**1.1. How HR-MAS works**

Materials that are crystalline or rigid solids (for example resins, ceramics, *etc.*) exhibit an extremely broad NMR signal due to extensive homo- and hetero-nuclear dipolar coupling, chemical shift anisotropy (CSA), and quadrupolar interactions. A variety of solid state NMR techniques have been developed to improve the resolution and sensitivity including the use of multiple pulse sequences, cross polarization, high power decoupling, multiple quantum NMR and fast MAS NMR. For example, solid state 1H MAS NMR has found a number of applications in material characterization.[17] Even with these advances, solution-like NMR spectra are rarely realized for solid samples.

For samples that are in the liquid/solid classification, motional averaging will partially reduce or remove many of these broadening interactions. It is also possible to swell or plasticize a material to increase the local mobility, thereby reducing the magnitude of these interactions.[18] Even for liquid environments in a heterogeneous sample, differences in the magnetic susceptibility within the material can drastically reduce the observed resolution. For these types of dynamically averaged or susceptibility broadened systems, MAS even at moderate speeds will produce high resolution NMR spectra: this is the niche of HR-MAS NMR.

This improved resolution due to MAS arises because the Hamiltonians describing the dipolar, CSA and magnetic susceptibility interactions all contain an orientational component that scales as (3cos2θ-1), where θ is the angle between the rotor spinning axis and the magnetic field (Figure 1). When a sample is spun about an axis that is at the "magic angle" (θ = 54.7o) these interactions vanish. A simplistic schematic of a HR-MAS stator is pictured in Figure 1. As mentioned before, the main difference between a standard MAS stator and an HR-MAS stator is the magic angle gradient coil that can produce a gradient along the rotor spinning axis.

modified nanoparticles.

NMR.

rotor spinning axis.

**1.1. How HR-MAS works** 

spectra are rarely realized for solid samples.

roasting process.[12, 13] These types of biological and foodstuff HR-MAS NMR investigations

The application of HR-MAS NMR to material science was initially focused almost exclusively on the analysis of solid-phase (*i.e.* utilizing support resins) organic and peptide synthesis, or analysis of combinatorial solid-phase results. In these studies, the material was swollen in appropriate solvents such that the mobility of the attached ligands was increased, allowing high resolution NMR spectra to be obtained. The application of HR-MAS to solid state synthetic chemistry remains an active area of research, but will not be discussed in detail. The readers are encouraged to consult several very extensive reviews in this area.[14- 16] In comparison to the numerous HR-MAS NMR studies on biological and solid-phase synthetic chemistry systems, there are fewer examples that focus on the use of this technique to material science. This chapter will review the application of HR-MAS NMR to a wide range of systems, including ceramics, zeolites, liquid crystals, ionic liquids, and surface

Materials that are crystalline or rigid solids (for example resins, ceramics, *etc.*) exhibit an extremely broad NMR signal due to extensive homo- and hetero-nuclear dipolar coupling, chemical shift anisotropy (CSA), and quadrupolar interactions. A variety of solid state NMR techniques have been developed to improve the resolution and sensitivity including the use of multiple pulse sequences, cross polarization, high power decoupling, multiple quantum NMR and fast MAS NMR. For example, solid state 1H MAS NMR has found a number of applications in material characterization.[17] Even with these advances, solution-like NMR

For samples that are in the liquid/solid classification, motional averaging will partially reduce or remove many of these broadening interactions. It is also possible to swell or plasticize a material to increase the local mobility, thereby reducing the magnitude of these interactions.[18] Even for liquid environments in a heterogeneous sample, differences in the magnetic susceptibility within the material can drastically reduce the observed resolution. For these types of dynamically averaged or susceptibility broadened systems, MAS even at moderate speeds will produce high resolution NMR spectra: this is the niche of HR-MAS

This improved resolution due to MAS arises because the Hamiltonians describing the dipolar, CSA and magnetic susceptibility interactions all contain an orientational component that scales as (3cos2θ-1), where θ is the angle between the rotor spinning axis and the magnetic field (Figure 1). When a sample is spun about an axis that is at the "magic angle" (θ = 54.7o) these interactions vanish. A simplistic schematic of a HR-MAS stator is pictured in Figure 1. As mentioned before, the main difference between a standard MAS stator and an HR-MAS stator is the magic angle gradient coil that can produce a gradient along the

highlight the diverse range of information that can be obtained.

**Figure 1.** Schematic of a HR-MAS stator with a magic angle gradient along the rotor spinning axis.

For samples where residual dipolar interactions or differences in magnetic susceptibility are small, HR-MAS NMR reduces the observed line widths to be similar to those observed in solution high resolution NMR spectroscopy. For liquids adsorbed into/onto materials MAS speeds between 1 and 2 kHz may result in improved resolution, while for plasticized/swollen materials MAS speeds between 4 and 10 kHz may be necessary to obtain the desired resolution. Care must also be taken to ensure that the residual spinning sidebands (due to incomplete averaging) fall outside the spectral window of interest.

As an example of the enhanced resolution that can be achieved under HR-MAS two different swollen polymer systems are shown in Figure 2. These anion exchange membranes (AEM) operate in methanol solution, and readily adsorb significant amounts of water and methanol from the solution during use. Our group is interested in identifying the different chemical environments that these solvent molecules see within the membrane, as well as directly measuring the diffusion rates for these different species. Under static conditions, the water and methanol are not resolved in the NMR spectrum (Figure 2A, green). This lack of chemical shift resolution precludes any information about the local membrane environment being obtained. The broadening is caused by the magnetic susceptibility differences between the rigid AEM and the methanol solution. In this example, minimal MAS of ~750 Hz (Figure 2A, red) was sufficient to average the magnetic susceptibility allowing individual solvent environments to be easily observed. Increasing the spinning speed to 4 kHz removed the spinning side bands from the spectral region of interest, but did not dramatically increase the resolution of the solvent resonances. In this swollen AEM example, signals from the actual membrane are not observed at 4 kHz (only the swelling solvent is observed) indicating that the dipolar interactions present in the membrane remain much larger than the MAS speed. Since the protons from the membrane are not readily observable, it provides a unique opportunity to further study the solvent behavior in the membrane without interference. Additional discussion of this system is presented in Section 2.1.1. The second example, involves the swelling of a polyButadiene-AcryloNitrile (pBAN) polymer in CDCl3. Inspection of Figure 2B (green) shows that under static conditions this rather soft material produces a relatively unresolved NMR spectrum. Even after extensive solvent swelling, the NMR spectrum is essentially unchanged (Figure 2B, red). With HR-MAS (4 kHz spinning) the resolution is dramatically enhanced, enabling a more detailed analysis of the pBAN 1H NMRspectra.

**Figure 2.** The improved resolution observed using 1H HR-MAS NMR for the A) methanol swollen anion exchange membrane, and B) the CDCl3 swollen pBAN (polyButadiene-AcryloNitrile) polymer.

With the dramatic increase in resolution observed utilizing HR-MAS, the arsenal of standard solution NMR techniques can be implemented for HR-MAS experiments, including solvent suppression, gradient-assisted sequences, and multiple-dimensional experiments. These can include INEPT (**I**nsensitive **N**uclei **E**nhanced by **P**olarization **T**ransfer), COSY (**CO**rrelation **S**pectroscop**Y**), NOESY (**N**uclear **O**verhauser **E**ffect **S**pectroscop**Y**), TOCSY (**TO**tal **C**orrelation **S**pectroscop**Y**), HETCOR (**HET**eronuclear **COR**relation), HMQC (**H**eteronuclear **M**ultiple **Q**uantum **C**oherence), and HMBC (**H**eteronuclear **M**ultiple **B**ond **C**orrelation) to name a few. As an example, a gradientassisted 2D 1H-1H HR-MAS COSY NMR spectra for an ionic liquid adsorbed on an aluminum oxide membrane is shown in Figure 3. These types of correlation experiments could not be realized without the resolution afforded by HR-MAS. Additional discussion of this IL material is presented in Section 2.3.1.

It is also important to note that because anisotropic interactions (dipolar, CSA) are still present within these HR-MAS samples (though greatly reduced by molecular motion), it is also possible to apply some solid state NMR techniques during HR-MAS experiments. For example, to measure the residual homonuclear dipole-dipole interaction it is possible to incorporate a dipolar recoupling sequence to re-introduce this interaction under MAS. This allows the incoherent NOE exchange process to be replaced by a coherent polarization transfer process. These types of HR-MAS recoupling experiments have been demonstrated using DQ COSY for solid state synthesis samples,[19] while our group has incorporated a radio frequency dipolar recoupling (RFDR) sequence into the standard NOESY experiment for lipid membranes, including the extension of this dipolar recoupling into a mixing period during 1H-13C Heteronuclear correlation experiments.[20]

**Figure 3.** The gradient 2D 1H HR-MAS NMR COSY spectrum for the ionic liquid [MBPyrr]+[TFSI] adsorbed into an inorganic aluminum oxide membrane. Even though the individual *J* couplings were not resolvable, these types of correlation experiments can still be realized under HR-MAS.

#### **1.2. Experimental protocol and limitations**

282 Advanced Aspects of Spectroscopy

NMRspectra.

the MAS speed. Since the protons from the membrane are not readily observable, it provides a unique opportunity to further study the solvent behavior in the membrane without interference. Additional discussion of this system is presented in Section 2.1.1. The second example, involves the swelling of a polyButadiene-AcryloNitrile (pBAN) polymer in CDCl3. Inspection of Figure 2B (green) shows that under static conditions this rather soft material produces a relatively unresolved NMR spectrum. Even after extensive solvent swelling, the NMR spectrum is essentially unchanged (Figure 2B, red). With HR-MAS (4 kHz spinning) the resolution is dramatically enhanced, enabling a more detailed analysis of the pBAN 1H

**Figure 2.** The improved resolution observed using 1H HR-MAS NMR for the A) methanol swollen anion exchange membrane, and B) the CDCl3 swollen pBAN (polyButadiene-AcryloNitrile) polymer.

this IL material is presented in Section 2.3.1.

With the dramatic increase in resolution observed utilizing HR-MAS, the arsenal of standard solution NMR techniques can be implemented for HR-MAS experiments, including solvent suppression, gradient-assisted sequences, and multiple-dimensional experiments. These can include INEPT (**I**nsensitive **N**uclei **E**nhanced by **P**olarization **T**ransfer), COSY (**CO**rrelation **S**pectroscop**Y**), NOESY (**N**uclear **O**verhauser **E**ffect **S**pectroscop**Y**), TOCSY (**TO**tal **C**orrelation **S**pectroscop**Y**), HETCOR (**HET**eronuclear **COR**relation), HMQC (**H**eteronuclear **M**ultiple **Q**uantum **C**oherence), and HMBC (**H**eteronuclear **M**ultiple **B**ond **C**orrelation) to name a few. As an example, a gradientassisted 2D 1H-1H HR-MAS COSY NMR spectra for an ionic liquid adsorbed on an aluminum oxide membrane is shown in Figure 3. These types of correlation experiments could not be realized without the resolution afforded by HR-MAS. Additional discussion of

It is also important to note that because anisotropic interactions (dipolar, CSA) are still present within these HR-MAS samples (though greatly reduced by molecular motion), it is also possible to apply some solid state NMR techniques during HR-MAS experiments. For example, to measure the residual homonuclear dipole-dipole interaction it is possible to incorporate a dipolar recoupling sequence to re-introduce this interaction under MAS. This Many HR-MAS samples contain liquids or liquid-like materials. These samples require special care to prevent dehydration that can occur in a standard rotor with only a rotor cap. Various inserts are commercially available for packing of HR-MAS samples and can be seen in Figure 4. These inserts provide a tight seal to prevent dehydration or loss of solvent during MAS experiments. The inserts used generally depend on the sample size and need of the user. Examples of various insert options from Bruker BioSpin for a standard 4 mm rotor are shown in Figure 4. For ~12 μL samples, a Kel-F® bottom spacer (or a half drilled out rotor) and a top spacer with a seal screw can be utilized (Figure 4B, 4F and 4G). The top spacer insert (Figure 4F) contains a small hole in the top. When placed into the rotor at the properly gauged distance, any additional liquid will protrude through this hole. A Kimwipe® can then be utilized to remove this liquid, followed by using a small Kel-F® screw to seal the insert (Figure 4B) and prevent dehydration. For ~50 μL volumes, a standard rotor can be utilized with the above top spacer and seal screw (Figure 4B, 4F, 4L). In addition to the spacers, a 30 μL disposable insert is also available (Figure 4I). The disposable Kel-F® inserts use a plug and screw cap to keep the sample well sealed (Figure 4B, 4H). The inserts are efficient if multiple samples need to be run, as samples can be packed into inserts and then easily placed into and removed from rotors, without having to wash the rotors between runs, and without having to own a large number of MAS rotors.

**Figure 4.** The tools and inserts used for HR-MAS NMR. These include A) the specialized tool for screw cap insertion, B) the sealing screw cap, C) the upper Teflon® insert, D) lower Teflon® insert for 30 μL volume, E) screw for insertion/extraction of top insert, F) top Kel-F® insert, G) bottom Kel-F® insert for 12 μL sample volume, H) plug for disposable insert, I) disposable 30 uL Kel-F® insert, J) 4 mm rotor cap, K) disposable inserted partially in a 4 mm rotor, L) 4 mm zirconia MAS rotor. All these parts are for the Bruker HR-MAS system, and may vary between vendors.

Temperature regulation is important in HR-MAS NMR for both sample purposes, and experimental reasons. The temperature of the bearing gas in many probes can be regulated to maintain a stable sample temperature; however, there is additional frictional heating that comes from spinning the rotor. To compensate for MAS related heating of the sample, the HR-MAS probe temperature should be calibrated at various MAS speeds and changes made to the temperature of the bearing gas to regulate the sample at the desired temperature. Temperature calibration of a HR-MAS probe has been discussed in literature.[21] This study utilized both methanol and glucose as NMR thermometers to determine the impact of MAS on temperature. Temperature changes measured at 2, 4, 6, and 8 kHz MAS where found to be 0.8 ± 0.1, 2.2 ± 0.1, 5.0 ± 0.1, and 7.9 ± 0.2 K in methanol and -0.3 ± 0.1, 1.2 ± 0.2, 3.0 ± 0.2, and 6.1 ± 0.1 K for glucose.[21]

Like all high resolution NMR techniques where resonances can be very narrow, it is important to be able to shim a sample to achieve the best possible resolution. Unlike standard high resolution NMR probes, the HR-MAS probes have the samples spinning at the magic angle, therefore the standard shimming protocols cannot be utilized. Instead, a combination of the standard shims must be used to address inhomogeneity of the *B0* field. By placing the probe so that the rotor is in the (x, z) plane in the laboratory frame, a combination of the laboratory shims can be used to express the Z shims along the magic angle. To obtain the equivalent response of a Z shim in standard shimming protocol, *i.e.* the Z shim along the magic angle ( *MAS ZB* ), the user needs to optimize a combination of the Z and X room temperature shims given by the linear relationship (1 / 3) ( 2 / 3) *MAS LAB LAB Z ZX B BB* . Likewise, the equivalent of a Z2 shim is given by 2 22 ( ) 2 2 *MAS LAB LAB Z XY XZ BB B* and the equivalent Z3 shim is defined using <sup>3</sup> <sup>3</sup> <sup>2</sup> 2 2 <sup>3</sup> ( ) (2 / 3 3) (1 / 6) (5 / 3) (5 / 3 6) *MAS LAB LAB LAB LAB <sup>Z</sup> <sup>Z</sup> XZ X YZ <sup>X</sup> B BBB B* . The higher order Z shims map directly as 4 4 (7 / 18) *MAS LAB Z Z B B* and 5 5 (1 / 6 3) *MAS LAB Z Z B B* .[22] For the experimental examples discussed in this chapter, the authors typically only adjusted the shims up through <sup>3</sup> *MAS ZB* . The Bruker Biospin Manual for High Resolution Magic Angle Spinning Spectroscopy points out that in theory shimming could be performed with just laboratory shims X, XZ,XZ2, Z4, and Z5.[23] In reality, additional shims can be used to compensate for any inefficiency in the shim coil. A previous study has shown that once good shims are obtained, those shims can be utilized for samples with the same detection volume and position, for example the same sample size in the same type of rotor.[24] The shims should be independent of solvent because any susceptibility differences caused by solvent will be averaged out by MAS and does not affect the shims.[24] Piotto *et al.* also demonstrated shimming issues involved in systems containing water. In many cases, improved shims will be detected in an increase in lock signal. Deuterated water (HDO), however, has a strong chemical shift dependence on the temperature. When a sample spins, there can be a temperature gradient across the sample which can cause a portion of the HDO resonances to shift and broaden. If the probe is shimmed again based on the lock level from the water resonance, then any other signal in the spectra will most likely be deshimmed as those signals do not have as large of a chemical shift temperature dependence. With this in mind, Piotto *et al.* emphasize that shimming should be performed based on line shape, not lock level and attention should be taken when working with materials containing water.[24] More recently the MAGIC SHIMMING method has been described which utilizes a conventional homospoil gradient pulse to perform gradient shimming on MAS probes. This techniques does not require a gradient along the magic angle, and will reduce some of the trial and error presently inherent with manual shimming of HR-MAS probes.[25]

284 Advanced Aspects of Spectroscopy

screw to seal the insert (Figure 4B) and prevent dehydration. For ~50 μL volumes, a standard rotor can be utilized with the above top spacer and seal screw (Figure 4B, 4F, 4L). In addition to the spacers, a 30 μL disposable insert is also available (Figure 4I). The disposable Kel-F® inserts use a plug and screw cap to keep the sample well sealed (Figure 4B, 4H). The inserts are efficient if multiple samples need to be run, as samples can be packed into inserts and then easily placed into and removed from rotors, without having to wash the rotors between runs, and without having to own a large number of MAS rotors.

**Figure 4.** The tools and inserts used for HR-MAS NMR. These include A) the specialized tool for screw cap insertion, B) the sealing screw cap, C) the upper Teflon® insert, D) lower Teflon® insert for 30 μL volume, E) screw for insertion/extraction of top insert, F) top Kel-F® insert, G) bottom Kel-F® insert for 12 μL sample volume, H) plug for disposable insert, I) disposable 30 uL Kel-F® insert, J) 4 mm rotor cap, K) disposable inserted partially in a 4 mm rotor, L) 4 mm zirconia MAS rotor. All these parts are for

Temperature regulation is important in HR-MAS NMR for both sample purposes, and experimental reasons. The temperature of the bearing gas in many probes can be regulated to maintain a stable sample temperature; however, there is additional frictional heating that comes from spinning the rotor. To compensate for MAS related heating of the sample, the HR-MAS probe temperature should be calibrated at various MAS speeds and changes made to the temperature of the bearing gas to regulate the sample at the desired temperature. Temperature calibration of a HR-MAS probe has been discussed in literature.[21] This study utilized both methanol and glucose as NMR thermometers to determine the impact of MAS on temperature. Temperature changes measured at 2, 4, 6, and 8 kHz MAS where found to be 0.8 ± 0.1, 2.2 ± 0.1, 5.0 ± 0.1, and 7.9 ± 0.2 K in methanol and -0.3 ± 0.1, 1.2 ± 0.2, 3.0 ± 0.2,

the Bruker HR-MAS system, and may vary between vendors.

and 6.1 ± 0.1 K for glucose.[21]

#### **2. HR-MAS NMR spectroscopy in materials characterization**

In this section a brief review of HR-MAS NMR studies involving the characterization of materials is presented. The majority of studies involved modified surfaces or surface interactions, and demonstrates the characterization need that HR-MAS NMR can fulfill.

#### **2.1. Polymers**

For rigid polymer materials the dipolar interactions are very large, leading to broadening of the NMR resonances, and can be characterized using standard solid state MAS NMR techniques. As noted before, these rigid polymers are not readily observed under HR-MAS conditions. For solvent swollen polymers the increased mobility of the polymer segments leads to a semi-solid regime where the moderate spinning speeds of HR-MAS are sufficient to produce high resolution NMR spectra. For example see the improved resolution observed for CDCl3 swollen pBAN (Figure 2B). This ability to obtain spectra for mobile domains or components is the basis for the majority of HR-MAS polymer studies. A variety of different solvents are employed in combinatorial and solid phase synthesis, many of these with multiple 1H resonances. In addition, there may be reasons to avoid having to dry the sample prior to introducing a deuterated solvent. In these cases, complicated solvent suppression techniques may be required.[26] It has been demonstrated that diffusion-filter HR-MAS spectra (see PFG discussion, Section 3.1) can provide suppression of solvent resonances based on differential diffusion behavior.[27]

One example of using HR-MAS NMR for characterizing polymer materials is the demonstration of the cyclic polyamide receptor threading onto the highly flexible polyethylene glycol (PEG) polymer chain attached to a polystyrene bead. Distinct PEG resonances for the threaded and non-threaded complexes could be easily resolved under HR-MAS conditions, while 2D NOESY spectra showed cross-peaks between the aromatic protons (δ ~ +8.87 ppm) of the rotaxane and the methylene protons (δ ~ +3.55 ppm) of the PEG polymer chain.[28] In another study, the complexation of Zn and Ru metalloporphyrins to beads functionalized with pyridyl ligands revealed that supermolecular interactions and changes in the dynamics are directly probed by HR-MAS techniques.[29]

The vulcanization of butadiene rubber (BR) with different curing systems has also been monitored by 1H and 13C HR-MAS NMR. This rubber is a very mobile system, with the increased resolution afforded by HR-MAS allowing the chemical identity of cross links to be determined, and revealing that α-substitution or addition depended on the disulfide crosslinkers employed.[30, 31] Other HR-MAS studies observed the impact of cross-linking in solvent swollen poly(amidoamine) polymers, the nature of water interactions in these same class of polymers, [32, 33] or the cross-linking performance in silicon-containing soybean-oil copolymers.[34] HR-MAS NMR has also been used to study the functionalization of poly hydroxyethyl methacrylate (HEMA) cryogels,[35] cyclomaltoheptaose polymers,[36] the synthesis of hyper-branched *bis* (hydroxymethyl) propionic acid (*bis*-MPA) polymers as a function of catalysts,[37] and polymers for nanoparticle stabilization.[38]

HR-MAS NMR has found use in investigation of porous polyalkylvinyl ether polymer particles being used for stationary phases in chromatographic applications. These measurements were performed using the same solvent as HPLC and allowed details about the polymer structure and mobility to be evaluated; these properties are expected to impact the chromatographic process.[39] Interactions between solution molecules and the polymer component of HPLC stationary phases (C18, C30 and PEAA)[40] or the interaction with molecularly imprinted polymers [41] have also been investigated using HR-MAS NMR. These studies used saturation transfer difference pulse sequences to identify those molecules that were involved in binding to the larger stationary phase.

286 Advanced Aspects of Spectroscopy

based on differential diffusion behavior.[27]

For rigid polymer materials the dipolar interactions are very large, leading to broadening of the NMR resonances, and can be characterized using standard solid state MAS NMR techniques. As noted before, these rigid polymers are not readily observed under HR-MAS conditions. For solvent swollen polymers the increased mobility of the polymer segments leads to a semi-solid regime where the moderate spinning speeds of HR-MAS are sufficient to produce high resolution NMR spectra. For example see the improved resolution observed for CDCl3 swollen pBAN (Figure 2B). This ability to obtain spectra for mobile domains or components is the basis for the majority of HR-MAS polymer studies. A variety of different solvents are employed in combinatorial and solid phase synthesis, many of these with multiple 1H resonances. In addition, there may be reasons to avoid having to dry the sample prior to introducing a deuterated solvent. In these cases, complicated solvent suppression techniques may be required.[26] It has been demonstrated that diffusion-filter HR-MAS spectra (see PFG discussion, Section 3.1) can provide suppression of solvent resonances

One example of using HR-MAS NMR for characterizing polymer materials is the demonstration of the cyclic polyamide receptor threading onto the highly flexible polyethylene glycol (PEG) polymer chain attached to a polystyrene bead. Distinct PEG resonances for the threaded and non-threaded complexes could be easily resolved under HR-MAS conditions, while 2D NOESY spectra showed cross-peaks between the aromatic protons (δ ~ +8.87 ppm) of the rotaxane and the methylene protons (δ ~ +3.55 ppm) of the PEG polymer chain.[28] In another study, the complexation of Zn and Ru metalloporphyrins to beads functionalized with pyridyl ligands revealed that supermolecular interactions and

The vulcanization of butadiene rubber (BR) with different curing systems has also been monitored by 1H and 13C HR-MAS NMR. This rubber is a very mobile system, with the increased resolution afforded by HR-MAS allowing the chemical identity of cross links to be determined, and revealing that α-substitution or addition depended on the disulfide crosslinkers employed.[30, 31] Other HR-MAS studies observed the impact of cross-linking in solvent swollen poly(amidoamine) polymers, the nature of water interactions in these same class of polymers, [32, 33] or the cross-linking performance in silicon-containing soybean-oil copolymers.[34] HR-MAS NMR has also been used to study the functionalization of poly hydroxyethyl methacrylate (HEMA) cryogels,[35] cyclomaltoheptaose polymers,[36] the synthesis of hyper-branched *bis* (hydroxymethyl) propionic acid (*bis*-MPA) polymers as a

HR-MAS NMR has found use in investigation of porous polyalkylvinyl ether polymer particles being used for stationary phases in chromatographic applications. These measurements were performed using the same solvent as HPLC and allowed details about the polymer structure and mobility to be evaluated; these properties are expected to impact

changes in the dynamics are directly probed by HR-MAS techniques.[29]

function of catalysts,[37] and polymers for nanoparticle stabilization.[38]

**2.1. Polymers** 

The NMR spectroscopic characterization of polymer degradation is commonly directed towards analysis of the small degradation fragments that are solvent soluble. By using HR-MAS NMR the hydrolytic degradation of the biodegradable photo-initiated cross linked poly(DL-lactide)-dimethacrylate (PDLLA) polymer network was directly monitored. Swelling these polymers in the solvent dimethyl sulfoxide (DMSO) and a combination of 1D and 2D (COSY, NOESY) experiments allowed the different signals in the PDLLA chain to be identified. Degradation in a base was shown to occur through hydrolysis of the ester bonds within the poly(lactide) segment.[42] HR-MAS has also been used to study the role of partial hydrolysis in controlling the composition of hydrophobic polyacrylamide gels.[43]

#### *2.1.1. Example: HR-MAS NMR investigations of anion exchange membranes*

The increased resolution achieved with 1H HR-MAS on the solvent swollen anion exchange membrane (AEM) was demonstrated in Figure 2. Due to differences in the magnetic susceptibility between the swelling solution and the polymer membrane, the water and methanol resonances were not resolved under static conditions. Under HR-MAS four distinct NMR resonances were observed for the solvent species; two water resonances and two methanol resonances. Based on line widths and PFG HR-MAS diffusion measurements (Section 3.2.1) the higher ppm shifted water and methanol were assigned to bulk (or free) methanol or water within the membrane, while the lower ppm shifted water and methanol resonances exhibit slower diffusion rates and were assigned to water or methanol associated (bound) within the membrane. The increased resolution obtained under HR-MAS was further exploited to explore the interchange of water and methanol between the different binding environments within the AEM using 2D HR-MAS NMR NOESY exchange experiments (Figure 5). At short mixing times (1 ms) no correlations were observed between any of the resonances. With increasing mixing time (> 10 ms), cross peaks were observed between free water and membrane-associated water, plus cross peaks between free methanol and membrane-associated methanol, resulting from physical exchange of water (red dashed lines) or methanol (red dashed lines) between the free and unbound environments. With long mixing times (> 200 ms), NOE correlation between methanol and water were observed (green dashed line), and result from through space NOE magnetization exchange (not physical spatial exchange). Interestingly, these NOE correlations were only observed between associated water and associated methanol, as well as free water and free methanol, supporting the assignment and indicating that the spatial contact of water and methanol is maintained in these associate membrane environments. Additional exchange studies are ongoing.

**Figure 5.** 2D 1H HR-MAS NMR NOESY spectrum (500 ms mixing time) of an anion exchange membrane swollen in a 1N methanol solution. The increased spectral resolution obtained for the swelling solvent made it possible to detect exchange between free and associated water and between free and associated methanol (red dashed lines), as well as NOE magnetization exchange between free water and methanol and associated water and methanol (green dashed lines).

#### **2.2. Ceramics, zeolites, catalysts surfaces and composites**

Due to the rigid or crystalline nature of ceramics, zeolites, catalysts or inorganic/organic composites, solid state MAS NMR spectroscopy remains the dominant NMR characterization method, and can involve a range of nuclei and multi-pulse NMR techniques. In contrast, the limited number of HR-MAS NMR studies is directed towards the investigation of gas/solid adsorption or intercalation within these materials. For example, the degradation of the pesticide diethyl parathion and the chemical warfare agent (CWA) simulant diisopropyl fluorophosphates in functionalized montmorillonite clay using 31P HR-MAS NMR has been reported. The HR-MAS removed susceptibility effects present from the clay material, and allowed a direct measure of the decomposition kinetics. The adsorption of organosilanes on the MgCl2 in Ziegler-Natta catalysts has been investigated using 1H HR-MAS, and demonstrates differential binding (differential mobility) to the Mg surface with changing surface coverage and as a function of the degree of saturation in the Mg coordination sites.[44]The polymer mobility in the condensed sol-gels produced from organosilanes and phenyl siloxanes was also evaluated.[45]

## **2.3. Ionic liquids and liquid crystals**

HR-MAS NMR should be an ideal tool for the characterization of ionic liquids (ILs) and liquid crystals (LC) due to the high viscosities commonly encountered in ILs, and the thermal- or concentration-induced ordering transitions of LCs. Ionic liquids are interesting compounds, and continue to be used for a wide range of material science applications. ILs are used as liquid electrolytes in energy storage and production devices, solvents for CO2 capture and green chemistry, solutions for biomass processing, nanostructured synthesis and solvents for catalysis reactions. For many of these applications room temperature ionic liquids (RTIL) are utilized as neat solutions without another solvent. Strong attractions between the IL cation and anion component may lead to the formation of important structural motifs that could be missed or changed with the introduction of a solvent. For NMR characterization, solvent-IL interactions may also mask subtle changes in the chemical shift produced by other molecular interactions of interest. In the case of ILs, it is now recognized that HR-MAS NMR provides a powerful tool for the characterization of these systems.

288 Advanced Aspects of Spectroscopy

**Figure 5.** 2D 1H HR-MAS NMR NOESY spectrum (500 ms mixing time) of an anion exchange membrane swollen in a 1N methanol solution. The increased spectral resolution obtained for the swelling solvent made it possible to detect exchange between free and associated water and between free and associated methanol (red dashed lines), as well as NOE magnetization exchange between free

Due to the rigid or crystalline nature of ceramics, zeolites, catalysts or inorganic/organic composites, solid state MAS NMR spectroscopy remains the dominant NMR characterization method, and can involve a range of nuclei and multi-pulse NMR techniques. In contrast, the limited number of HR-MAS NMR studies is directed towards the investigation of gas/solid adsorption or intercalation within these materials. For example, the degradation of the pesticide diethyl parathion and the chemical warfare agent (CWA) simulant diisopropyl fluorophosphates in functionalized montmorillonite clay using 31P HR-MAS NMR has been reported. The HR-MAS removed susceptibility effects present from the clay material, and allowed a direct measure of the decomposition kinetics. The adsorption of organosilanes on the MgCl2 in Ziegler-Natta catalysts has been investigated using 1H HR-MAS, and demonstrates differential binding (differential mobility) to the Mg surface with changing surface coverage and as a function of the degree of saturation in the Mg coordination sites.[44]The polymer mobility in the condensed sol-gels produced from

HR-MAS NMR should be an ideal tool for the characterization of ionic liquids (ILs) and liquid crystals (LC) due to the high viscosities commonly encountered in ILs, and the

water and methanol and associated water and methanol (green dashed lines).

**2.2. Ceramics, zeolites, catalysts surfaces and composites** 

organosilanes and phenyl siloxanes was also evaluated.[45]

**2.3. Ionic liquids and liquid crystals** 

One of the earliest reports involving HR-MAS NMR of ILs studied silica-immobilized ILs suspended in DMSO,[46] which produced nearly liquid-like resolution. In this same study, they were also able to obtain 2D 1H-13C HR-MAS NMR HMQC spectra of these attached ILs. Another study explored the use of ILs as a solvent or chemical reaction media. The standard solution NMR of compounds dissolved in these ILs suffered from low resolution, produced by the high viscosities of the ILs, dynamic range issues due to the lack of deuterated ILs, and concerns about chemical shift referencing due to the high magnetic susceptibility of ILs. To demonstrate the capabilities of HR-MAS NMR, Rencurosi and co-workers[47] dissolved *para*-methoxy benzyl acetate and a glucopyranoside in a series of different ILs. Even at moderate spinning speeds there was significant improvement in resolution, and the chemical shift referencing became consistent with that observed in solution NMR using traditional deuterated solvents. In addition, they were able to follow acetylation of *para*methoxybenzyl alcohol directly in the ILs. The increased resolution inherent in this technique was also important for investigations of CO2 interactions with imidazolium based ILs using 13C HR-MAS NMR.[48] 1H HR-MAS NMR has also been used to probe the interaction of the RTIL *N*-methylimidazolium chloride ([Hmim]+Cl- ) with the silica surface of Aerosil. This adsorption was monitored through changes in chemical shift with increased IL loading and Cl salt concentration. Based on the magnitude of the chemical shift variations, the interaction was found to preferentially involve the H(2) position of Hmim+, and was a physisorption versus a chemisorption process.[49] In another study, the dynamics of IL's confined in monolithic silica ionogels were followed by 1H HR-MAS NMR and relaxation experiments, and demonstrated that the IL maintained liquid like behavior with very little reduction in motions even as the pore size diminished from 12 nm to 1.5 nm.[50] Surprisingly, only a single 1H HR-MAS NMR diffusion study of a non-biological LC material confined in nanopores has been reported.[51]

#### *2.3.1. Example: HR-MAS NMR investigation of ionic liquids on surfaces*

While many RTILs are "liquids" at room temperature, they can be highly viscous, making standard solution NMR analysis on the neat ILs challenging (as noted above). In our laboratory, a novel series of quaternary ammonium and cyclic pyrrolidinium RTIL have been studied using different NMR techniques. This includes pulse field gradient (PFG) NMR to measure diffusion and 14N NMR relaxation experiments to determine IL molecular reorientation times.[52] More recently, we have explored using 1H HR-MAS NMR to characterize the interactions between IL and metal oxide surfaces. Figure 6A shows the 1H HR-MAS NMR spectra of the neat ionic liquid, *N*-methyl-*N*-(n-butyl) pyrrolidinium bis(trifluoromethanesufonyl) imide ([MBPyrr]+[TFSI]- ). The changes in the NMR spectrum for the neat solution under static conditions (top) and under MAS conditions (bottom) demonstrate the simple resolution enhancing properties of HR-MAS (previously discussed in Section 1.1). The static NMR spectrum revealed distorted line shapes reflecting poor magnetic field homogeneity across the sample volume, along with broadening (FWHM ~ 115 Hz) due to differences in local magnetic susceptibility within the sample. The broad line width is commonly encountered when working with highly viscous RTIL that are not dissolved into solvents. Increasing the temperature (within the limitations of the instrumentation) will improve the resolution for these RTIL, but may not be desirable in some instances. Not surprisingly, even slow spinning (~1 kHz) of the neat RTIL provides an immediate improvement in the resolution. For the ([MBPyrr]+[TFSI]- ) example, the line width are reduced to ~ 3 Hz (~40 fold reduction), such that small 1H-1H spin-spin *J* coupling are clearly resolved (Figure 6A, bottom).

**Figure 6.** The 1H HR-MAS NMR of the ionic liquid [MBPyrr]+TFSI at 298K as a A) neat liquid and B) adsorbed (20 wt%) into an aluminum oxide ANOPORE inorganic membranes (200 nm pore diameter). The spectra in the top portion of each figure were obtained under static conditions, with the spectra in the bottom portions of each figure obtained under MAS conditions.

The loss of resolution is even more dramatic when [MBPyrr]+[TFSI] is adsorbed into the small pores (200 nm) of an inorganic aluminum oxide membrane. The 1H NMR spectrum of the static sample is now a broad (~2780 Hz) featureless line providing no chemical shift resolution. Under HR-MAS the individual 1H resonances become resolved, with a line width on the order of 37 Hz. While the individual *J* couplings are no longer resolved as they were in the neat IL case, the resolved chemical shifts allow one to compare directly with the neat IL. In the adsorbed [MBPyrr]+[TFSI]- , the *n*-butyl methyl group protons (H-8) and the ring methylene protons (H-3) show a very small chemical shift change (Δδ = 0.007 ppm) due to adsorption with the ANOPORE membrane. In contrast the methyl group (H-4) and the *n*butyl methylene protons (H-5) reveal an asymmetric line shape with chemical shift variations ranging from Δδ = +0.01 to +0.02 ppm. This result suggests that while the adsorption process is weak, it occurs preferentially through the IL N+ with the surface. The ability to resolve such small differences under HR-MAS NMR is important in identifying these surface interactions.

Even though the 1H HR-MAS NMR spectra for the surface adsorbed [MBPyrr]+[TFSI] does not reveal resolvable *J* coupling, it is still possible to obtain a 2D COSY spectrum for this material, as shown in Figure 3. This correlation experiment immediately confirms the chemical shift assignments for the neat IL sample, and clarifies differences seen for the IL dissolved in a solvent. The 2D 1H HR-MAS NOESY NMR spectra for the IL [MBPyrr]+[TFSI] adsorbed into the ANAPORE membrane is shown in Figure 7. Interestingly there are numerous cross peaks observed even at moderate mixing times, reminiscent of the liquidordered phase observed in lipid bilayers (For example see Ref. [20, 53, 54]). These throughspace correlations are stronger than those observed in the neat IL (not shown), and suggest that an increase in the dipolar-dipolar interactions is occurring for the surface adsorbed species. This argues that local motions have become reduced for the adsorbed species. There are also some missing or weak correlations (dashed circle) implying distinct conformations present for the *n*-butane chain on the adsorbed species. Also of interest is the appearance of new chemical environments for the adsorbed IL. This includes a weak resonance at δ = +4.8 ppm which is assigned to residual water, but shows no correlations with the IL. There are two additional 1H environments at δ = +5.2 and +5.3 ppm, which are attributed to additional surface water species on the aluminum oxide surface. The environment at δ = +5.2 ppm shows a through space NOE correlation with a new resonance near 2 ppm. This small resonance is buried under the shoulder of the dominant methylene (H-2) resonance and has not been presently assigned. These types of correlation experiments demonstrate the capabilities and information that can be obtained from HR-MAS studies of ILs, and suggest future efforts along these lines are warranted.

#### **2.4. Surface modified nanoparticles**

290 Advanced Aspects of Spectroscopy

bis(trifluoromethanesufonyl) imide ([MBPyrr]+[TFSI]-

are clearly resolved (Figure 6A, bottom).

immediate improvement in the resolution. For the ([MBPyrr]+[TFSI]-

**Figure 6.** The 1H HR-MAS NMR of the ionic liquid [MBPyrr]+TFSI-

the bottom portions of each figure obtained under MAS conditions.

laboratory, a novel series of quaternary ammonium and cyclic pyrrolidinium RTIL have been studied using different NMR techniques. This includes pulse field gradient (PFG) NMR to measure diffusion and 14N NMR relaxation experiments to determine IL molecular reorientation times.[52] More recently, we have explored using 1H HR-MAS NMR to characterize the interactions between IL and metal oxide surfaces. Figure 6A shows the 1H HR-MAS NMR spectra of the neat ionic liquid, *N*-methyl-*N*-(n-butyl) pyrrolidinium

for the neat solution under static conditions (top) and under MAS conditions (bottom) demonstrate the simple resolution enhancing properties of HR-MAS (previously discussed in Section 1.1). The static NMR spectrum revealed distorted line shapes reflecting poor magnetic field homogeneity across the sample volume, along with broadening (FWHM ~ 115 Hz) due to differences in local magnetic susceptibility within the sample. The broad line width is commonly encountered when working with highly viscous RTIL that are not dissolved into solvents. Increasing the temperature (within the limitations of the instrumentation) will improve the resolution for these RTIL, but may not be desirable in some instances. Not surprisingly, even slow spinning (~1 kHz) of the neat RTIL provides an

width are reduced to ~ 3 Hz (~40 fold reduction), such that small 1H-1H spin-spin *J* coupling

adsorbed (20 wt%) into an aluminum oxide ANOPORE inorganic membranes (200 nm pore diameter). The spectra in the top portion of each figure were obtained under static conditions, with the spectra in

). The changes in the NMR spectrum

at 298K as a A) neat liquid and B)

) example, the line

HR-MAS NMR studies of surface modified nanoparticles (NP) are closely related to those of the polymer resin studies mentioned in the previous sections, with accurate

characterization of the surface-attached ligands being the primary objective. For NPs the solvent plays the role of suspending or dispersing the material, in contrast to the role of swelling that the solvent plays in analysis of resins. For HR-MAS NMR studies of NPs, the optimal solvent provides both high solubility, and good dispersion (prevents aggregation) of the NPs.

**Figure 7.** The 2D 1H HR-MAS NOESY NMR correlation spectra for the ionic liquid [MBPyrr]+[TFSI] adsorbed into an inorganic aluminum oxide membrane (pore size 200 nm). The spectrum in A) shows numerous through space correlation between the protons of the IL, while B) shows the spectral expansion for the water and surface associate-water species and corresponding NOE correlations.

HR-MAS NMR studies of surface modified NPs include the 1D and 2D 1H NMR investigation of modified gold (Au) NPs using a wide range of high resolution correlation experiments: COSY, TOCSY and HMQC.[55] This revealed that the relative signal sensitivity (intensity) depends on the distance between the detected 1H and the surface of the Au NP. This distance correlation is either a function of reduced local dynamics, or spin-spin T2 relaxation effects governed by the Au surface. Another study involved the active molecular compounds Aloin A and Aloesin extracted from the leaf of Cape Aloe, and was able to show how these compounds selectively stabilize Au NPs preferentially through the glucose component.[56] HR-MAS NMR has also been used to identify the binding motif of peptides on the Au NP surface.[57] While HR-MAS NMR is typically employed to overcome anisotropy of magnetic susceptibility or residual dipolar interactions, Poito and coworkers[58, 59] have reported an interesting set of experiments that demonstrated HR-MAS can be employed to overcome paramagnetic effects present in iron oxide NPs. Through a careful analysis of these iron oxide systems, the surface structure and ligand binding (chelation) configuration was determined, and in several cases were quite different from the standard single point attachment proposed by others.[58]

1H HR-MAS NMR has also been used to look at the surface modification in polymer based NPs, including the monitoring of multiple synthetic steps employed during the peptidesurface modification of poly(vinylidene fluoride) (PVDF) nanoparticles,[60] or the surface modification of Dendron based NPs.[61] Polystyrene embedded silver clusters produced by a thermolysis reactions have also been characterized using HR-MAS NMR.[62]
