Fast Magic Angle Spinning Solid State 1H NMR Reveals Structural

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C: Physical Processes in Nanomaterials and Nanostructures

Fast Magic Angle Spinning Solid State 1H NMR Reveals Structural Relationships in the High Explosive 2,6Diamino-3,5-Dinitropyrazine-1-Oxide (LLM-105) Harris E. Mason, Ginger J. Guillen, and Alexander E Gash J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06180 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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The Journal of Physical Chemistry

Fast magic angle spinning solid state 1H NMR reveals structural relationships in the high explosive 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) Harris E. Mason1*, Ginger J. Guillen1, Alexander E. Gash1 1Physical

and Life Sciences Directorate, Lawrence Livermore National Laboratory, 7000 East Ave,

Livermore CA 94550 *Corresponding

author: [email protected]

Abstract: 1H

nuclear magnetic resonance (NMR) experiments were performed on the proposed insensitive high

explosive 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) at high speed magic angle spinning rates of up to 60 kHz. These rapid speeds produce well resolved spectra that aid in the study of the chemical and structural properties of this material. Extraordinarily long 1H T1 values were observed for the main amine peaks from the LLM-105 explosive and can be used to differentiate between peaks from the main compound and those of impurities. Further, advanced NMR measurements and simulations reveal unique spectral properties due to the strong network of intramolecular and intermolecular hydrogen bonding, and provide insight in the structure of LLM-105.

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Introduction: The compound 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) has been developed as a potential insensitive high explosive1, 2 (Figure 1). Insensitive explosives provide added safety due to their low reaction to external insult such as applied force or temperature. The high stability and low sensitivity of LLM-105 and other insensitive high explosive materials has been suggested to be due to the network of strong intra- and intermolecular hydrogen H ··· O bonds.1, 3-5 The solid crystal structure of LLM-105 is formed as a layered series of zig zagging sheets of intermolecular H-bonds between neighboring molecules (Figure 2).1, 5 The LLM-105 molecules also have strong intramolecular H-bonds between the amine protons and neighboring N-oxide and nitro oxygens.

O N+ -

O N

N+ O-

O

H 2N

N+

NH2

O-

Figure 1. Molecular structure of LLM-105


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Figure 2. Crystal structure of LLM-105. a. viewing parallel to the sheet structure to show the characteristic “herring bone” layering of the structure. b. View perpendicular to the sheet showing the hydrogen bonding between the molecules as dashed lines with intramolecular bonds labeled in blue and intermolecular bonds labeled in orange.

Solid state nuclear magnetic resonance (NMR) is an elementally specific, non-destructive technique that can provide detailed information about a wide range of materials. In fact, 15N magic angle spinning (MAS) NMR studies of some high explosive materials have provided insight into the structure of high explosives previously. These works have shown that, combined with computational methods, the 15N chemical shift anisotropy (CSA) can be used to provide insight into the structure of these materials.6, 7 Solid-state 1H NMR studies are complementary for studying the interactions in these strongly hydrogen bonded materials due to their sensitivity to changes in local structure.8-11 However, we are aware of only



three solid-state 1H NMR studies on explosives and these were performed under static conditions.12-14 One recent study focuses only on the relaxation properties of the 1H due to 14N cross relaxation.14 The other two focus on the effects of structural changes on the 1H NMR properties.12, 13 The work of Landers et al.12 present the most comprehensive study of polymorphic RDX and HMX compounds. In all cases, they observed exceedingly broad 1H lineshapes, but were able to measure homonuclear 1H dipolar couplings of 29 to 31 kHz. These results suggest that magic angle spinning (MAS) studies on these materials must be performed at spinning rates greater than 30 kHz. Here, we present the first study of a high explosive material utilizing 1H MAS NMR spectroscopic methods. We observe unique spectral signatures in these materials that can be attributed to the strong network of hydrogen bonding present in these materials. This work provides insight into how this network of bonds forms and leads to the stability of this material. Methods: Materials The sample of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) used in the study was synthesized via the DMP process where 2,6-dimethoxypyrazine (DMP) is used to make 2,6-dimethyoxy-3,5dintropyranine (ANPZ) which is subsequently oxidized by a solution 50% hydrogen peroxide in trifluoroacetic acid to form LLM-105.15 Previous wet chemical analysis of samples formed by this process show potential impurities can remain such as small amounts (3-5 wt%) of ANPZ, the solvent trifluoroacetic acid, and potentially the reaction product 2-amino-6-methoxy-3-nitropyridine. Previous safety tests of the specific LLM-105 material studied reveal a drop hammer (ΔH50) value if 110 cm, a thermal decomposition temperature of 628.15 K (as measured by differential scanning calorimetry), and no spark sensitivity at 1 J of energy and a resistance of 510 ohms. In all cases when handling high explosives, care should be taken. However, frictional heating due to rotor spinning (vide infra) is far lower than the thermal decomposition temperature and not enough to initiate deflagration of the sample. Further, the small amounts of material analyzed in this study (< 10 mg) also preclude detonation due to the potential shock from unintending rotor crash. NMR spectroscopy All NMR experiments were collected on a 600 MHz Bruker Avance III spectrometer using a HX Bruker Very Fast MAS probe configured for 1.3 mm (o.d) rotors. Samples of ~2 mg of LLM-105 were loaded into 1.3 mm (o.d.) rotors and spun from 15 to 60 kHz. 1H spin-lattice relaxation (T1) measurements were made using a saturation recovery sequence. 1H exchange spectroscopy (EXSY) experiments were


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performed at 50 kHz using a 10 s recycle delay for a range of mixing rates of 50 us to 1 s. A total of 48 points were collected in the t1 domain at a 33 µs time delay corresponding to a 30 kHz spectral width in the indirect dimension. A 71.4 kHz radio frequency (r.f.) was used for all the pulses used in the above experiments corresponding to a 90° excitation pulse of 3.5 μs. Further a 1H/1H anisotropic/isotropic correlation experiment was implemented using a R1245 (270°90°) recoupling sequence at a spinning rate of 50 kHz16. In this sequence the duration, r.f. and phasing of the pulses were synchronized to such as to be multiples of rotor spinning speed. The r.f. of the pulses used in this sequence was set to 120 kHz. A total of 32 points were collected in the indirect dimension using a 100 µs time delay corresponding to a 10 kHz spectral width. The sequence used for this experiment is provided in the supporting material. The magic angle of the probe was carefully set using the 79Br NMR signal of KBr prior and the magnet shimmed using the 27Al lineshape of a concentrated AlCl3 solution prior to collecting the 1H NMR spectra. 1H spectra were referenced with respect to TMS using an external standard of α-glycine and setting the -NH2 resonance to δH = 8.2 ppm. The temperature increases due to frictional heating at different spinning rates were calibrated with 207Pb NMR measurements of Pb(NO3)2 (Figure S1)17. The probe set temperature read 293 K for the calibration experiment. The 207Pb chemical shift scale was set using the frequency ratio (Ξ) between 1H and 207Pb.18 Variable temperature 1H MAS NMR experiments on LLM-105 were also performed at 296 to 351 K at a fixed spinning rate of 50 kHz. These temperatures were calculated from the probe set temperature using the probe calibration and the sample was equilibrated for 10 min at each temperature prior to measurement. This temperature range spans that of the frictional heating. No changes in the 1H spectra were noted as a function of temperature (Figure S2). Simulation and analysis Spectral simulations were performed using the SIMPSON NMR simulation software package.19 Values for the isotropic resonances were derived from the NMR data collected at a 60 kHz spinning rate. Dipolar couplings and bond angles were taken from the published crystal structure.3 Simulations of the 1H/1H anisotropic/isotropic experiment were performed to extract CSA parameters from slices taken along the anisotropic dimension. In these experiments the coupling between 1H and 14N was considered and set to 11.4 kHz which corresponds to a N-H distance of 0.80 Å. A quadrupole coupling constant of 3 MHz was chosen for these simulations based on prior work on amino acids.20 For variable spinning rate simulations of the 1H spectra, simulations were performed at different spinning rates using the fitted parameters from the experimental data while keeping the spectral width of the simulations constant. Results and Discussion:



Figure 3. 1H SP/MAS NMR spectra collected at 50 kHz at the indicated recycle delay (rd). a. Spectra are normalized to the most intense peak. b. spectra are all normalized with respect to that collected at rd = 200.

The 1H SP/MAS spectra collected at a 50 kHz spinning rate for LLM-105 at different recycle delays are plotted in Figure 3. At short recycle delays the spectra are dominated by peaks located at 0 to 5 ppm, but at longer times the spectra are dominated by two main peaks at 7.06 and 9.00 ppm. The dominance of these peaks and their similar chemical shifts to NH groups in amino acids indicate these represent the main resonances for the amine protons in LLM-105. A saturation recovery sequence was used to measure the T1 of the main peaks at a spinning rate of 50 kHz. The intensity growth of both peaks can be described with a stretched exponential with an exceptionally long T1 of 1234.1 s with an exponential stretching factor of 0.5. This long value for the 1H T1 is consistent with those observed for other strongly H-bound compounds under fast MAS conditions.21-23 Further, the value for the exponential stretching factor indicates dipolar mediated spin diffusion is slow and has little influence on T1 relaxation.24-26 Previous work has indicated that 1H spin diffusion diminishes as the spinning rate is increased.27, 28 The smaller peaks located at 0 to 5 ppm are fully obscured by the main peaks at longer recycle delays, so we could not measure their respective T1’s, however it is clear they have a much shorter T1 due to their prominence in spectra collected at short recycle delays. Based on the T1’s of these two peak populations, these likely represent two independent populations of species present in the LLM-105 sample. It is important to discuss the above observed relaxation properties of the main LLM-105 peaks as they relate to the structure and dynamics of the protons in this material. Typically, the observation of a non-


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exponential T1 relaxation is a strong indicator of the presence of a paramagnetic center influencing the relaxation behavior.24-26, 29, 30 In that instance, the presence of the paramagnet suppresses spin energy exchange through dipole-dipole coupling mediated spin diffusion. However, the exceedingly long relaxation times we observe rule out the influence of paramagnets in this material. Further chemical analyses of these materials do not show paramagnetic metals or organic radical species in enough abundance to cause the observed effect. In the LLM-105 structure, the amine protons are separated on average by 1.60 Å which corresponds to a 1H homonuclear dipole coupling constant of 29 kHz.5 In these experiments, the applied 50 kHz spinning rate mostly averages out this strong dipole coupling and hence suppresses spin diffusion between neighboring protons. Molecular reorientation of the amine groups could provide a route to provide relaxation, but the long T1 observed here would indicate that reorientation is limited in this material.21

Figure 4. 1H EXSY spectra collected at mixing times (τ) of a. 2 ms and b. 500 ms. Spectra were each collected at a spinning rate of 50 kHz and a 10 s recycle delay The presence of two separate populations is further confirmed by the 1H EXSY experiments performed on this sample. This experiment probes the spin diffusion between chemically different sites in a molecule. The spectra produced by these experiments exhibit peaks at the 1:1 diagonal at short mixing times where little spin exchange is expected. At longer mixing times when 1H magnetization is transferred via spin diffusion; off diagonal peaks will occur between peaks that represent species that are in close spatial proximity to one another. These data were collected at a shorter, 10 s, recycle delay which allows for the



peaks at 0 to 5 ppm to be observed clearly without being obscured by the main LLM-105 peaks. At short mixing times, the 1H peaks for the LLM-105 spectra are well separated, and little exchange between the sites is observed (Figure 4a). At longer mixing times (Figure 4b, Figure S3), the peaks from 7 ppm to 9 ppm exhibit strong exchange cross peaks. Further, the peaks closer to 0 ppm still exhibit no exchange cross peaks which indicate they are well isolated from one another and those from the main peaks at 7 to 9 ppm. Those resonances occurring from 0 to 5 ppm are assigned to minor amounts of entrained solvent or other minor defects identified above. We did not attempt to assign these peaks further. We note here that contributions from defects and even polymer binders can be easily differentiated from the main explosive due to their vastly different relaxation properties. However, care should be taken when quantifying the amounts of defects or binder due to the extraordinarily long recycle delays needed to collect quantitative spectra for the explosive. Spectra collected at insufficiently short delays may provide the appearance that the defect or binder concentrations are too large.


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Figure 5. a. 1H MAS NMR and b-c. SIMPSON simulated lineshapes as a function of the indicated spinning rates. b. Simulation including both dipolar coupling and CSA. c. Simulation including only dipolar coupling. d. Simulation including only CSA. The red dashed lines are placed at the isotropic chemical shifts determined for the two main peaks at 60 kHz spinning rate



Figure 6. The 1H chemical shift for the two main peaks in LLM-105 plotted as a function of the MAS spinning rate. The blue circles are values measured from 1H MAS NMR and those in orange are from SIMPSON simulations.

Table 1: Spectral parameters derived from deconvolutions of the spectra collected as a function of MAS spinning rate compared. δH represents the position of the peak centers of mass and not necessarily the isotropic chemical shift. The peak near δH = 8.1 ppm was only observed in the two spectra collected at the highest spinning rates. Simulated values are derived from SIMPSON simulations of the spin system. Uncertainties in the δH measured and FWHM were less than the 0.1 ppm resolution of the measurement. Spinning Rate (kHz)

δH measured (ppm)

FWHM (ppm)

15

6.4 9.5 6.6 9.3 6.7 9.3 7.0 9.1 7.1 8.2 9.1 7.1 8.1

4.1 2.8 3.3 2.5 2.8 2.3 2.1 1.8 1.9 0.7 1.5 1.6 0.9

20 25 40 50 60

δH simulated (ppm) 6.4 9.6 6.7 9.4 6.7 9.3 6.9 9.2 7.0 9.1 7.1 -


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9.0

1.3

9.0

Figure 7. Slice taken at the indicated chemical shift from the 1H EXSY experiment collected at a 1 s mixing time.

Using the T1 measured for the LLM-105, we measured spectra as a function of the MAS spinning rate from 15 to 60 kHz. In each case, a series of saturation pulses were applied to remove any equilibrium magnetization and a single scan was acquired after a 1 hr delay. This procedure ensures a fully quantitative spectrum is collected. The results of these experiments are presented in Figure 5. We observe both a gradual sharpening and shifting of the two main peaks (Figures 5, 6, Table 1). Additionally, we observe the emergence of a third peak at 8.1 ppm in the 1H spectra collected at 60 kHz that is not apparent at slower spinning rates. Transects taken through the 1H EXSY data set collected at a 1 s mixing time show that all three peaks are in contact with one another through spin diffusion (Figure 7). Significant intensity can be observed in the slices taken at 9.1 and 7.1 ppm as well as these peaks showing prominently in the transect taken at 8.1 ppm. This result indicates that the peak at 8 ppm is a constituent of the LLM-105 and not an additional defect. In addition to the ability of NMR to detect defects in this material, the unique 1H NMR relaxation properties and lineshapes produced hint at interesting structural relationships of the amine protons in LLM-105. The long relaxation times and the lineshape changes due to increased spinning rates can be explained if we consider the structure of the amine protons in the LLM-105 structure (Figure 2). In this



structure both amine protons have strong intramolecular and intermolecular hydrogen bonds to neighboring oxygens. This strong network of hydrogen bonds significantly restricts the motion of these groups and explains the long relaxation times observed for these sites. Further, these two sites are located on average of 1.60 Å apart which produces an average 1H homonuclear dipole coupling of 29.3 kHz. This strong dipole coupling leads to the broad lineshapes that are observed despite the application of the fast MAS. However, the changes in the peak positions and the more complex lineshapes at lower spinning rates are due to the combination of large chemical shift anisotropies (CSA’s) with the homonuclear dipole coupling.31-33 These effects can cause the peak positions to strongly deviate from their true isotropic chemical shift at slower spinning rates. In order to ensure that the temperature effects due to increased frictional heating from the fast spinning rates used, we also measured these spectra as a function of temperature at a fixed spinning rate of 50 kHz. We note that no changes in either the peak widths or positions were observed over a wide range of temperatures (Figure S2).

Figure 8. Results from the 1H/1H anisotropic/isotropic correlation experiment for LLM-105. a. Full contour plot. The top projection is a corresponding spectrum collected at 50 kHz. b. Slices taken at 9.00 ppm (top) and 7.06 ppm (bottom) and corresponding SIMPSON simulations (orange lines) using the best fit parameters obtained from the variable spinning rate experiments.

The large CSA values are confirmed with the 1H/1H anisotropic/isotropic correlation experiment (Figure 8). This experiment is used to isolate the anisotropic contributions from the NMR chemical shift so that the CSA can be measured directly. These experiments have been useful to determine the structural


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relationships of organic components since the CSA is intrinsically linked to conformation of the protons in these materials. In the case of LLM-105, the more commonly used R1878 recoupling sequence was insufficient to measure the full lineshape in the indirect dimension due to the more limited spectral width (6.25 kHz) afforded by these sequences using a 50 kHz spinning rate.16, 20, 34 Instead, a R1245 sequence was required since it permits a larger, 10.0 kHz spectral width. Using SIMPSON, we have simulated the indirect spectra. The effects of 14N-1H dipolar coupling were considered in these simulations using a value of 11.4 kHz (corresponding to a 0.80 Å N-H distance). The best fit values for the CSA data for each of the three sites are presented in Table 2. As can be observed they have large anisotropies (ζ) and are consistent with prior work that indicates large CSA’s for strongly H-bound networks.35 The effects of the 1H-14N coupling are known to strongly distort these lineshapes and the application of 14N decoupling are known to ameliorate these issues.20 However, the current probe hardware used does not permit decoupling at the low 14N frequency. More measurements utilizing 14N decoupling combined with abinitio simulations would be required to provide crystallographic constraints on the amine proton positions.36-38 Table 2: 1H CSA parameters derived from SIMPSON fits of the CSA recoupling data. The δiso values were taken directly form the measured values. Root mean squared errors (RMS) of the fits are reported directly from the SIMPSON simulations δiso (ppm)

ζ (ppm)

η

RMS

7.1

41.4

0.8

12.7

8.0

38.6

0.8

16.9

9.0

38.6

0.8

11.3

An asymmetry parameter of 0.8 was found to best fit to the CSA data (Table 2). Most studies that report 1H

CSA parameters report values much closer to 0 corresponding to a more axial pattern that indicates a

localization of the spin density along the X-H bond.31, 36-39 In the case of LLM-105, the strong network of H-bonding present strongly delocalizes the spin density and produces the asymmetric CSA pattern. This delocalization also leads to the large anisotropies (ζ) also observed in this system. Work on peptides has also shown that the strong intramolecular hydrogen bonds also contribute to the increase in the anisotopy.40 The delocalization observed leads to the formation of strong inter and intramolecular bonds that provide the structural and chemical stability observed for this compound. It is likely that the breaking of these strong bonds contributes significantly to its overall performance as a high explosive material.



We used the SIMPSON package to simulate the lineshapes for this system as a function of the MAS spinning rate (Figure 5). Using the isotropic chemical shifts at 60 kHz, the average homonuclear dipole coupling, an average H-N-H bond angle of 123.8° from the crystal structure,5 and the CSA parameters obtained from the above measurements we can produce a series of spectra as function of the MAS spinning rate. Plotting the centers of mass for these two peaks from the SIMPSON simulations, we find excellent agreement with their positions and the overall trend displayed to those obtained experimentally (Figure 6, Table 1). The peak at 8 ppm was not considered in these simulations since it is obscured by the two prominent peaks at most spinning rates. To probe the interaction between CSA and dipole coupling, the simulations were run again with the same parameters but with either the CSA or dipolar coupling values set to zero. These results indicate that lineshapes only arise through a combined effect of a large homonuclear dipolar coupling and a large CSA (Figure 5b, c). These results imply that despite a 2 ppm difference in their chemical shift, the peaks in the 1H NMR spectrum represent two different amine protons residing on the same functional group. The LLM-105 structure consists of long sheets that are each two molecules wide which are subsequently bound to neighboring sheets in a zigzag fashion. In this structure there are amines which lie in the plane of the sheet and those which lie on the edge of the sheet. The strong intermolecular bonds between the amines and neighboring oxygens provide the framework of this structure. These amine protons can be subsequently broken down by how they intramolecularly interact with neighboring oxygens. One site is H-bound to a neighboring N-oxide group and the other to a neighboring nitro group. In the work of Yamauchi and co-workers,10, 11 they illustrated a progressive increase in the 1H chemical shift as the NH···O bond distance decreases in peptides and polypeptides that mirrors the trends observed for that in the O-H···O system.41 Using this rational, we can assign the various chemical shifts we observe to specific amine protons that lie within the sheet and those on the edge of the sheet. Within the sheet those intramolecularly bound to nitro groups have an average N···O distance of 2.90 Å whereas those bound to N-oxide groups have an average N···O distance of 2.78 Å. We therefore assign these two sites to the 7.1 and 9.0 ppm sites, respectively, since these sites represent the majority protons sites in the structure. As far as the edge sites, both amine protons have a similar N···O distance of 2.83 and 2.84 Å for those bound to the nitro and N-oxide groups, respectively. Since these distances fall intermediate between the two other sites, we assign both sites to the peak at 8.2 ppm. Unlike the other sites, these are not differentiable since they occur at the same chemical shift. Conclusions:


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Here, we have demonstrated the ability of 1H Fast MAS NMR to elucidate chemical and structural properties of the high explosive LLM-105. The relaxation properties of this material allow the facile observation of impurities and defects in this material and will prove valuable for investigating the final plastic bonded formulations. Further, the strong network of hydrogen bonds in this material contributes to unique NMR characteristics that we have observed. The immobilization of the protons in the structure leads to extraordinarily long T1 relaxation times with motion so restricted that protons on the same amine group can exhibit up to a 2 ppm difference in their chemical shift. Further, the stability and formation of the crystalline structure of this material comes from a significant delocalization of the proton spin density among the various hydrogen bonds. Future studies utilizing solid-state NMR on these materials will provide valuable insight into the structure and function of this unique class of materials. Supporting Information: Temperature calibration as a function of sample spinning speed, variable temperature 1H NMR spectra for LLM-105, slices from the 1H EXSY NMR experiment, and Bruker pulse sequence for the CSA recoupling sequence used. Acknowledgements: The authors would like to thank Dr. Brian L. Phillips of Stony Brook University for his insight on the interpretation of the variable spinning rate experiments, and Dr Ayyalusamy Ramorthy of the University of Michigan and Dr. Rongchun Zhang of South China University of Technology for sharing the SIMSPON code for the anisotropic/isotropic correlation experiment. This work was prepared by LLNL under contract DE-AC52-07NA27344. References: 1. Pagoria, P., A Comparison of the Structure, Synthesis, and Properties of Insensitive Energetic Compounds. Propell. Explos. Pyrot. 2016, 41 (3), 452-469. 2. Zuckerman, N. B.; Shusteff, M.; Pagoria, P. F.; Gash, A. E., Microreactor flow synthesis of the secondary high explosive 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105). J. Flow Chem. 2015, 5 (3), 178-182. 3. Stavrou, E.; Riad Manaa, M.; Zaug, J. M.; Kuo, I. F. W.; Pagoria, P. F.; Kalkan, B.; Crowhurst, J. C.; Armstrong, M. R., The high pressure structure and equation of state of 2,6-diamino-3,5dinitropyrazine-1-oxide (LLM-105) up to 20 GPa: X-ray diffraction measurements and first principles molecular dynamics simulations. J. Chem. Phys. 2015, 143 (14), 144506. 4. He, W.-D.; Zhou, G.; Wong, N.-B.; Tian, A. M.; Long, X.-P., Intramolecular H-bonds in LLM105 and its derivatives: a DFT study. Compt. Theor. Chem. 2005, 723 (1), 217-222. 5. Averkiev, B. B.; Antipin, M. Y.; Yudin, I. L.; Sheremetev, A. B., X-ray structural study of three derivatives of dinitropyrazine. J. Mol. Struct. 2002, 606 (1), 139-146.



6. Clawson, J. S.; Anderson, K. L.; Pugmire, R. J.; Grant, D. M., 15N NMR Chemical Shift Tensors of Substituted Hexaazaisowurtzitanes: The Intermediates in the Synthesis of CL-20. J. Phys. Chem. A 2004, 108 (14), 2638-2644. 7. Hu, J. Z.; Facelli, J. C.; Alderman, D. W.; Pugmire, R. J.; Grant, D. M., 15N Chemical Shift Tensors in Nucleic Acid Bases. J. Am. Chem. Soc. 1998, 120 (38), 9863-9869. 8. Uldry, A.-C.; Griffin, J. M.; Yates, J. R.; Pérez-Torralba, M.; Santa María, M. D.; Webber, A. L.; Beaumont, M. L. L.; Samoson, A.; Claramunt, R. M.; Pickard, C. J.; Brown, S. P., Quantifying Weak Hydrogen Bonding in Uracil and 4-Cyano-4‘-ethynylbiphenyl: A Combined Computational and Experimental Investigation of NMR Chemical Shifts in the Solid State. J. Am. Chem. Soc. 2008, 130 (3), 945-954. 9. Goward, G. R.; Schnell, I.; Brown, S. P.; Spiess, H. W.; Kim, H.-D.; Ishida, H., Investigation of an N···H hydrogen bond in a solid benzoxazine dimer by 1H–15N NMR correlation techniques under fast magic-angle spinning. Magn. Reson. Chem. 2001, 39 (S1), S5-S17. 10. Yamauchi, K.; Kuroki, S.; Fujii, K.; Ando, I., The amide proton NMR chemical shift and hydrogen-bonded structure of peptides and polypeptides in the solid state as studied by high-frequency solid-state 1H NMR. Chem. Phys. Lett. 2000, 324 (5), 435-439. 11. Yamauchi, K.; Kuroki, S.; Ando, I., The amide proton NMR chemical shift and hydrogen-bonded structure of glycine-containing peptides and polypeptides in the solid state as studied by multi-pulseassociated high-speed MAS 1H NMR. J. Mol. Struct. 2002, 602-603, 9-16. 12. Landers, A. G.; Apple, T. M.; Dybowski, C.; Brill, T. B., 1H nuclear magnetic resonance of αhexahydro-1,3,5-trinitro-s-triazine (RDX) and the α-, β-, γ- and δ-polymorphs of octahydro-1,3,5,7tetranitro-1,3,5,7-tetrazocine (HMX). Magn. Reson. Chem. 1985, 23 (3), 158-160. 13. Mesaros, D. V.; Oyumi, Y.; Brill, T. B.; Dybowski, C., NMR investigation of the premelt phase of hexahydro-1,3,5-trinitroso-s-triazine. J. Phys. Chem. 1986, 90 (9), 1970-1973. 14. Smith, J. A. S.; Blanz, M.; Rayner, T. J.; Rowe, M. D.; Bedford, S.; Althoefer, K., 14N quadrupole resonance and 1H T1 dispersion in the explosive RDX. J. Magn. Reson. 2011, 213 (1), 98-106. 15. Wang, H.; Wang, Y.; Li, Y.; Liu, Y.; Tan, Y., Scale-up synthesis and characterization of 2,6diamino-3,5-dinitropyrazine-1-oxide. Defence Technology 2014, 10 (4), 343-348. 16. Pandey, M. K.; Malon, M.; Ramamoorthy, A.; Nishiyama, Y., Composite-180° pulse-based symmetry sequences to recouple proton chemical shift anisotropy tensors under ultrafast MAS solid-state NMR spectroscopy. J. Magn. Reson. 2015, 250, 45-54. 17. Guan, X.; Stark, R. E., A general protocol for temperature calibration of MAS NMR probes at arbitrary spinning speeds. Solid State Nucl. Mag. 2010, 38 (2), 74-76. 18. Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.; Goodfellow, R.; Granger, P., NMR Nomenclature: Nuclear Spin Properties and Conventions for Chemical Shifts: IUPAC Recommendations 2001. Solid State Nucl. Mag. 2002, 22 (4), 458-483. 19. Bak, M.; Rasmussen, J. T.; Nielsen, N. C., SIMPSON: A General Simulation Program for SolidState NMR Spectroscopy. J. Magn. Reson. 2000, 147 (2), 296-330. 20. Pandey, M. K.; Nishiyama, Y., Determination of NH proton chemical shift anisotropy with 14N– 1H heteronuclear decoupling using ultrafast magic angle spinning solid-state NMR. J. Magn. Reson. 2015, 261, 133-140. 21. Hayashi, S.; Jimura, K., Detailed mechanisms of 1H spin-lattice relaxation in ammonium dihydrogen phosphate confirmed by magic angle spinning. Solid State Nuclear Magnetic Resonance 2017, 87, 24-28. 22. Hayashi, S.; Jimura, K., Hydrogen Bond Networks in Cs2(HSO4)(H2PO4) As Studied by SolidState NMR. J. Phys. Chem. C 2017, 121 (23), 12643-12651. 23. Hayashi, S.; Jimura, K., Spin diffusion and 1H spin-lattice relaxation in Cs2(HSO4)(H2PO4) containing a small amount of ammonium ions. Solid State Nucl. Mag. 2017, 88, 15-21. 24. Bakhmutov, V. I., The 29Si T1 and T2 NMR relaxation in porous paramagnetic material SiO2MnO-Al2O3. Solid State Nuclear Magnetic Resonance 2008, 34 (4), 197-201.


Page 16 of 27

Page 17 of 27 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


25. Bakhmutov, V. I.; Shpeizer, B. G.; Prosvirin, A. V.; Dunbar, K. R.; Clearfield, A., On 29Si NMR relaxation as a structural criterion for studying paramagnetic supermicroporous silica-based materials: Silica-based materials incorporating Mn2+ ions into the silica matrix of SiO2-Al2O3-MnO. Solid State Nucl. Mag. 2009, 36 (3), 129-136. 26. Bakhmutov, V. I., Strategies for Solid-State NMR Studies of Materials: From Diamagnetic to Paramagnetic Porous Solids. Chem. Rev. 2011, 111 (2), 530-562. 27. Jia, Z.; Zhang, L.; Chen, Q.; Hansen, E. W., Proton Spin Diffusion in Polyethylene as a Function of Magic-Angle Spinning Rate. A Phenomenological Approach. J. Phys. Chem. A 2008, 112 (6), 12281233. 28. Sorte, E. G.; Abbott, L. J.; Frischknecht, A. L.; Wilson, M. A.; Alam, T. M., Hydrophilic domain structure in polymer exchange membranes: Simulations of NMR spin diffusion experiments to address ability for model discrimination. J. Polym. Sci. Pol. Phys. 2018, 56 (1), 62-78. 29. Bakhmutov, V. I., On Hahn-echo measurements of short 29Si T2 times in some silica-based materials. Solid State Nucl. Magn. 2009, 36 (4), 164-166. 30. Bakhmutov, V. I.; Shpeizer, B. G.; Prosvirin, A. V.; Dunbar, K. R.; Clearfield, A., Supermicroporous silica-based SiO2-Al2O3-NiO materials: Solid-state NMR, NMR relaxation and magnetic susceptibility. Micropor. Mesopor. Mat. 2009, 118 (1-3), 78-86. 31. Phillips, B. L.; Burnley, P. C.; Worminghaus, K.; Navrotsky, A., 29Si and 1H NMR spectroscopy of high-pressure hydrous magnesium silicates. Phys. Chem. Miner. 1997, 24 (3), 179-190. 32. Zilm, K. W.; Grant, D. M., Carbon-13 dipolar spectroscopy of small organic molecules in argon matrixes. J. Am. Chem. Soc. 1981, 103 (11), 2913-2922. 33. Harris, R. K.; Packer, K. J.; Thayer, A. M., Slow magic-angle rotation 13C NMR studies of solid phosphonium iodides. The interplay of dipolar, shielding, and indirect coupling tensors. J. Magn. Reson. (1969) 1985, 62 (2), 284-297. 34. Pandey, M. K.; Yarava, J. R.; Zhang, R.; Ramamoorthy, A.; Nishiyama, Y., Proton-detected 3D 15N/1H/1H isotropic/anisotropic/isotropic chemical shift correlation solid-state NMR at 70 kHz MAS. Solid State Nucl. Mag. 2016, 76-77, 1-6. 35. Berglund, B.; Vaughan, R. W., Correlations between proton chemical shift tensors, deuterium quadrupole couplings, and bond distances for hydrogen bonds in solids. J. Chem. Phys. 1980, 73 (5), 2037-2043. 36. Miah, H. K.; Bennett, D. A.; Iuga, D.; Titman, J. J., Measuring proton shift tensors with ultrafast MAS NMR. J. Magn. Reson. 2013, 235, 1-5. 37. Miah, H. K.; Cresswell, R.; Iuga, D.; Titman, J. J., 1H CSA parameters by ultrafast MAS NMR: Measurement and applications to structure refinement. Solid State Nucl. Mag. 2017, 87, 67-72. 38. Damron, J. T.; Kersten, K. M.; Pandey, M. K.; Mroue, K. H.; Yarava, J. R.; Nishiyama, Y.; Matzger, A. J.; Ramamoorthy, A., Electrostatic Constraints Assessed by 1H MAS NMR Illuminate Differences in Crystalline Polymorphs. J. Phys. Chem. Lett. 2017, 8 (17), 4253-4257. 39. Aliev, A. E.; Harris, K. D. M., Probing Hydrogen Bonding in Solids Using Solid State NMR Spectroscopy. In Supramolecular Assembly via Hydrogen Bonds I, Mingos, D. M. P., Ed. Springer Berlin Heidelberg: Berlin, Heidelberg, 2004; pp 1-53. 40. Pichumani, K.; George, G.; Hebbar, S.; Chatterjee, B.; Raghothama, S., Effects of hydrogen bonding on amide-proton chemical shift anisotropy in a proline-containing model peptide. Chem. Phys. Lett. 2015, 627, 126-129. 41. Yesinowski, J. P.; Eckert, H.; Rossman, G. R., Characterization of hydrous species in minerals by high-speed 1H MAS NMR. J. Am. Chem. Soc. 1988, 110 (5), 1367-1375.




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Fast Magic Angle Spinning Solid State 1H NMR Reveals Structural

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