Article pubs.acs.org/JPCC
Temperature Dependence of the Infrared Spectrum of Ammonia Borane: Librations, Rotations, and Molecular Vibrations A. Paolone,*,†,‡ F. Teocoli,§ S. Sanna,‡ O. Palumbo,†,‡ and T. Autrey∥ †
CNR-ISC, U.O.S. Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy Dipartimento di Fisica and §Centro di Ricerca Hydro-Eco, Sapienza Università di Roma, Piazzale A. Moro 5, 00185 Roma, Italy ∥ Pacific Northwest National Laboratory, 908 Battelle Blvd., Richland, Washington 99352, United States ‡
ABSTRACT: The absorbance of solid ammonia borane (AB) was measured in the energy range between 30 and 5000 cm−1 and in the temperature range between 10 and 300 K. The intramolecular vibrations and their evolution through the structural phase transition around Tt ≈ 220 K fairly agree with previous measurements performed by means of Raman spectroscopy. In addition, we observed new vibrations centered in the far-infrared range, which can be tentatively ascribed to rotations and librations. The number of such modes does not agree with the calculations based on the group theory for both the tetragonal and the orthorhombic crystal structure of AB. We suggest that such a discrepancy is due to local reduction of the crystal symmetry compared to the one reported by X-ray diffraction and used to compute the number of IR active modes.
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INTRODUCTION Ammonia borane (NH3BH3, in short AB) has attracted much attention in recent years due to its high hydrogen content (19.4 wt %)1,2 and the moderate dehydrogenation temperatures.3−5 However, there are still some drawbacks which should be overcome before AB could be used as a practical hydrogen storage material: the dehydrogenation kinetics below 85 °C should be improved, unwanted secondary gaseous products of the decomposition (such as borazine and diborane6) should be avoided, and the regeneration procedure of the spent material should be more energy efficient. Good improvements of the kinetics and suppression of the release of byproducts have been obtained by infiltration of AB in nanoporous silica.7 The interest in the study of AB is also motivated by the unusual network of dihydrogen bonds which is present in the material: the hydridic hydrogen attached to the boron acts as a hydrogen acceptor for the protonic hydrogen attached to the nitrogen.8 Indeed, the occurrence of dihydrogen bonding makes NH3BH3 a crystalline solid up to ∼370 K, while its isostructural molecule ethane (C2H6) melts around 89 K. A deeper knowledge of the dihydrogen bond network in AB would allow a better understanding also of the release of hydrogen, which occurs more easily in the liquid phase, where dihydrogen bond is disrupted, thus providing a way to tailor the release of hydrogen from ammonia borane. Moreover, AB presents a structural phase transition around Tt ≈ 220 K from a high-temperature tetragonal phase to a low T orthorhombic structure, which has been widely studied by means of diffraction measurements, anelastic spectroscopy, and NMR and Raman spectroscopy.9−17 According to diffraction data, below Tt, hydrogen atoms occupy three well-defined lattice positions around B and N atoms; on the other hand, above the phase transition, temperature disorder of the H © 2012 American Chemical Society
atoms should be considered in order to fulfill the crystal symmetry.9 The same measurements indicated that the intramolecular geometry changes little as a function of temperature, that the phase transition is accompanied by a rotation of the B−N bond parallel to the c-axis, and finally that it is apparent that the dihydrogen bonding network is significantly weaker in the tetragonal phase than in the orthorhombic one.9 In order to gain a better knowledge of the structure of solid AB, we performed infrared spectroscopy measurements of the intraand intermolecular vibrations of ammonia borane as a function of temperature. Indeed, infrared spectroscopy is a powerful technique to study rotations, librations, and molecular vibrations in solids. The previous studies of AB by means of infrared spectroscopy reported the vibration spectrum only at room temperature and above 500 cm−1.18−20 A detailed investigation of the vibrations of solid AB through the structural phase transition is based on Raman spectroscopy measurements and was reported by Hess et al. in ref 13. That contribution has become a benchmark for the subsequent studies of the vibration modes of solid AB. However, this investigation is concerned with the vibrations above 600 cm−1, and hence, it is limited to the intramolecular modes. Later on, ab initio molecular dynamics calculations21 pointed out that phonons occurring below 1000 cm−1 probe the energy transfer to the protonic and hydridic atoms, thus giving information about the dihydrogen bonds. In addition, Kathmann et al.21 provided evidence of the importance of anharmonicity for the low-energy vibration modes. Received: June 26, 2012 Revised: December 13, 2012 Published: December 17, 2012 729
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In the present work, we will show that the intramolecular vibrations measured by means of IR are in good agreement with those reported by means of Raman spectroscopy,13 as expected. In addition, in the far-infrared region we could observe the intermolecular vibrations, i.e., rotations and the librations. Some information about the dihydrogen bond network and the local structure of ammonia borane is inferred from those spectra.
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EXPERIMENTAL SECTION Ammonia borane (AB, NH3BH3) powder (99%) obtained from Aviabor was purified by vacuum sublimation. Small quantities of AB (4 to 7 mg) were pressed in a circular 13 mm die in order to obtain thin compacted samples with a thickness ranging between 50 and 90 μm. The infrared spectroscopy measurements were performed in transmission mode by means of a Bruker IFS 125 spectrometer at the AILES beamline at Soleil Synchrotron.22,23 The temperature was varied between 10 and 300 K by means of a cryorefrigerator Cryomec PT405. Various beamsplitters (mylars and KBr) and detectors (bolometer and MCT) allowed coverage of the infrared range between 30 and 5000 cm−1. A resolution of 0.1 cm−1 was used in the spectral range between 30 and 100 cm−1 and above 600 cm−1. In the intermediate spectral range, measurements were performed with a resolution of 1 cm−1.
Figure 1. Temperature dependence of the absorbance of NH3BH3 in the NH stretching range.
BH Stretching Region. In Figure 2, we reported the infrared absorption of AB as a function temperature in the BH stretching
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RESULTS AND DISCUSSION The report of the experimental data and their discussion will proceed in the following starting from high frequencies and moving toward lower energies. Indeed, the temperature evolution of the mid-infrared region, which is dominated by the intramolecular vibrations, was widely studied by Hess et al. by means of Raman spectroscopy.13 The group theory for molecular vibration (ref 13 and references therein) predicted that intramolecular vibrations are both Raman and IR active, and we will provide evidence that the temperature dependence of the mid-IR spectrum of AB fairly agrees with the Raman one. Incidentally, this agreement further validates the procedure we adopted to perform our measurements. We will discuss separately each region corresponding to each molecular vibration, taking in mind the attribution of spectral lines to molecular movements discussed in the benchmark ref 13. Finally, in the last part of the paper we will show the far-infrared spectrum of NH3BH3, with the attributions of the spectral lines to the torsions and the librations, and we will discuss the information which can be obtained about the local structure of NH3BH3. NH stretching region. The temperature dependence of the absorption of AB in the spectral region of the NH stretching vibrations (3100−3400 cm−1) is reported in Figure 1. At room temperature one can observe three large and intense peaks around 3180, 3253, and 3323 cm−1. On cooling, the same structures are preserved down to 220 K. At 215 K, just below the transition temperature to the orthorhombic phase, the peak at 3180 cm−1 splits into two components and on further cooling also the other peaks split. At 10 K, eight absorption bands centered at 3164, 3200, 3230, 3252, 3296, 3309, 3332, and 3341 cm−1 are shown by AB. Moreover less intense features around 3060, 3530, and 3540 are also visible. Hess et al.13 attributed the room temperature Raman active modes centered at 3248 and 3310 cm−1 to the NH symmetric and asymmetric vibrations respectively, and suggested that the peak at 3168 cm−1 is an overtone. The position of the Raman active modes reported by Hess et al. at T = 88 K fairly agrees with the position of the infrared active bands here reported for T = 10 K.
Figure 2. Absorbance of NH3BH3 as a function of temperature in the BH stretching range of frequencies.
region (2000−2600 cm−1). At 270 K, AB presents three broad peaks centered around 2117, 2281, and 2360 cm−1. The two modes at lower energies were attributed to the BH symmetric and asymmetric vibrations by the previous literature.13 Below the phase transition temperature, a low energy shoulder of the peak at ∼2100 cm−1, a weak mode at 2169 cm−1, and a high energy shoulder of the peak at ∼2400 cm−1 appear. At T = 10 K, one can identify absorption bands at 2103, 2120, 2169, 2231, 2261, 2291, 2313, 2344, 2380, 2433, and 2546 cm−1. Only five of them coincide with those reported in ref 13 as due to BH stretching. However, also in the Raman spectra reported13 one can clearly identify in this frequency range additional vibration modes which are not attributed to a definite movement of atoms. Deformations and Rocks. Hess at al.13 suggested that the atomic displacements associated to the deformations primarily consist of umbrella- and scissor-like movements; moreover, the deformation of the NH3 groups occurs at frequencies (1597 and 1377 cm−1 at room temperature) higher than the deformations of BH3 groups (1157 and 1187 cm−1, at ambient temperature). In the energy range of the NH3 deformations, a mode attributed to an overtone is observed by means of Raman spectroscopy around 1450 cm−1.13 The infrared spectra reported show similar 730
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absorption bands at 1160, 1173, 1182, and 1214 cm−1 are clearly visible. In the frequency range between 1000 and 1100 cm−1, one of the rocking modes of BNH is expected according to the previous literature.13 Indeed, at the highest temperature one can observe in Figure 4 a broad asymmetric band around 1072 cm−1. Below the phase transition, the peak splits at least into three components centered around 1014, 1055, and 1087 cm−1. A similar splitting of the rocking mode was reported by Hess et al. in ref 13.
features: at room temperature, one can observe (see Figure 3) a broad peak which can be decomposed into two components
Figure 3. Temperature dependence of the absorbance of AB in frequency range corresponding to the deformations of the NH3 groups.
centered around 1375 and 1393 cm−1, an absorption around 1452 cm−1, a weak band around 1546 cm−1, and a strong absorption around 1602 cm−1. Below the temperature of the phase transition, the two peaks at the lowest energies move toward 1368 and 1395 cm−1, with an unusual decrease of the peak frequency below T = 210 K. The band around 1450 cm−1 splits into two components, which at T = 10 K are centered around 1445 and 1467 cm−1. The peak which at 270 K is observed at 1546 cm−1 displaces toward higher energies below Tt reaching 1572 cm−1 at 10 K. The absorption centered around 1602 cm−1 at 270 K splits into three components at 1584, 1609, and 1622 cm−1 at the lowest temperature. The deformations of the BH3 groups occur at lower frequencies. According to the Raman measurements reported by Hess et al., at room temperature the umbrella and the scissor movements are centered around 1157 and 1187 cm−1, respectively.13 In the infrared spectra of Figure 4, at 270 K one can observe a broad peak at 1182 cm−1 and a less intense absorption around 1225 cm−1. The asymmetry of the band at 1182 cm−1 might be related to a double structure. Below the temperature of the phase transition, the peaks split and at 10 K
Figure 5. Temperature dependence of the absorbance of AB in the region of BN stretching movements and to the NBH rocking motions.
At T = 270 K, the absorption spectrum of ammonia borane presents two absorption peaks at 784 and 799 cm−1 whose position is in good agreement with those attributed to the 11BN and 10BN stretching movements (ref 13 and references therein). At T = 10 K, one can observe that the shapes of these peaks are asymmetric and are constituent with a splitting into four components at 795, 799, 811, and 814 cm−1, in agreement with the Raman modes.13 Moreover, at the highest temperature we measured an intense absorption at 726 cm−1, which splits upon cooling below Tt into four bands at 712, 720, 731, and 740 cm−1. The absorption at the lowest energy is very weak and consists of a shoulder of the adjacent peak. In the Raman spectrum, only three components could be resolved at T = 10 K, possibly due to the extremely low intensity of the fourth peak. Hess et al.13 attributed the peak around 726 cm−1 to the E component of the NBH rocking motion. The attribution of the spectral lines in the frequency range between 600 and 1000 cm−1 was the most debated in the literature: in the paper of Smith et al.,18 which studied matrix isolated ammonia borane, the stretching vibration of BN and the rock motion were ascribed to the bands around 970 and 600 cm −1 , respectively. However, other works on solid NH3BH313,24−26 identified the stretching and the rocking motion with the bands located around 780 and 720 cm−1, respectively. Indeed, ab initio calculations of the vibrations of ammonia borane indicate that the stretching frequency decreases by 200 cm−1 in the solid state due to the shortening of the BN bond.27 One should note that, besides the expected absorption bands which have been reported and widely studied in the previous literature, one can also observe in the infrared spectra reported in the present paper some peaks which do not correspond to the molecular vibrations already known, for example, the bands at 887, 923, 664, 693, 3050, and 3550 cm−1, which can be possibly
Figure 4. Deformations of the BH3 groups: dependence of the absorbance of AB on the temperature. 731
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ascribed to overtones or combination bands due to anharmonicity. Except for those lines, both the good agreement of the calculations of the number of intramolecular vibrations with the experiments, both below and above the phase transition, and the agreement of the solid state spectrum with that of the matrix isolated are remarkable. These facts suggest that the intramolecular characteristics are preserved in the crystal and are not altered by the crystal environment. Librations and Lattice Modes. The temperature dependence of the far-infrared spectrum of solid ammonia borane is reported in Figure 6. At room temperature, it presents a very
Figure 7. Detail of the absorbance of solid ammonia borane as a function of temperature.
Figure 6. Far-infrared absorbance of solid ammonia borane in the temperature range between 300 and 10 K.
broad peak with structures around 83, 208, and 345 cm−1. Above 100 cm−1, the structures remain practically unchanged on cooling down to 180 K. At 120 K, peaks centered around 120, 160, 250, and 360 cm−1 become clearly visible. At T = 10 K, one can observe absorption bands at 128, 141, 166, 209, 235, 252, 319, 337, 354, and 363 cm−1. The less intense structures present between 100 and 200 cm−1 are probably due to interferences caused by the diamond window of the synchrotron beamline. The structures previously enumerated cannot be attributed to interferences, due to their large intensity and to the lack of periodicity. The most unusual behavior in the infrared spectrum is displayed by the absorption located around 83 cm−1 at room temperature (see Figure 7). This peak is very broad at high temperature and remains unchanged when the sample is cooled down to 220 K (this is more evident in Figure 6 than in Figure 7 due to the reduced extension of the wavenumber scale in the latter). On further cooling, the peak shrinks, and at 210 K, a double structure with maxima at 77.9 and 80.1. cm−1 is detected. The two peaks move apart on further cooling, and at 195 K, the maxima are located at 77.7 and 80.7 cm−1. On decreasing temperatures, the two bands tend to come closer, and at 150 K, only one maximum at 79.8 cm−1 is visible. At lower T values, the peak displaces toward higher energies, and at T = 10 K, the position of the maximum is at 86.2 cm−1. For a more quantitative analysis of the data, we performed a fit of the far-infrared absorbance measurements at 300 (Figure 8) and 10 K (Figure 9) by means of the Lorenz model. At 300 K, three peaks are needed in order to reproduce the data between 50 and 450 cm−1 (see Table 1). At 10 K, we used the minimum
Figure 8. Absorbance data and best fit curve of the absorbance of ammonia borane at 300 K.
Figure 9. Comparison of the absorbance data and best fit curve of the absorbance of ammonia borane at 10 K.
number of Lorenz oscillator to fit the data between 50 and 400 cm−1, i.e., 11 (see Figure 9 and Table 1). The fit is acceptable, but more contributions should be included in order to better reproduce the tails of the main peaks and to include the small features above 400 cm−1. One can wonder about the physical origin of the vibrations observed in the far-infrared spectrum. The factor group analysis performed in the gas phase indicates that the molecular rotations 732
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180, 200, and 340 cm−1.21 Kathmann et al. again observed that the low frequency anharmonic modes are highly coupled to internal coordinated, and therefore, no simplifications of the modes into the clear spectral assignment found at higher frequencies are possible. The above-mentioned experiments provide indications of the existence of some Raman active vibrations centered in the farinfrared range. As the cell of AB lacks a center of symmetry, the infrared phonons are also Raman active. Moreover, INS measurements show that, in the far-infrared frequency range, AB possesses a quite complex vibration spectrum at low T. Dillen and Verhoeven27 calculated the vibration spectrum of AB in both the gas and solid phases. The crystal environment of each AB molecule was simulated with several models in which the number of surrounding molecules is progressively increased. It was shown that short-range interactions can change the vibration spectrum of solid AB, mainly the frequency of the BN stretching mode. Dillen and Verhoeven27 evidenced that two A2 modes corresponding to the torsions of the ammonia borane molecule should be observed at 188 and 328 cm−1 (for the case of the isotope 11B). The results of ref 27 in part contradict the calculation of the number of vibrations performed by means of the theory in ref 13. In view of the close similarity between the torsion frequency calculated in ref 27 and the frequency of two of the IR modes that we measured at room temperature, we can tentatively ascribe the vibrations centered around 215 and 340 cm−1 to the torsions of AB, and the mode around 84 cm−1 to a libration. The presently reported measurements indicate that the number of vibrations clearly detected in the far-infrared spectrum of AB, and which could be due to rotation and librational modes of the AB molecule, exceeds the number of vibrations calculated by means of the theory in ref 13, both above and below the phase transition. At T = 300 K, we detected three vibrations, which are not overtones of each other. Also in the case of the spectrum at T = 10 K, the high intensity of the (n > 11) modes at higher frequencies cannot be easily explained in a framework in which some of them could be interpreted as overtones. We suggest that such an increase in the number of modes observed is due to a local reduction of the crystal symmetry compared to that obtained by means of diffraction. A disagreement of local symmetry with the symmetry of the crystal obtained by means of diffraction data was reported also by Cho et al.:33 the electric field gradient tensor obtained by means of nuclear magnetic resonance measurements indicated a different symmetry of the ammonia borane molecules in the low temperature orthorhombic phase compared with diffraction data.33 Recent Raman spectroscopy measurements and DFT calculations taking into account the low temperature Pmn21 crystal cell indicated by diffraction data34 suggested that at 10 K one would expect 2A1+4A2+2B1+3B2 lattice modes. However, from symmetry considerations, the A2 modes are not infrared active.35 Therefore, an inconsistency of the group theory predictions and the present experimental results is still evident. It is interesting to note that the splitting of the vibration modes in the spectral range between 100 and 400 cm−1 occurs at temperatures much lower (T ≤ 160 K) than the temperature of the structural phase transition (Tt ≈ 220 K) and that the mode around 80 cm−1 has a temperature evolution which starts when the sample is cooled below the structural phase transition temperature, but it evolves in a complex way below Tt. The previous calculations reported in the literature21,32 pointed out that the phonons below 500 cm−1 are strongly influenced by the
Table 1. Frequency (in cm−1) of the Lorenz Oscillators Used to Fit the Absorbance Data at T = 300 and 10 K in the Range between 50 and 400 cm−1 T = 300 K
T = 10 K
84
87 128 141 175 209 234.5 256 319 337 359 380
215
340
should be present; however, they are not infrared active.18 In the solid phase, the factor group analysis of the tetragonal structure of AB13 predicts that the vibrational modes are all acoustic (therefore not observable by infrared spectroscopy). In addition, two libration modes with symmetry E and A2 are expected.13 In the orthorhombic phase, three phonons of symmetry A1, A2, and B2 and six librations of symmetry A1+2A2+2B1+B2 are predicted.13 The far-infrared spectrum of ammonia borane measured by means of infrared spectroscopy has not been reported until now. However, few experimental reports about the vibrations occurring in this frequency region are available, but in the major part of those works, a detailed investigation and discussion is lacking. Indeed, the minimum energy of the previous infrared studies is around 500 cm−1.18−20 On the contrary, some Raman spectra extend to very low frequency, and in many cases, some broad bands in the spectral region below 500 cm−1 are clearly visible, but poorly explained.26,28−30 Liu et al.29 observed at room temperature a broad peak centered around 100 cm−1, which upon cooling splitted into two lattice modes at 94 and 212 cm−1. Custelcean et al.26 were more interested in the pressure dependence of the Raman spectrum and reported that upon increasing the pressure up to 40 kbar a peak at 175 cm−1 develops. Actually, these authors26 did not comment on the fact that the spectrum at ambient pressure also displays a broad peak around 190 cm−1. Inelastic neutron scattering (INS) is also a valid technique to study the phonon density of states which does not suffer from the limitations due to selection rules, but, on the other hand, has a lower frequency resolution compared to both infrared and Raman spectroscopy. Allis et al.31 measured the INS spectrum at a fixed temperature of 30 K and found phonon bands at 93.5, 150.1, 170.9, 202.4, and 333.7 cm−1. Among them, the phonon at higher energy was attributed to the torsions of the AB molecule. A comparison of the INS spectrum of ammonia borane with ab initio molecular dynamics calculation was reported by Kathmann et al.32 These authors identified two rotation peaks centered around 340 and 200 cm−1. Moreover, some additional collective motion was found at lower frequencies. The visual examination of the modes below 210 cm−1 showed highly coupled whole molecule translations, torsions, and rotations, with no clearly dominant NH3 or BH3 motion. A more recent paper by Kathmann et al.21 reported the comparison of the INS spectrum with the calculated vibrational spectrum also considering anharmonic contributions. At 12 K, the INS of AB displays a very structured spectrum with bands evident around 100, 150 733
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(9) Hess, N. J.; Schenter, G. K.; Hartman, M. R.; Daemen, L. L.; Proffen, T.; Kathmann, S. M.; Mundy, C. J.; Hartl, M.; Heldebrant, D. J.; Stowe, A. C.; et al. J. Phys. Chem. A 2009, 113, 5723−5735. (10) Hoon, C. F.; Reynhardt, E. C. J. Phys. C 1983, 16, 6129−6136. (11) Penner, G. H.; Chang, Y. C. P.; Hutzal, J. Inorg. Chem. 1999, 38, 2868−2873. (12) Gunaydin-Sen, O.; Achey, R.; Dalal, N. S.; Stowe, A.; Autrey, T. J. Phys. Chem. B 2007, 111, 677−681. (13) Hess, N. J.; Bowden, M. E.; Parvanov, V. M.; Mundy, C.; Kathmann, S. M.; Schenter, G. K.; Autrey, T. J. Chem. Phys. 2008, 128, 034508. (14) Bowden, M. E.; Gainsford, G. J.; Robinson, W. T. Aust. J. Chem. 2007, 60, 149−153. (15) Yang, J. B.; Lamsal, J.; Cai, Q.; James, W. J.; Yelon, W. B. Appl. Phys. Lett. 2008, 92, 091916. (16) Paolone, A.; Palumbo, O.; Rispoli, P.; Cantelli, R.; Autrey, T. J. Phys. Chem. C 2009, 113, 5872−5878. (17) Paolone, A.; Palumbo, O.; Rispoli, P.; Cantelli, R.; Autrey, T.; Karkamkar, A. J. Phys. Chem. C 2009, 113, 10319−10321. (18) Smith, J.; Seshadri, K. S.; White, D. J. Mol. Spectrosc. 1973, 45, 327−337. (19) Sams, R. L.; Xantheas, S. S.; Blake, T. A. J. Phys. Chem. A 2012, 116, 3124−3136. (20) Zhang, J.; Zhao, Y.; Akins, D. L.; Lee, J. W. J. Phys. Chem. C 2010, 114, 19529−19534. (21) Kathmann, S. M.; Mundy, C. J.; Schenter, G. K.; Autrey, T.; Aeberhard, P. C.; David, B.; Jones, M. O.; Ramirez-Cuesta, T. J. Phys. Chem. C 2012, 116, 5926−5931. (22) Mathis, Y.-L.; Roy, P.; Tremblay, B.; Nucara, A.; Lupi, S.; Calvani, P.; Gerschel, A. Phys. Rev. Lett. 1998, 80, 1220−1223. (23) Roy, P.; Cestelli Guidi, M.; Nucara, A.; Marcouille, O.; Calvani, P.; Giura, P.; Paolone, A.; Mathis, Y.-L.; Gerschel, A. Phys. Rev. Lett. 2000, 84, 483−486. (24) Goubeau, J.; Ricker, E. Z. Anorg. Allg. Chem. 1961, 310, 123−142. (25) Sawodny, W.; Goubeau, J. Z. Phys. Chem. Neue Folge 1965, 44, 227−241. (26) Custelcean, R.; Dreger, Z. A. J. Phys. Chem. B 2003, 107, 9231− 9235. (27) Dallen, J.; Verhoeven, P. J. Phys. Chem. A 2003, 107, 2570−2577. (28) Nylén, J.; Sato, T.; Soignard, E.; Yarger, J. L.; Stoyanov, E.; Haussermann, U. J. Chem. Phys. 2009, 131, 104506. (29) Liu, A.; Song, Y. J. Phys. Chem. C 2012, 116, 2123−2131. (30) Trudel, S.; Gilson, D. F. R. Inorg. Chem. 2003, 42, 2814−2816. (31) Allis, D. G.; Kosmowski, M. E.; Hudson, B. S. J. Am. Chem. Soc. 2004, 126, 7756−7757. (32) Kathmann, S.; Parvanov, V.; Schenter, G. K.; Stowe, A. C.; Daemen, L. L.; Hartl, M.; Linehan, J.; Hess, N. J.; Karkamkar, A.; Autrey, A. J. Chem. Phys. 2009, 130, 024507. (33) Cho, L.; Shaw, W. J.; Parvanov, V.; Schenter, G. K.; Karkamkar, A.; Hess, N. J.; Mundy, C.; Kathmann, S.; Sears, J.; Lipton, A. S.; et al. J. Phys. Chem. A 2008, 112, 4277−4283. (34) Ziparo, C.; Colognesi, D.; Giannasi, A.; Zoppi, M. J. Phys. Chem. A 2012, 116, 8827−8832. (35) Kroumova, E.; Aroyo, M. I.; Perez-Mato, J. M.; Kirov, A.; Capillas, C.; Ivantchev, S.; Wondratschek, H. Phase Transitions 2003, 76, 155− 170.
dihydrogen bond network. Therefore, we propose that the occurrence of the reduction of symmetry of the AB molecules due to mutual interaction through the dihydrogen bonds, whose network changes as a function of temperature. Also, Cho et al.33 evidenced that the exchange rate of the NH3 group show an apparent discontinuity between 80 and 85 K. This fact may be connected to changes in the far-infrared spectrum presently reported, which displays the appearance of well-defined, fine structures below 120 K (see Figure 6). Further experimental and computational studies would be needed in order to investigate the changes of the dihydrogen bond network from room temperature down to 10 K.
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CONCLUSIONS The absorbance spectrum of solid ammonia borane in the midinfrared range is dominated by the intermolecular vibrations. Around 220 K, which is the temperature of the structural phase transition of AB, the absorption lines split because of the reduced symmetry of the low temperature orthorhombic phase, compared to the tetragonal high temperature structure. The infrared spectroscopy data of the intermolecular vibrations are in close agreement with the previous Raman spectroscopy measurements,13 which have become a benchmark in their field. The absorbance measurements also provide clear evidence for the appearance of intermolecular vibrations in the far-infrared range, which provide information about the dihydrogen bond network of NH3BH3. The number of such modes is not properly accounted for by previous group theory calculations. Moreover, the evolution and splitting of the intermolecular vibrations occurs well below the temperature of the structural phase transition. We suggest that our results can be interpreted as the evidence that the local symmetry of the AB molecules is lower than that proposed by diffraction data and that the dihydrogen bond network is changed as a function of temperature.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The present work was partially supported by the Project Industria 2015 “HYDROSTORE” funded by the Italian Ministry of Economic Development. We wish to thank P. Roy and J.-B. Brubach for assistance at the AILES beamline at Synchrotron Soleil.
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REFERENCES
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