J. M.; Gutmann, T.; Buntkowsky, G. Characterization of V–Mo–W Mixed Oxide Catalyst Surface Species by 51v Solid-State Dynamic Nuclear Polarization NMR. The Journal of Physical Chemistry C 2017, 121, 20857-20864. (38) Hope, M. A.; Halat, D. M.; Magusin, P. C. M. M.; Paul, S.; Peng, L.; Grey, C. P. Surface-Selective Direct 17o DNP NMR of Ceo2 Nanoparticles. Chem. Commun. 2017, 53, 2142-2145. (39) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; et al. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012-1057. (40) Owen, J. The Coordination Chemistry of Nanocrystal Surfaces. Science 2015, 347, 615-616. (41) Protesescu, L.; Rossini, A. J.; Kriegner, D.; Valla, M.; de Kergommeaux, A.; Walter, M.; Kravchyk, K. V.; Nachtegaal, M.; Stangl, J.; Malaman, B.; et al. Unraveling the Core-Shell Structure of Ligand-Capped Sn/SnOx Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mossbauer, and X-Ray Absorption Spectroscopies. ACS Nano 2014, 8, 2639-2648.


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(42) Piveteau, L.; Ong, T. C.; Rossini, A. J.; Emsley, L.; Coperet, C.; Kovalenko, M. V. Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy. J. Am. Chem. Soc. 2015, 137, 13964-13971. (43) Piveteau, L.; Ong, T.-C.; Walder, B. J.; Dirin, D. N.; Moscheni, D.; Schneider, B.; Bär, J.; Protesescu, L.; Masciocchi, N.; Guagliardi, A.; et al. Resolving the Core and the Surface of Cdse Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced Pass–Pieta NMR Spectroscopy. ACS Central Sci. 2018, DOI: 10.1021/acscentsci.8b00196. (44) Viger‐Gravel, J.; Berruyer, P.; Gajan, D.; Basset, J. M.; Lesage, A.; Tordo, P.; Ouari, O.; Emsley, L. Frozen Acrylamide Gels as Dynamic Nuclear Polarization Matrices. Angew. Chem. Int. Ed. 2017, 56, 8726-8730. (45) van der Wel, P. C. A.; Hu, K. N.; Lewandowski, J.; Griffin, R. G. Dynamic Nuclear Polarization of Amyloidogenic Peptide Nanocrystals: GNNQQNY, a Core Segment of the Yeast Prion Protein Sup35p. J. Am. Chem. Soc. 2006, 128, 10840-10846. (46) Rossini, A. J.; Zagdoun, A.; Hegner, F. S.; Schwarzwälder, M.; Gajan, D.; Copéret, C.; Lesage, A.; Emsley, L. Dynamic Nuclear Polarization NMR Spectroscopy of Microcrystalline Solids. J. Am. Chem. Soc. 2012, 134, 16899−16908. (47) Rossini, A. J.; Widdifield, C. M.; Zagdoun, A.; Lelli, M.; Schwarzwälder, M.; Copéret, C.; Lesage, A.; Emsley, L. Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations. J. Am. Chem. Soc. 2014, 136, 2324-2334. (48) Zhao, L.; Pinon, A. C.; Emsley, L.; Rossini, A. J. DNP-Enhanced Solid-State NMR Spectroscopy of Active Pharmaceutical Ingredients. Magn. Reson. Chem. 2018, 56, 583-609. (49) Ni, Q. Z.; Yang, F.; Can, T. V.; Sergeyev, I. V.; D’Addio, S. M.; Jawla, S. K.; Li, Y.; Lipert, M. P.; Xu, W.; Williamson, R. T.; et al. In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced Mas Nmr. J. Phys. Chem. B 2017, 121, 8132-8141. (50) Märker, K.; Pingret, M.; Mouesca, J.-M.; Gasparutto, D.; Hediger, S.; De Paëpe, G. A New Tool for NMR Crystallography: Complete 13C/15N Assignment of Organic Molecules at Natural Isotopic Abundance Using DNP-Enhanced Solid-State NMR. J. Am. Chem. Soc. 2015, 137, 13796-13799. (51) Zhao, L.; Hanrahan, M. P.; Chakravarty, P.; DiPasquale, A. G.; Sirois, L. E.; Nagapudi, K.; Lubach, J. W.; Rossini, A. J. Characterization of Pharmaceutical Cocrystals and Salts by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy. Crystal Growth Des. 2018, 18, 2588-2601. (52) Rossini, A. J.; Zagdoun, A.; Lelli, M.; Canivet, J.; Aguado, S.; Ouari, O.; Tordo, P.; Rosay, M.; Maas, W. E.; Copéret, C.; et al. Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy of Functionalized Metal-Organic Frameworks. Angew. Chem. Int. Ed. 2012, 51, 123-127. (53) Kobayashi, T.; Perras, F. A.; Goh, T. W.; Huang, W.; Pruski, M. DNP-Enhanced Ultra-Wideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks. J. Phys. Chem. Lett. 2016, 7, 2322–2327. (54) Lafon, O.; Thankamony, A. S. L.; Kobayashi, T.; Carnevale, D.; Vitzthum, V.; Slowing, I. I.; Kandel, K.; Vezin, H.; Amoureux, J. P.; Bodenhausen, G.; et al. Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning C-13 and Si-29 NMR Enhanced by Dynamic Nuclear Polarization. J. Phys. Chem. C 2013, 117, 1375-1382. (55) Pump, E.; Viger-Gravel, J.; Abou-Hamad, E.; Samantaray, M. K.; Hamzaoui, B.; Gurinov, A.; Anjum, D. H.; Gajan, D.; Lesage, A.; Bendjeriou-Sedjerari, A.; et al. Reactive Surface Organometallic Complexes Observed Using Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy. Chem. Sci. 2017, 8, 284-290. (56) Gunther, W. R.; Michaelis, V. K.; Caporini, M. A.; Griffin, R. G.; Roman-Leshkov, Y. Dynamic Nuclear Polarization NMR Enables the Analysis of Sn-Beta Zeolite Prepared with Natural Abundance Sn-119 Precursors. J. Am. Chem. Soc. 2014, 136, 6219-6222. (57) Wolf, P.; Valla, M.; Rossini, A. J.; Comas-Vives, A.; Núñez-Zarur, F.; Malaman, B.; Lesage, A.; Emsley, L.; Copéret, C.; Hermans, I. NMR Signatures of the Active Sites in Sn-Β Zeolite. Angew. Chem. Int. Ed. 2014, 53, 10179-10183.



(58) Wolf, P.; Valla, M.; Núñez-Zarur, F.; Comas-Vives, A.; Rossini, A. J.; Firth, C.; Kallas, H.; Lesage, A.; Emsley, L.; Copéret, C.; et al. Correlating Synthetic Methods, Morphology, Atomic-Level Structure, and Catalytic Activity of Sn-Β Catalysts. ACS Catal. 2016, 6, 4047-4063. (59) Kumar, A.; Walder, B. J.; Kunhi Mohamed, A.; Hofstetter, A.; Srinivasan, B.; Rossini, A. J.; Scrivener, K.; Emsley, L.; Bowen, P. The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate. J. Phys. Chem. C 2017, 121, 17188-17196. (60) Leroy, C.; Aussenac, F.; Bonhomme-Coury, L.; Osaka, A.; Hayakawa, S.; Babonneau, F.; CoelhoDiogo, C.; Bonhomme, C. Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization. Anal. Chem. 2017, 89, 10201-10207. (61) Björgvinsdóttir, S.; Walder, B. J.; Pinon, A. C.; Emsley, L. Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface. J. Am. Chem. Soc. 2018, 140, 7946-7951. (62) Brownbill, N. J.; Gajan, D.; Lesage, A.; Emsley, L.; Blanc, F. Oxygen-17 Dynamic Nuclear Polarisation Enhanced Solid-State NMR Spectroscopy at 18.8 T. Chem. Commun. 2017, 53, 2563-2566. (63) Thurber, K. R.; Wai-Ming, Y.; Tycko, R. Low-Temperature Dynamic Nuclear Polarization at 9.4 T with a 30 mW Microwave Source. J. Magn. Reson. 2010, 204, 303-313. (64) Matsuki, Y.; Ueda, K.; Idehara, T.; Ikeda, R.; Ogawa, I.; Nakamura, S.; Toda, M.; Anai, T.; Fujiwara, T. Helium-Cooling and -Spinning Dynamic Nuclear Polarization for Sensitivity-Enhanced Solid-State NMR at 14 T and 30 K. J. Magn. Reson. 2012, 225, 1-9. (65) Lee, D.; Bouleau, E.; Saint-Bonnet, P.; Hediger, S.; De Paëpe, G. Ultra-Low Temperature MasDNP. J. Magn. Reson. 2016, 264, 116-124. (66) Sesti, E. L.; Alaniva, N.; Rand, P. W.; Choi, E. J.; Albert, B. J.; Saliba, E. P.; Scott, F. J.; Barnes, A. B. Magic Angle Spinning NMR Below 6 K with a Computational Fluid Dynamics Analysis of Fluid Flow and Temperature Gradients. J. Magn. Reson. 2018, 286, 1-9. (67) Mathies, G.; Jain, S.; Reese, M.; Griffin, R. G. Pulsed Dynamic Nuclear Polarization with Trityl Radicals. J. Phys. Chem. Lett. 2016, 7, 111-116. (68) Can, T. V.; Weber, R. T.; Walish, J. J.; Swager, T. M.; Griffin, R. G. Frequency-Swept Integrated Solid Effect. Angew. Chem. Int. Ed. 2017, 56, 6744-6748. (69) Saliba, E. P.; Sesti, E. L.; Scott, F. J.; Albert, B. J.; Choi, E. J.; Alaniva, N.; Gao, C.; Barnes, A. B. Electron Decoupling with Dynamic Nuclear Polarization in Rotating Solids. J. Am. Chem. Soc. 2017, 139, 6310-6313.


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