Mechanism of Solid-State Thermolysis of Ammonia Borane: A 15N

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Mechanism of Solid-State Thermolysis of Ammonia Borane: A 15N NMR Study Using Fast Magic-Angle Spinning and Dynamic Nuclear Polarization Takeshi Kobayashi,*,† Shalabh Gupta,† Marc A. Caporini,‡ Vitalij K. Pecharsky,*,†,§ and Marek Pruski*,†,∥ †

Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, United States Bruker BioSpin Corporation, 15 Fortune Drive, Billerica, Massachusetts 01821, United States § Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, United States ∥ Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States ‡

S Supporting Information *

ABSTRACT: The solid-state thermolysis of ammonia borane (NH3BH3, AB) was explored using state-of-the-art 15N solid-state NMR spectroscopy, including 2D indirectly detected 1H{15N} heteronuclear correlation and dynamic nuclear polarization (DNP)-enhanced 15N{1H} cross-polarization experiments as well as 11B NMR. The complementary use of 15N and 11B NMR experiments, supported by density functional theory calculations of the chemical shift tensors, provided insights into the dehydrogenation mechanism of ABinsights that have not been available by 11B NMR alone. Specifically, highly branched polyaminoborane derivatives were shown to form from AB via oligomerization in the “head-to-tail” manner, which then transform directly into hexagonal boron nitride analog through the dehydrocyclization reaction, bypassing the formation of polyiminoborane.

1. INTRODUCTION Chemical hydrides are promising media for efficient hydrogen storage due to their high gravimetric capacities.1−5 Among various chemical hydrides, ammonia borane (NH3BH3, henceforth referred to as AB) has continuously attracted significant attention owing to its particularly high hydrogen content of 19.6 wt %.6,7 Although pristine AB is stable under ambient temperature, ∼12 wt % of hydrogen is released below 200 °C in a solid-state thermolysis, reportedly via a multiple-step decomposition to acyclic polyaminoborane (PAB) (∼90 °C), polyiminoborane (∼140 °C), and finally, polyborazylene (at or above 150 °C).6,8−14 Arguments about the dehydrogenation mechanism of AB have often relied on the intermediates inferred from the weight loss of samples and volatilized species, yet details of these reactions have not been well understood. Direct analysis of remnant amorphous solid-state species, which is critical to obtaining further insights into the mechanism, can be best performed using solid-state nuclear magnetic resonance (SSNMR). Indeed, in several recent studies 11B SSNMR has been successfully employed to identify some of the intermediates during solid-state thermolysis of AB and its derivatives.12,13,15−17 For example, Stowe et al. explored the intermediates directly by using in situ SSNMR and revealed that diammoniate of diborane plays an important role in the initial dehyrogenation step around ∼90 °C.12 In most previous studies, only 1D magic-angle spinning (MAS) 11B NMR spectroscopy has been used. The resonance © XXXX American Chemical Society

frequencies in the MAS NMR spectra of quadrupolar nuclei such as 11B (I = 3/2) depend on both the chemical shifts (δcs) and quadrupolar-induced shifts (δQIS), which complicates the spectral assignments.18 15 N nuclei have spin I = 1/2, yielding simple spectra that are free from interferences from the quadrupolar effects. However, 15 N NMR is plagued by low sensitivity due to the low natural abundance (0.37%) and low gyromagnetic ratio (γN/γH = 0.10). These difficulties often interfere with spectroscopic access to the dehydrogenation products. One of the promising solutions to the 15N NMR experiments using natural abundance samples is offered by the indirect detection technique. For over two decades, the indirect detection of insensitive nuclei (i.e., 15N) via high-γ nuclei (mostly 1H) has been universally used in 2D heteronuclear correlation protocols, so-called idHetcor.19,20 Applications of this approach in SSNMR have been limited until recently because 1H nuclei could only be observed under high resolution by supplementing MAS with radio frequency (RF) multiple-pulse homonuclear decoupling or by partial deuteration. However, both strategies lower the sensitivity in idHetcor applications. (RF decoupling requires the use of stroboscopic observation, whereas deuteration dilutes the 1H spin network and can be difficult to achieve.) The development Received: May 2, 2014 Revised: August 4, 2014

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dx.doi.org/10.1021/jp504328x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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(CP). The DNP-enhanced 15N{1H} CPMAS spectra were obtained at B0 = 14.1 T using a Bruker BioSpin DNP NMR spectrometer, equipped with a triple-resonance low-temperature (∼100 K) MAS probe and a 395 GHz gyrotron. For the conventional SSNMR measurements, the samples were packed in MAS zirconia rotors in a glovebox under an argon atmosphere to minimize contact with the moisture. For DNP-enhanced SSNMR, the samples were quickly packed into 3.2 mm sapphire MAS rotors under ambient atmosphere and immediately soaked with a 10 mM bTbK (bis-TEMPObisketal, TEMPO = 2,2,6,6-tetramethylpiperidin-1-oxyl)39 solution in 1,1,2,2-tetrachloroethane by incipient wetness impregnation.40 Note that (i) AB is practically insoluble in 1,1,2,2-tetrachloroethane41 and (ii) the short exposure to ambient conditions during this process did not affect the spectra. The experimental parameters are given in the figure captions using the following symbols: νR denotes the MAS rate, νRF(X) is the magnitude of the RF magnetic field applied to X spins, τCP is the cross-polarization time, τRR is the rotary resonance recoupling, τRD is the recycle delay, Δt1 is the time interval of t1 during 2D acquisition, NS is the number of scans, and AT is the total acquisition time. The chemical shifts of 11B and 15N are reported in ppm referenced to boron trifluoride diethyl etherate and nitromethane, respectively. 2.3. Theoretical Calculations. The chemical shift tensors were theoretically calculated using the ORCA program package (ver. 2.8).42 The geometries of models were first optimized at B3LYP level with 6-311G(d,p) basis set. An IGLO-II basis set43 was then employed for calculating the shielding tensors. The computed shielding tensors were converted to chemical shifts relative to nitromethane and boron trifluoride diethyl etherate for 15N and 11B, respectively.

of fast MAS has led to improved spectral resolution in a straightforward manner. Ishii et al. reported the first application of an idHetcor experiment in solids over a decade ago, yielding two- to three-fold sensitivity enhancements in the naturally abundant 13C polymer and 15N-labeled peptides.21 The sensitivity enhancement by idHetcor even allowed the collection of 2D 1H−15N correlation spectra from the surface species without isotopic enrichment.22 In the studies of such species, the technique offered better signal-to-noise ratio (S/N) per unit time than the conventional 1D cross-polarization (CP)MAS, in addition to the correlation information.23 Dynamic-nuclear-polarization (DNP)-enhanced NMR is another rapidly developing technique, which drastically enhances the sensitivity by irradiating unpaired electrons near their electron paramagnetic resonance frequency and subsequently transferring the polarization to the observed nuclei.24 The potential sensitivity gain can be on the order of the ratio of the electron and nuclear gyromagnetic ratios, for example, up to ∼6500 for transfer to 15N spins. This type of hyperpolarization was theoretically predicted by Overhauser and confirmed by Slichter in the 1950s;25,26 however, its application has been limited by the lack of microwave sources compatible with modern high-field NMR. During the past decade, a series of advances, especially in the gyrotron technology,27,28 cryogenic MAS29,30 and the use of exogeneous sources of unpaired electrons (biradical polarizing agents),31−37 have enabled DNPenhanced SSNMR experiments at high magnetic fields. Herein, we apply the 2D 1H{15N} idHetcor and DNPenhanced 15N{1H} CPMAS NMR spectroscopies in addition to the 1D 11B direct polarized (DP) and 2D 11B triple-quantum (3Q)MAS NMR38 to the analysis of products and mechanisms involved in solid-state thermolysis of AB. We demonstrate that DNP offers a substantial sensitivity advantage over the conventional SSNMR methods, including the indirect detection under fast MAS, despite the fact that the DNP-enhanced 1H polarization must be propagated within the samples to reach nuclei at remote locations from the polarizing biradical molecules. We carry out theoretical calculations of 11B and 15 N chemical shifts to facilitate the interpretation of NMR data. Taken together, these spectroscopic and theoretical tools provide definite structures of hexagonal boron-nitrides resulting from decomposition of samples heated to 200 °C.

3. RESULTS AND DISCUSSION We first examine the 11B and 15N NMR spectra in sections 3.1 and 3.2, respectively. Then, we assign the signals based on both

2. EXPERIMENTAL SECTION 2.1. Sample Preparations. In a typical experiment, ∼2 g of AB (≥97 wt % purity, Sigma-Aldrich) was placed in a fusedsilica tube (∼0.6 in. i.d.) and heated to various temperatures between 100 and 200 °C under dynamic vacuum (5 × 10−2 Torr or better). Unless noted otherwise, all samples were equilibrated at the desired temperature until hydrogen release ceased and the pressure dropped to the predesorption level, which required 20 h at 100 and 125 °C and 10 h at 150 and 200 °C. To observe the transition states, we also heated the samples at 125 °C for only 3 h. The samples are denoted as ABx-y, etc., where “x” and “y” indicate the dehydrogenation temperature in degrees Celsius and exposure time, respectively. 2.2. Solid-State NMR Experiments. 1D 11B DPMAS and 2D 11B 3QMAS NMR spectra were obtained at B0 = 9.4 T using an Agilent DDR-2 spectrometer, equipped with a 3.2 mm MAS probe. The 2D 1H{15N} idHetcor experiments were performed at B0 = 14.1 T on a Varian VNMRS spectrometer equipped with a 1.6 mm FastMAS probe, using dipolar interaction for 1H → 15N and 15N → 1H polarization transfers

Figure 1. 11B DPMAS spectra of AB treated under various conditions, obtained at 9.4 T. The spectra were measured using νR = 16 kHz, νRF(11B) = 125 kHz, and νRF(1H) = 64 kHz for TPPM 1H decoupling, τRD = 2 s, and NS = 64. The spectra are normalized to a constant height. B



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Figure 2. 2D 11B 3QMAS spectra of AB treated under various conditions, obtained at 9.4 T. The spectra were measured using νR = 16 kHz, νRF(11B) = 125 and 10 kHz for hard and soft (Z-filter) pulses, respectively, and νRF(1H) = 64 kHz for CW 1H decoupling. The data were acquired in 128 rows with Δt1 = 12.5 μs, τRD = 2 s, and NS = 96. The asterisk denotes the spinning sideband.

Table 1. NMR Line Shape Parameters Obtained from 11B 3QMAS Spectra for AB Treated under Various Conditions signal

δiso (ppm)

δcs (ppm)a

δQIS (ppm)a

PQ (MHz)b

assignment

1 2 3 4 5 6 7

−37 −22 −10 −4 1 35 40

−37 −23 −12 −6 1 27 31

0 3 4 3 0 14 16

0.4 1.4 1.5 1.3 0.4 3.0 3.2

BH4− BH3N BH2N2 BHN3 BN4 B(−NBH)2(−NB2)c BH(−NBH)2c

Chemical shifts (δcs) and quadrupole induced shifts (δQIS) were obtained from the observed isotropic shifts δ and δiso.18 bPQ denotes the secondorder quadrupolar effect parameter and is given by PQ = CQ(1 + η2Q/3)1/2, where CQ and ηQ are the quadrupole coupling constant and the asymmetry of the electric field gradient tensor, respectively. cChemical structures of corresponding boron sites are shown in Figure 6a. a

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B and 15N experiments and theoretical calculations and discuss the dehydrogenation mechanism in section 3.3. Because the spectra for AB150-10h and AB200-10h were nearly identical, the latter are shown in Supporting Information. 3.1. 11B NMR. Figure 1 shows the 11B DPMAS NMR spectra of AB treated under various conditions. The AB100-20h sample yielded multiple signals between 0 and −40 ppm, which are attributed to tetrahedrally coordinated boron sites (BIV).44 In AB125-3h, the signals of BIV decreased, while a new signal

appeared in the range between 10 and 30 ppm, exhibiting a MAS-averaged second-order quadrupolar powder pattern that is typical for trigonally coordinated boron species (BIII). The signals of BIV further decreased with extended heating time (AB125-20h) and completely disappeared in AB150-10h. The 11B 3QMAS spectra of corresponding samples are shown in Figure 2, and the line-shape parameters obtained from the analysis of these spectra are listed in Table 1. As expected, the BIII sites exhibit much stronger quadrupolar C



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Figure 3. 2D 1H{15N} idHetcor spectra of AB samples, obtained at 14.1 T. The spectra were measured using νR = 25 kHz, νRF(1H) = 125 kHz during short pulses, νRF(1H) = 100 kHz during tangent ramped CP and TPPM 1H decoupling, νRF(1H) = 12.5 kHz during rotary recoupling, νRF(15N) = 75 kHz during CP, νRF(15N) = 100 kHz during short pulses and TPPM 15N decoupling, τCP = 3 ms, τRR = 16 ms, 64 rows with Δt1 = 25 μs, 40 scans per row, τRD = 30 s, and AT = 43 h.

interaction than BIV, as evidenced by the larger values of δQIS and the second-order quadrupolar effect parameter (PQ). The 3QMAS spectra provide additional information about the dehydrogenation products. First, the spectrum of AB100-20h resolved the BIV signal, which was convoluted in the 1D 11B DPMAS spectrum, into five peaks centered at around δcs = −37, −23, −12, −6, and 1 ppm (represented as 1 to 5, respectively). Second, two BIII sites at δcs = 27 and 31 ppm (referred to as 6 and 7) were detected in the samples heated at and above 125 °C. The signals from BIV sites decreased with increasing dehydrogenation temperature (with the peak at 1 ppm disappearing last of all), while the line shape representing BIII sites was not changed except for a slight decrease in peak 7. 3.2. 15N NMR. The 2D 1H{15N} idHetcor spectra of AB samples are shown in Figure 3. By using 1H detection, the spectra were measured without isotopic enrichment, albeit with a very long acquisition time of 43 h each. The spectrum of AB100-20h shows a broad 15N signal centered around −355 ppm (denoted as a), which is a typical chemical shift for tetracoordinated nitrogen atoms (NIV). In AB125-3h and AB125-20h, three signals appeared around −304, −288, and −269 ppm (b, c, and d, respectively), representing tricoordinated nitrogen atoms (NIII).45,46 Meanwhile, dehydrogenation

resulted in decline (in AB125-3h) and complete disappearance (in AB125-20h and AB150-10h) of signal a. The DNP-enhanced 15N{1H} CPMAS spectra of corresponding samples are shown in Figure 4. Note that the SEM micrograph of a partially dehydrogenated sample (AB150-10h, see Figure S2 in Supporting Information) shows a complex morphology with typical particle sizes of 1−8 μm. Therefore, the DNP-enhanced 1H polarization must be transported into the bulk of the material by 1H−1H spin diffusion before being transferred to 15N nuclei via 1H−15N cross-polarization. Such propagation of polarization can occur at considerable distances of hundreds of nanometers, as has been demonstrated in previous DNP studies of polymers, peptides, and microcrystalline solids.47−50 Given the size of the particles studied here, their DNP responses are biased toward the outer, subsurface regions. Nevertheless, the observed line shapes are very similar to the skyline projections of corresponding 1H{15N} idHetcor spectra onto the 15 N axis (which indicates chemical homogeneity of the sample), and their S/N per unit of experimental time was significantly improved. We note that the DNP sensitivity enhancement is affected by the experimental temperature, the presence of frozen solvent, paramagnetic effects provided by the radicals, morphology of an individual D



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attributable to shorter τRD delay under the DNP conditions. Undoubtedly, higher sensitivity gains can be achieved in these samples by further optimization of experimental conditions and improved sample formulations. 3.3. Signal Assignments and Dehydrogenation Scheme. The concomitant changes in the 11B and 15N spectra yield invaluable mechanistic insights into the solid-state thermolysis of AB. The 11B chemical shifts obtained by the analysis of 2D 11B 3QMAS spectra allow us to directly assign the signals 1−5 in AB100-20h to BH4, BH3, BH2, BH, and nonprotonated B sites, respectively.12,16,52,53 The BH2 and BH sites have been often attributed to BNxH4−x (x = 2, 3) by assuming the formation of B−N bonds; however, direct spectroscopic evidence of B−N connectivity is yet to be presented. Namely, 11B DPMAS NMR cannot discriminate between B−B and B−N structures, as demonstrated by the theoretical calculations of 11B chemical shifts (Figure 5a). In contrast with 11B NMR, the 15N chemical shifts are sensitive to the presence of N−N bonds. For the models without N−N bond, the computed 15N chemical shifts are in the range between −350 and −400 ppm (Figure 5b). All feasible structural motifs containing N−N bond(s) yield 15N shifts between −220 and −310 ppm (Figure 5c). Thus, the signal a in AB100-20h is unambiguously assigned to NIV sites without N− N bond, that is, NBxH4−x. The simultaneous appearance of NIII (b, c, and d) and BIII (6, 7) sites in AB125-3h and the absence of B−B and BB bonds in the final products (see later) strongly suggests the absence of B−B bond in AB100-20h. Accordingly, the signals 1∼5 are assigned to BNxH4−x (x = 0− 4), as listed in Table 1. These NMR results indicate that the dehyrogenation at 100 °C results from intermolecular oligomerization of AB in the “head-to-tail” manner during the first dehydrogenation state, followed by the intramolecular dehydrogenation at 125 °C. We note that the dominant 15N peaks b and c in AB125-3h and AB125-20h have very similar chemical shift values (−304 and −288 ppm) to those computed for hexagonal boron nitride (h-BN) structure A (Figure 6a) in the previous study by Gastreich and Marian,54 where the signals with chemical shifts similar to b and c are assigned to NH(−BN2)2 and NH(−BHN)(−BN2), respectively. Among ∼30 tricoordinated boron nitride model compounds studied by these authors using Hartree−Fock level theory, this is the only one that yielded the set of 15N chemical shifts observed in the present study. Our own DFT calculations of 15N chemical shifts performed for several selected structures produced very similar results, suggesting that hydrogen-saturated h-BN model A well represents the product of the dehydrocyclization of PAB derivatives. Further corroboration of this result is obtained from our theoretical calculations of 11B chemical shifts, which for model A yielded δCS = 24.4 ppm for B(−NHB)2(−NB2) and 28.5 ppm for BH(−NHB)2, in close agreement with the values measured for sites 6 and 7 (Table 1). Finally, we computed the chemical shifts for models involving clustered h-BN units of type A (Figure 6b,c). The chemical shifts of “cross-linking” nitrogen atoms, N(−BN2)3 (Figure 6b) and N(−BHN)(−BN2)2 (Figure 6c), are estimated to be −291 and −270 ppm, respectively. These resonances are in accord with the signals c and d. The final products suggested here have few BH sites, which is consistent with a previous study, showing that the creation of BH prior to the hydrazine treatment is indispensable for efficient regeneration of AB.55,56 It is also important to note that similar sets of BIII and NIII

Figure 4. DNP-enhanced 15N{1H} CPMAS NMR spectra of AB samples, obtained at 14.1 T. The spectra were measured at ∼100 K using νR = 10 kHz (untreated AB, AB100-20h) or 8 kHz (AB125-3h, AB125-20h, and AB150-10h); νRF(1H) = 100 kHz during short pulses; νRF(1H) = 55−60 kHz during linear ramped CP; νRF(1H) = 83 kHz during SPINAL-64 1H decoupling; νRF(15N) = 75 kHz during CP; τCP = 2 ms; τRD = 1.3 s (the T1 relaxation time measured for AB150-10h sample was ∼1 s, note that a similar τRD delay has been used in other studies utilizing 10 mM bTbK solution in tetrachloroethane51); NS = 2048 (untreated AB and AB100-20h), 8192 (AB125-3h), 16 384 (AB125-20h), or 40 960 (AB150-10h); AT = 45 min (untreated AB and AB100-20h), 3 h (AB125-3h), 6 h (AB125-20h), or 15 h (AB15010h).

Figure 5. Computed 11B (a) and 15N (b,c) chemical shifts for AB oligomer models. (b,c) Models without and with the N−N bond(s), respectively.

sample, as well as changes in the relaxation and crosspolarization behavior,23 yet the ultimate efficacy of DNP is best judged by comparing the spectral S/N ratio per unit time with one obtained using state-of-the-art conventional SSNMR. To this end, we notice that the S/N in the DNP spectrum of AB100-20h exceeds that of the 2D 1H{15N} idHetcor experiment, despite a factor of 60 shorter acquisition time. Taking into account the experimental time and the sample amounts, the DNP-enhanced SSNMR demonstrated an order of magnitude higher sensitivity per unit time than the 2D 1 H{15N} idHetcor experiment, with much of the gain being E



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Figure 6. (a) Hydrogen-saturated h-BN model, A, and (b,c) the derivative models which consist of two units of A. The chemical shift values (in ppm) result from our own DFT calculations.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The research was performed at the Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract no. DE-AC0207CH11358.

signals were observed in the samples heated between 120 and 200 °C. These last results shed light on the second step of dehydrogenation. First, the PAB directly transforms to the cyclic products, whose structure is reminiscent of h-BN, without yielding polyiminoborane. This type of reaction is well known as dehydrocyclization and is observed in various organic systems.57 Second, this transformation simultaneously occurs in multiple locations of the PAB chains. Third, once the BIV and NIV sites are depleted, further dehydrogenation between the h-BN-like units A hardly occurs below 200 °C. A small contribution from the BN4 site (signal 5) remained in AB150-10h. Most likely, the transformation from BN4 to the cyclic products needs higher activation energy than the dehydrocyclization because the BN bond must be cleaved.

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(1) Zuttel, A. Materials for Hydrogen Storage. Mater. Today 2003, 6, 24−33. (2) Sandrock, G. A Panoramic Overview of Hydrogen Storage Alloys from a Gas Reaction Point of View. J. Alloys Compd. 1999, 293, 877− 888. (3) Schuth, F.; Bogdanovic, B.; Felderhoff, M. Light Metal Hydrides and Complex Hydrides for Hydrogen Storage. Chem. Commun. 2004, 2249−2258. (4) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M. Complex Hydrides for Hydrogen Storage. Chem. Rev. 2007, 107, 4111−4132. (5) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B-N Compounds for Chemical Hydrogen Storage. Chem. Soc. Rev. 2009, 38, 279−293. (6) Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Acid Initiation of Ammonia-Borane Dehydrogenation for Hydrogen Storage. Angew. Chem., Int. Ed. 2007, 46, 746−749. (7) Smythe, N. C.; Gordon, J. C. Ammonia Borane as a Hydrogen Carrier: Dehydrogenation and Regeneration. Eur. J. Inorg. Chem. 2010, 509−521. (8) Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W. ThermalDecomposition of Ammonia-Borane. Thermochim. Acta 1978, 23, 249−255. (9) Sit, V.; Geanangel, R. A.; Wendlandt, W. W. The ThermalDissociation of NH3BH3. Thermochim. Acta 1987, 113, 379−382. (10) Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-Rossler, K.; Leitner, G. Thermal Decomposition of B-N-H Compounds Investigated by Using Combined Thermoanalytical Methods. Thermochim. Acta 2002, 391, 159−168. (11) Wolf, G.; Baumann, J.; Baitalow, F.; Hoffmann, F. P. Calorimetric Process Monitoring of Thermal Decomposition of BN-H Compounds. Thermochim. Acta 2000, 343, 19−25. (12) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. In Situ Solid State B-11 MAS-NMR Studies of the Thermal Decomposition of Ammonia Borane: Mechanistic Studies of the Hydrogen Release Pathways from a Solid State Hydrogen Storage Material. Phys. Chem. Chem. Phys. 2007, 9, 1831−1836. (13) Al-Kukhun, A.; Hwang, H. T.; Varma, A. Mechanistic Studies of Ammonia Borane Dehydrogenation. Int. J. Hydrogen Energy 2013, 38, 169−179. (14) Frueh, S.; Kellett, R.; Mallery, C.; Molter, T.; Willis, W. S.; King’ondu, C.; Suib, S. L. Pyrolytic Decomposition of Ammonia Borane to Boron Nitride. Inorg. Chem. 2011, 50, 783−792.

4. CONCLUSIONS The solid-state thermolysis of AB was explored using a combination of 11B and 15N SSNMR spectroscopy with theoretical calculations. Despite the good agreement between the measured and calculated 11 B chemical shifts, the intermediate and final products cannot be identified without the help of 15N chemical shift information. The 2D 1H{15N} idHetcor and DNP-enhanced 15N{1H} CPMAS spectra, in concert with theoretical calculations, revealed not only the structure of these products but also the reaction mechanism leading to their formation. Results presented here indicate that even though a variety of chemical hydride systems based on ammonia and its derivatives have been studied in the past by SSNMR, the use of 15N NMR spectroscopy, in particular, the DNP method, adds a new dimension to the field and opens up new research pathways, potentially adding critical insights into those systems.

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ASSOCIATED CONTENT

S Supporting Information *

SSNMR spectra of AB200-10h and the SEM micrograph of AB150-10h. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*T.K.: Tel: (515) 294-6823. E-mail: [email protected]. *V.K.P.: Tel: (515) 294-8220. E-mail: [email protected] *M.P.: Tel: (515) 294-2017. E-mail: [email protected]. Notes

The authors declare no competing financial interest. F



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