Vapor Phase Infrared Spectroscopy and Ab Initio Fundamental

Article pubs.acs.org/JPCA

Vapor Phase Infrared Spectroscopy and Ab Initio Fundamental Anharmonic Frequencies of Ammonia Borane Robert L. Sams, Sotiris S. Xantheas, and Thomas A. Blake* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-88, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: The infrared absorption spectrum of ammonia borane vapor has been recorded between 3600 and 600 cm−1. Of the eleven infrared active fundamental vibrational modes, seven modes of NH311BH3 and four modes of NH310BH3 were observed. The spectra were recorded with sufficient resolution to observe the rotational structure of the bands, which allowed for preliminary least-squares fitting of the band origins and rotational constants. First-principles electronic structure calculations were performed to obtain anharmonic band origins and their intensities. The band assignments are discussed in relation to other spectroscopic techniques that have been previously used to study this molecule. A semi-empirical estimate of the vapor pressure of ammonia borane at room temperature (22 °C) was made and found to be ∼1 × 10−4 Torr. The assignment of the measured modes was aided by the calculated anharmonic frequencies and their infrared intensities. The combination of the CCSD(T) harmonic frequencies with the B3LYP anharmonicities, obtained from second-order vibrational perturbation theory, was found to produce an overall best agreement with the measured band origins.

I. INTRODUCTION Ammonia borane has garnered considerable interest in recent years as a chemical hydrogen storage material for vehicle fuel cells.1−6 It is attractive for this purpose because of its large gravimetric hydrogen density, chemical stability, and commercial availability.5,6 Researchers are studying several mechanisms for facilitating hydrogen release from the compound: solid state thermal decomposition,7−11 metal catalyzed dehydrogenation,12−18 ionic liquid catalyzed dehydrogenation,19 solution phase thermal decomposition,20,21 Lewis and Bronsted acidcatalyzed decomposition,22,23 and thermal decomposition from ammonia borane encapsulated in a support material.24−26 Detailed studies of these mechanisms have focused interest on a number of fundamental properties of the compound such as structure, dipole moment, charge transfer, internal rotation, force field, and how these properties are ultimately related to bulk thermodynamic values.1−6 These properties of the ammonia borane molecule, which is typically found in some complex environment (such as crystal, solvent, oligomer, etc.) and under a range of temperatures and pressures, have been studied using a wide variety of spectroscopic techniques such as Raman,27−33 infrared,34,35 NMR,36−40 inelastic neutron scattering (INS),41,42 X-ray43,44 and neutron diffraction,45−48 and bulk techniques such as inelastic spectroscopy,49 differential scanning calorimetry50 and thermogravimetry.7,11,51 Mass spectrometry has also been used to monitor the decomposition products of ammonia borane.51−53 Despite the intense current interest in this molecule and the vast armamentarium of techniques arrayed against it, only three gas phase absorption spectroscopic studies of ammonia borane © 2012 American Chemical Society

have been reported to date. In 1981, Suenram and Thorne reported a preliminary microwave study of 10B and 11B ammonia borane,54 and in 1983, Thorne, Suenram, and Lovas reported the microwave spectroscopy of nine isotopic species of ammonia borane.55 From this data they were able to determine substitution (rs) and zero point (r0) structures and a dipole moment of 5.216(17) D for the gas phase molecule. For two of its isotopomers, ND2H11BH3 and NH311BD2H, they were able to determine the torsional barrier about the N−B bond, V3, to be 2.047(9) and 2.008(4) kcal/mol, respectively. Their derived structure of the molecule is analogous to that of ethane, with which ammonia borane is isoelectronic, although there was insufficient information in their data set to determine whether the ground state structure is in a staggered or eclipsed conformation. Finally, in 1991, Vorman and Dreizler recorded microwave spectra of NH311BH3 and ND311BH3 with sufficient resolution to observe, for a limited number of transitions, the quadrupole splittings of these compounds in the gas phase.56 In the infrared, no gas phase spectrum of ammonia borane has yet been reported. Instead, much of the literature on this molecule refers back to the matrix isolation study of Smith, Seshadri, and White,35 wherein they recorded the vibrational spectra of NH3BH3, NH3BD3, and ND3BH3 in an argon matrix at liquid hydrogen temperatures with argon-to-ammonia borane mixing ratios of between 400 and 800 to 1. From the vibrational assignments of these isotopomers and an assumed Received: December 1, 2011 Revised: February 14, 2012 Published: February 15, 2012 3124

dx.doi.org/10.1021/jp2115753 | J. Phys. Chem. A 2012, 116, 3124−3136

The Journal of Physical Chemistry A

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II. EXPERIMENTAL AND THEORETICAL APPROACH Experimental Setup. The White cell used for these measurements is custom-made following a design by Olson et al.63 The cell is optically mated to a Bruker IFS 120HR Fourier transform spectrometer through a foreoptics box, which is separated from the White cell by a pair of KBr windows. The spectrometer and the foreoptics box were evacuated during the spectral measurements. Approximately 100 mg of ammonia borane powder was placed in the bottom of a 10 cm long, 1 cm diameter stainless steel tube. The tube was connected to a port on the cell by an O-ring seal, centering ring, and clamp. The White cell and sample tube remained at room temperature (22 °C) for the duration of the measurements. A high precision capacitance manometer connected to the cell was used to monitor the pressure during the measurements. The cell and sample were evacuated to less than 10−4 Torr by a diffusion pump for 1 h before the sample tube was valved off from the White cell and background spectra of empty cell were recorded. The sample tube valve was then opened, the vacuum valve closed, and sample scans were started. It took approximately 2 h from the time the sample tube valve was opened for the intensity of the ammonia borane transitions to reach a stable value (the cell’s volume is ∼114 L). The White cell path length for spectra recorded with the ammonia borane sample with isotopic distributions in natural abundance was 68 m, and the spectrometer’s setup parameters for these spectra are listed in Table S1 (contained in the Supporting Information). The White cell path length for spectra recorded with the 11Benriched sample was 92 m, and the spectrometer’s setup parameters for these spectra are shown in Table S2 (contained in the Supporting Information). Both the natural abundance sample and 11B-enriched sample were purchased from BoroScience International, Inc. Apart from pumping on the samples when connected to the White cell, no further purification of the samples was performed. The spectra shown in the figures are displayed as absorbance −ln(I/I0). To show a NH310BH3 absorbance spectrum, the NH311BH3 absorbance spectrum was subtracted from the corresponding natural abundance spectrum. Considerable effort went into removing ammonia, water, and carbon dioxide transitions64−66 from the spectra to enhance the appearance of the ammonia borane transitions. The molecules leading to these parasitic transitions arose from the sample, the interferometer chamber, the White cell, or some combination of the three. During the course of the measurements, amino borane (NH2BH2) transitions67 grew in, but were not subtracted from the absorbance spectra. Theoretical Details. The structure of NH3BH3 was optimized at the DFT,68,69 MP2,70 and CCSD(T)71,72 levels of theory using Dunning’s augmented correlation consistent basis sets of double (aug-cc-pVDZ) and triple (aug-cc-pVTZ) zeta quality.73,74 The B3LYP functional75 was employed in all DFT calculations. The minimum geometry corresponds to the staggered conformation as seen in Figure 1. Harmonic and anharmonic frequencies were computed for this geometry at all three levels of theory [DFT (B3LYP), MP2, and CCSD(T)]. The anharmonic corrections were estimated with second-order perturbation theory,76−81 which provides closed expressions for most of the spectroscopic constants needed for obtaining anharmonic frequencies. Starting from the analytical second derivatives of the electronic energy with respect to the nuclear coordinates, the third and semidiagonal fourth derivatives were

structure, the authors obtained a force field for the molecule. Since that time, a number of publications have presented ab initio treatments of the molecule either in some molecular environment or in the “gas phase.”57−61 A few of these publications have called into question some of the band assignments in the Smith, Seshardri, and White paper.35 For example, and perhaps most importantly, Smith, Seshardi, and White had assigned a band at 968 cm−1 to the N−11B stretch of NH311BH3.35 Dillen and Verhoeven calculated the structure and vibrational spectrum of ammonia borane in the gas phase, cluster, and crystal environments at the SCF and MP2 levels of theory with the 6-31G(d) basis set.59 With the aid of those calculations, these authors reassigned the experimental value of 603 cm−1 from the matrix isolation data set as the N−11B stretch (their calculated value for this mode was 539 cm−1). From a variety of measurements, it has been shown that the N−B stretching frequency of ammonia borane is a significant indicator of its molecular environment (see refs 3, 30, and 59 and the references therein). Charge transfer between the NH3 and BH3 moieties affects the N−B bond length and, in turn, the N−B stretching frequency.59 The experimental 11BH3 rocking vibration was reassigned to be 968 cm−1 in their work (calculated as 1011 cm−1 at the MP2 level of theory), and the NH3 deformation was reassigned from 1343 to 1301 cm−1 (calculated as 1325 cm−1 at the MP2 level). Because the vibrational band centers from the Smith, Seshardri, and White paper35 were quoted in so many subsequent papers as representing the isolated molecule, but were in fact from a matrix isolation experiment where interactions with the matrix (inductive, electrostatic, dispersive) come into play and affect the measured frequencies,62 and because some doubt still exists about their assignments,59 we have initiated a joint experimental−theoretical study to address these issues. In particular, we have performed spectroscopic measurements on vapor phase ammonia borane to determine the band centers of the fundamentals of the molecule using a sample with natural isotopic abundance and another sample that is enriched in 11B. We have conducted these measurements at resolutions of between 0.112 and 0.0035 cm−1 that allow for the observation of the rotational structure of the bands to determine preliminary band centers and upper state rotational constants. Further analysis of the high-resolution spectra will be reported in subsequent publications. In order to aid the assignments of the experimental spectra, we have also performed ab initio electronic structure calculations to obtain the anharmonic vibrational band origins. We relied on a scheme to obtain the harmonic frequencies at the Coupled Cluster Singles and Doubles with a perturbative estimate of the Triple excitations [CCSD(T)] level of theory combined with the anharmonicities (obtained from the higher energy derivatives) from density functional (DFT) and secondorder Møller−Plesset perturbation (MP2) calculations. Details of our calculations will be outlined in the next section. In this study we present the measured NH311BH3 band centers and rotational constants for 7 of the 12 fundamentals. Four bands are too weak to be observed under current conditions while the torsion is infrared inactive. We also report four band centers for NH310BH3. The calculated harmonic and anharmonic spectra of the gas phase ammonia borane are also presented and compared with the experimental results in an attempt to aid the assignment of the experimentally measured spectra. 3125



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the observed values during previous joint experimental− theoretical studies. As noted earlier, this approach requires the availability of analytic second derivatives of the energy as a function of the nuclear coordinates, a capability that is available in Gaussian 03 at the DFT and MP2 levels of theory. Our best estimate of the anharmonic band origins at the CCSD(T)/augcc-pVTZ level of theory (for which analytical second derivatives are not currently available) was based on the scheme ν[CCSD(T)] ≈ ω[CCSD(T)] + {ν[MP2] − ω[MP2]} (1)

Figure 1. Geometry and normal modes of ammonia borane. See Table 1 for the mode descriptions.

where ω and ν correspond to the harmonic and anharmonic frequencies, respectively. This scheme relies on the assumption that the anharmonicities are converged at a lower level of theory (in this case, MP2/aug-cc-pVTZ), and these estimates can be combined with the CCSD(T) harmonic frequencies to estimate anharmonicities. The validity of this assumption was tested by comparing the anharmonicities obtained at the B3LYP and MP2 levels of theory with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The DFT and MP2 geometry optimizations and harmonic and anharmonic calculations were carried out using the Gaussian 03 suite of electronic structure software103 at the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory. The CCSD(T) geometry optimizations and subsequent harmonic frequency calculations, based on the numerical evaluation of the first and second derivatives, were performed using the NWChem suite of electronic structure software104 at the Molecular Sciences Computing Facility at Pacific Northwest National Laboratory.

obtained by a finite difference approach following the method suggested by Barone and co-workers.82−84 This approach has been previously found85 to produce similar results with gridbased methods86−93 based on vibrational second-order perturbation theory (VPT2)94 or vibrational configuration interaction (VCI) expansions.95−99 Its application to the vibrational spectra of ammonia clusters100 as well as the band origins and spectroscopic constants for cyclobutane101 and perfluorocyclobutane102 resulted in an excellent agreement with

III. RESULTS The initial spectral measurements of ammonia borane were performed using a cell with a 20 cm path length hard mounted in the sample compartment of a Bruker IFS 120HR Fourier transform spectrometer. The cell was jacketed so that fluid from a temperature bath could be circulated through the jacket and temperatures up to 95 °C could be attained. We assumed that the ammonia borane powder had to be heated above room temperature to achieve sufficient vapor pressure to observe the

Table 1. Observed [11B/10B] and Best Calculated Anharmonic [at the CCSD(T)/aug-cc-pVTZ Level of Theory and MP2 Anharmonicities Using Eq 1, See Text] Fundamental Modes of NH311BH3 and Their Descriptiona observed mode ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12

sym.

description

a1 sym. N−H str. a1 sym. B−H str. a1 sym. NH3 def. 2ν12 resonating with ν3 a1 sym. BH3 def. a1 N−B str. a2 torsion e asym. N−H str. e asym. B−H str. e asym. NH3 def. e asym. BH3 def. e asym. BH3 rock e asym. NH3 rock

calculated

matrix isolationb

gas phase, this work 11B/10B

anharmonic frequency

intensity (km/mol)

3337 2340 1301

− 2298.861(8) 1288.6384(3)/1289.5c 1277.7231(7) 1177.56(3)/1183.5c − inactive 3417.81(6) 2405.58(2)/2418.14(2)c 1610.62(2)/1611.90(1)c − 1042.316(2) −

3321 2449 1297 1297 1208 630 252 3410 2392 1608 1165 1027 648

4 58 128

1052 603 inactive 3386 2415 1608 1186 968 ?

147 13 0 90 516 52 14 58 2

a

Calculated intensities are obtained at the MP2/aug-cc-pVTZ level. The intensities of the doubly degenerate modes have been multiplied by 2 based on the discussion in ref 105. bReference 35, as amended by ref 59. cNH310BH3 band center. 3126



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set (the rotational constants of the equilibrium structure were calculated from the half sums of the vibration−rotation interaction constants). Based on the microwave spectroscopy results,54,55 ammonia borane has an ethane-like structure, although it is not clear whether the ground state is in the eclipsed or staggered conformation. Both conformers of ammonia borane would have C3v symmetry, and so ro-vibrational selection rules alone would not distinguish between staggered and eclipsed forms. Ammonia borane is a prolate symmetric top with the a1 symmetry vibrational modes producing parallel band rotational structure with ΔJ = 0, ± 1, ΔK = 0 rotational selection rules, and the e symmetry modes producing perpendicular band rotational structure with ΔJ = 0, ± 1, ΔK = ± 1. Intensity alternation of the ro-vibrational transitions due to the spin statistics of the protons can be observed in the parallel and perpendicular bands for both the 10B and 11B species. To obtain the reported band centers and upper state rotational constants, a small subset of approximately a dozen ro-vibrational transitions for each band was fit to term value expressions for either a parallel band, E(J, K) = ν0 + BJ(J + 1) + (A − B)K2, or a perpendicular band, E(J, K, l) = ν0 + BJ(J + 1) + (A − B)K2 ∓ 2AζK. The ground state rotational term values E(J, K) = BJ(J + 1) + (A − B)K2 were calculated using the constants given in ref 55. For NH311BH3, B″ = 0.5843102(47) cm−1, and DJ″ = 1.62(32) × 10−6 cm−1; and for NH310BH3, B″ = 0.6022306(33) cm−1, and DJ″ = 1.67(22) × 10−6 cm−1. These measured constants55 are in good agreement with the calculated values that are listed in Table 2. It is not possible to determine the ground state A value (A″) for a symmetric top from either a parallel or perpendicular band analysis; instead we assume an A″ value and hold it fixed in the fits. The ground state A value for NH311BH3 was calculated from the ab initio structure described above as A″ = 2.4502 cm−1 (vibrationally averaged value) The ground state A value for NH310BH3 was estimated from the NH311BH3 structure to be A″ = 2.530 cm−1. With one exception (ν4), the ground state energies were held fixed in the fits. Even a cursory examination of the rotational structure of the bands showed a pattern far more complex than that expected from a semirigid molecule. This complexity is almost certainly due to internal rotation, and a detailed analysis of these thickets of lines will be saved for future publications. Only those transitions that we believe to belong to the lowest internal rotor state are assigned and fit here. The results of these fits are to be found in Tables S3 through S13 of the Supporting Information. The highest energy fundamental of ammonia borane (11B), which is the N−H asymmetric stretch of e symmetry (ν7), is predicted at 3410 cm−1 with medium absorption intensity, and the 0.05 cm−1 resolution natural abundance sample spectrum (Figure 2) exhibits a series of Q-branch band heads centered near 3420 cm−1. By measuring the areas under each of these band heads, we assign the one band head with the largest area to the rQ0 branch, the others toward lower wavenumbers as pQK and to higher wavenumbers as rQK, accordingly. A fit of these transitions gives the band origin as 3417.81(6) cm−1 and the upper state B value as 0.61(2) cm−1, A′ = 2.523(5) cm−1, and Aζ′ = 0.24(1) cm−1 (see Table S3). The next highest fundamental is the symmetric N−H stretch of a1 symmetry (ν1), and its calculated band center is 3321 cm−1. This band would be in the vicinity of the strong ν1 parallel band of ammonia centered at 3334 cm−1.64−66 Searching this region before and after subtracting off the

infrared spectrum over this short path length. At temperatures between 25 and 30 °C, we did not see ammonia borane bands. For temperatures above 40 °C, we did briefly observe bands from the molecule, but these were either quickly obscured by ammonia64−66 or amino borane (NH2BH2) bands,67 or the ammonia borane bands disappeared altogether. We found that a better approach was to keep the ammonia borane powder sample at room temperature and use a much longer path length. Subsequently, a variable optical path length White cell was interfaced to the spectrometer, and a path length of 68 or 92 m through the vapor phase sample (22 °C) was used to make the measurements. Of the 11 infrared-active fundamental bands of ammonia borane (11B),35,59 seven bands were observed and assigned using data from the natural abundance sample at room temperature. For some spectral regions, additional measurements were made using an 11B-enriched sample of ammonia borane in a manner that allowed the 11B-enriched spectrum to be subtracted from the natural abundance sample spectrum so that the 10B spectrum could be revealed. Four ammonia borane (10B) fundamental bands were observed and assigned in this manner. Table 1 gives the band center wavenumbers (cm−1) of the measured fundamentals, as well as the band centers from the Smith et al. paper,35 as amended by Dillen and Verhoeven.59 The table also lists the best ab initio anharmonic band centers and intensities as determined in this present work (vide inf ra). The irreducible representation of the vibrational modes of the C3v ammonia borane molecule is 5A1 + A2 + 6E. Of the 12 unique modes, 11 are infrared active. Figure 1 shows the minimum energy geometry (staggered conformation) and pictorial representations of the fundamental modes ν1 through ν12. The calculated equilibrium and vibrationally averaged (at T = 0 K, in parentheses) spectroscopic constants and structural parameters of ammonia borane are listed in Table 2 at various levels of electronic structure theory with the aug-cc-pVTZ basis Table 2. Equilibrium and Vibrationally Averaged (at T = 0 K, in Parentheses) Spectroscopic and Structural Constants of BH3NH3 Estimated at the B3LYP, MP2, and CCSD(T) Levels of Theory with the aug-cc-pVTZ Basis Seta

A (cm−1) B = C (cm−1) DJ (cm−1) DJK (cm−1) DK (cm−1) ZPE (harmonic), (cm−1) ZPE (fundamental), (cm−1) ZPE (anharmonic), (cm−1) r(B−N), Å r(B−H), Å r(N−H), Å α(H−B−N), deg. β(H−N−B), deg.

B3LYP/aug-ccpVTZ

MP2/aug-ccpVTZ

CCSD(T)/ aug-cc-pVTZ

2.4698 (2.4502) 0.5885 (0.5744) 1.678 × 10−6 5.717 × 10−11 7.747 × 10−12 15 284.4

2.4689 (2.4507) 0.5933 (0.5795) 1.546 × 10−6 1.222 × 10−12 3.131 × 10−14 15 499.5

2.4559 0.5896

14 715.6

14 981.0

15 050.2

15 258.6

1.6591 1.2070 1.0150 104.98 111.00

(1.6835) (1.2141) (1.0153) (104.90) (111.01)

1.6520 1.2068 1.0140 104.76 110.96

(1.6753) (1.2138) (1.0142) (104.71) (110.98)

15 349.8

1.6574 1.2109 1.0157 104.75 110.10

a

DJ, DJK, and DK correspond to the Wilson centrifugal distortion constants. 3127



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sharp peaks belong to the ν8 fundamental of the less abundant NH310BH3 species (see Figure 4, upper trace). Whereas the

Figure 2. Infrared absorption spectrum of the ν7 (e) asymmetric N−H stretch region of ammonia borane (natural abundance sample). The spectrum was recorded with an optical path length through the 22 °C sample of 68 m and a resolution was 0.05 cm−1. Ammonia transitions have been subtracted from this spectrum.

Figure 4. Expanded view of the ν8 region of ammonia borane. The top trace is of the NH310BH3 ν8 spectrum. This spectrum was obtained by recording the spectrum of an ammonia borane sample with natural isotopic abundance at 0.004 cm−1 instrument resolution, a 68 m path length at 22 °C, and then subtracting from it the spectrum of an ammonia borane sample that is enriched in 11B recorded at 0.004 cm−1 instrument resolution and 92 m path length at 22 °C (bottom trace).

ammonia lines in both the natural abundance and 11B-enriched sample spectra revealed no ammonia borane band. Given that the calculated absorption intensity of this band (4 km/mol) is so low, it is unobservable. Overall, our threshold for detecting ammonia borane bands with this experimental arrangement seems to be a calculated intensity of approximately 15 km/mol or higher. The ν8 (e) asymmetric 11B−H stretch fundamental is calculated to be at 2392 cm−1 and have the strongest absorption intensity of all the fundamentals (516 km/mol). [The calculated intensities for the doubly degenerate fundamental modes have been multiplied by 2 based on the discussion in ref 105. These “doubled” intensities are given in the rightmost column of Table 1.] This band was initially recorded with a resolution of 0.05 cm−1 (cf. Figure 3) for the natural abundance

rotational structure of each of the rQK and pQK bands in the ν8 NH310BH3 fundamental is collapsed, the rotational structure of the Q branches of the NH311BH3 ν8 fundamental is spread out (see Figure 4 lower trace). Assigning rQ0 as the most intense feature in the NH310BH3 ν8 spectrum (upper trace Figure 4) and fitting nine other rQK and pQK transitions, we get an estimate for the band origin of 2418.14(2) cm−1 with B′ = 0.602(6) cm−1, A′ = 2.515(2) cm−1, and Aζ′ = 0.373(5) cm−1 (see Table S4). Because of proton spin statistics, the ν8 rovibrational transitions of ammonia borane originating from ground state levels with K = 0, 3, 6, 9, ... are more intense than the K = 1, 2, 4, 5, ... transitions, and fifteen rQK = 0, 3, 6, 9 transitions were assigned and fit to yield a ν8, NH311BH3 band origin of 2405.58(2) cm−1 and B′ = 0.5812(2) cm−1, A′ = 2.464(1) cm−1, and Aζ′ = 0.295(6) cm−1 (see Table S5). On the red side of the ν8 band, a weak parallel band is observed at 2298.8 cm−1 (see Figure 3). The ν2 (a1) symmetric 11 B−H stretch band is calculated to be at 2449 cm−1, and if we assign the observed parallel band with the fit origin at 2298.861(8) cm−1 as ν2, this gives the largest percentage difference between observed and predicted band centers, ∼ 6.5%. The high-resolution spectra recorded in this range were searched for a parallel band that might be assigned to ν2 that is closer to the predicted anharmonic value, but none was found. Ten transitions in this band were fit to determine the band origin, B′ = 0.58510(6) cm−1, and (A′ − B′) − (A″ − B″) = 4(1) × 10−4 cm−1 (see Table S6). No spectral information for NH310BH3 was recovered for this band. The ν9 (e) asymmetric NH3 deformation band, recorded with the natural abundance sample and a 0.05 cm−1 resolution, is shown in Figure 5. The calculated origin of this band is at 1608 cm−1. The band was partially obscured by the strong ν2 water transitions,64−66 and these were subtracted from the experimental spectrum. This perpendicular band shows sharp Q-branch heads for both the 11B and 10B isotopomers, and these were fit to the perpendicular band term value expression. For NH311BH3, thirteen rQK and pQK transitions were fit to give the ν9 band origin at 1610.62(2) cm−1 with B′ = 0.582(5) cm−1, A′ = 2.457(1) cm−1, and Aζ′ = −0.705(3) cm−1 (see Table S7).

Figure 3. Infrared absorption spectrum of the ν8 (e) asymmetric B−H stretch region of ammonia borane (natural abundance). The ν2 (a1) symmetric B−H stretch is also shown. During the 2 h needed to record this spectrum, the ν10 (2564 cm−1) and ν2 (2495 cm−1) bands of amino borane67 were observed to grow in. The spectrum was recorded with an optical path length through the 22 °C sample of 68 m and an instrument resolution of 0.05 cm−1. Carbon dioxide transitions have been subtracted from this spectrum.

sample, and then recorded again with a resolution of 0.004 cm−1. The ν10 (2564 cm−1) and ν2 (2461 cm−1) bands67 from NH2BH2 grew in over the 2 h required to record the spectrum. The ν8 ammonia borane band region shows a series of strong Q-branch band heads centered at 2419 cm−1 on the high wavenumber side of what appears to be the center of the band. When the 11B-enriched spectrum was recorded with a 0.004 cm−1 resolution and subtracted from the high-resolution natural abundance sample spectrum, it was determined that the 3128



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Figure 5. Infrared absorption spectrum of the ν9 (e) asymmetric NH3 deformation region of ammonia borane (natural abundance sample). The spectrum was recorded with an optical path length through the 22 °C sample of 68 m and an instrument resolution of 0.05 cm−1. Water transitions have been subtracted from this spectrum.

Figure 7. Infrared absorption spectrum of ammonia borane between 980 and 1390 cm−1 where the 11B-enriched spectrum has been subtracted from the natural abundance sample spectrum. Both spectra were recorded with an instrument resolution of 0.0035 cm−1. The natural abundance sample spectrum was recorded with a path length of 68 m, and that of the 11B-enriched sample was recorded with a path length of 92 m, both at 22 °C. The resulting spectrum shows the NH310BH3 ν3 and ν4 fundamental modes. Because of the varying amounts of amino borane in the two recorded spectra, the ν4 mode of 11 B amino borane is still observed.67.

Thirteen transitions for NH310BH3 were also fit to give ν0 = 1611.90(1) cm−1, B′ = 0.608(3) cm−1, A′ = 2.5167(9) cm−1, and Aζ′ = −0.630(2) cm−1 (see Table S8). Again, the initial assignment of rQ0 for both isotopomers was given to the band head with the largest area. Two medium intensity parallel bands, ν3 (a1) and ν4 (a1), symmetric NH3 and BH3 deformations, respectively, and a weak perpendicular band, ν11 (e), the asymmetric BH3 rock, are shown in Figure 6. These bands were initially recorded with the

Figure 8. Expanded view of the spectrum shown in Figure 6 showing the band center region of the ν3 fundamental mode (1289 cm−1) along with a resonating state at 1278 cm−1. This resonating state is 2ν12.

− (A″ − B″) = 8(5) × 10−6 cm−1 (see Table S9). These constants were fit with 27 P- and R-branch, K″ = 3, 6, 9, 12 transitions. On the low wavenumber side of the gap, there is a band centered at 1277.7231(7) cm−1 and B′ = 0.579370(7) cm−1, and (A′ − B′) − (A″ − B″) = 4(2) × 10−5 cm−1 (see Table S10). Twenty P-, and R-branch K″ = 3, 6, 9 transitions were used in these preliminary fits. Comparing the integrated line intensities of transitions with the same rotational quantum number assignment, we found that the intensities of the transitions from the band on the high wavenumber side of the gap were approximately 10% larger than those on the low wavenumber side of the gap. Consequently, we assign the band on the high wavenumber side of the gap to ν3 and the band on the low wavenumber side of the gap to a resonating state, which can be either 2ν5 or 2ν12. On the basis of the computed origins and anharmonicities of the overtones of the 2ν5 and 2ν12 bands, we assign the observed frequency of 1277.7231(7) cm−1 to 2ν12. This conclusion was based on the following information: the calculated anharmonic (harmonic) band positions for 2ν5 are 1172 (1288) cm−1 (B3LYP/aug-cc-pVTZ) and 1255 (1363) cm−1 (MP2/aug-cc-pVTZ), whereas the corresponding values for 2ν12 are 1291 (1296) cm−1 and 1276 (1291) cm−1. The computed anharmonicity of the 2ν5 overtone is therefore ∼110 cm−1, whereas the value for 2ν12 is just ∼10 cm−1. The

Figure 6. Infrared absorption spectrum of ammonia borane (natural abundance sample) between 980 and 1390 cm−1 recorded with an instrument resolution of 0.0035 cm−1 and 68 m path length at 22 °C. The spectrum shows the ν3 (a1) symmetric NH3 deformation, the ν4 (a1) BH3 deformation, and the ν11 (e) asymmetric rock of ammonia borane. While recording this spectrum, the strong ν4 band (1337 cm−1) of amino borane grows in.67.

natural abundance sample, a resolution of 0.0035 cm−1, and a 68 m path length. The same region was scanned at the same resolution using the 11B-enriched ammonia borane sample and a 92 m path length. When this spectrum was subtracted from the natural abundance spectrum, the resulting 10B spectrum for this region had a low signal-to-noise ratio, so 10B band origins were estimated. The ν3 band center for NH310BH3 is estimated to be at 1289.5 cm−1. The NH310BH3 ν4 band center was estimated to be located at 1183.5 cm−1, the most intense feature in this region of the subtracted spectrum (see Figure 7). The high-resolution spectrum of the ν 3 NH 3 11 BH 3 fundamental appears to consist of two near-equal intensity bands: one is on the high wavenumber side of a gap (see Figure 8) between the bands, and it has a fit band center of 1288.6384(3) cm−1 and B′ = 0.579491(3) cm−1, and (A′ − B′) 3129



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abundance sample at 612 cm−1, but the signal-to-noise is so poor that the assignment is dubious and that “bump” appears to rise and fall with changes in the amount of amino borane in the White cell rather than with changes in the partial pressure of ammonia borane. We assign the bump to the ν12 band of amino borane, which Gerry et al.67 estimated to be at 595 cm−1. The ν10 (e) asymmetric BH3 deformation is calculated to be centered at 1165 cm−1 and have weak (14 km/mol) absorption. That band is probably buried under the ν4 fundamental. The intensity of the ν12 (e) asymmetric NH3 rock is calculated to be roughly a factor of 6 weaker than the ν5, N−B stretch and was not observed in either the natural abundance or 11B-enriched sample. Finally, the torsional ν6 (a2) mode, predicted at 252 cm−1, is infrared inactive. An order of magnitude estimate of the room temperature (22 °C) vapor pressure of ammonia borane can be made using a combination of experimental and ab initio results. The vapor pressure can be determined from the relationship

use of the MP2 anharmonicity, when combined with the CCSD(T)/aug-cc-pVTZ harmonic calculations, furthermore yield 2ν5 = 1238 cm−1 and 2ν12 = 1297 cm−1. The combined estimates at all three levels of theory produce much smaller deviations if the measured value is assigned to the 2ν12 rather than to the 2ν5 overtone. Coincidentally, the 2ν12 = 1297 cm−1 value is equal to the CCSD(T)/aug-cc-pVTZ frequency for the ν3 fundamental with MP2 anharmonicity correction. Fourteen ro-vibrational transitions in the NH311BH3 ν4 band were fit to the parallel band term value expression to give ν0 = 1177.56(3) cm−1, B′ = 0.5802(3) cm−1, and (A′ − B′) − (A″ − B″) = 4.7(4) × 10−3 cm−1 (see Table S11). The same transitions were fit by also allowing the lower state B value to vary in the fit. In this case, ν0 = 1177.57(2) cm−1, B′ = 0.5811(6) cm−1, (A′ − B′) − (A″ − B″) = 4.2(4) × 10−3 cm−1 (see Table S12) and B″ = 0.5849(4) cm−1, which is within 1.5 standard deviations of the microwave value.55 The ν11 NH311BH3 perpendicular band is weak (Figure 6), but 21 transitions were assigned and fit to give ν0 = 1042.316(2) cm−1, B′ = 0.57957(3) cm−1, A′ = 2.4974(2) cm−1, and Aζ′ = 0.8111(4) cm−1 (see Table S13). There was no evidence of the NH310BH3 ν11 band in the subtracted spectrum. The ν5 (a1), N−B stretch band origin is calculated to be centered at 630 cm−1 and have weak (13 km/mol) absorption intensity. We observed a “bump” (see Figure 9) in the natural

p NH BH (Torr) = [Band Area (cm−1) 3

3

× (760 Torr/atm)] /[Path (cm) × Band Strength (cm−2 /atm)]

The measured band area for the ν8 band is 2.09 cm−1, the path length is 6815 cm, and the band strength, as calculated in this work, is 2131 cm−2/atm (using 1.2187 × 103/295.15 to convert from km/mol to cm−2/atm at 22 °C). Using these values in the equation above gives a vapor pressure for ammonia borane of 0.0001 Torr at 22 °C. Using the band strength of Dillen and Verhoeven,59 1330 cm−2/atm, gives a vapor pressure of 0.00017 Torr at 22 °C. A reasonable order of magnitude estimate for the vapor pressure of ammonia borane at room temperature is ∼1 × 10−4 Torr. The observed (gas phase, this work) and calculated fundamental modes of 11BH3NH3 are listed in Table 1 together with the description of their symmetry and the atomic motions along the corresponding normal modes. These motions are schematically shown in Figure 1. Also listed are the observed fundamentals during earlier matrix isolation studies,35 as amended by Dillen and Verhoeven.59 The matrix effect was found to be on the order of ≤60 cm−1 for most fundamental

Figure 9. A spectrum between 550 and 810 cm−1 of a sample of ammonia borane at 22 °C in a White cell with an optical path of 68 m and an instrument resolution of 0.05 cm−1. The feature at 612 cm−1 is more likely associated with amino borane rather than ammonia borane.67

Table 3. Calculated Harmonic Frequencies (cm−1) of 11BH3NH3 at the B3LYP, MP2, and CCSD(T) Levels of Theory and Their Variation with Basis Seta aug-cc-pVDZ mode ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 a

B3LYP 3450 2433 1313 1181 641 257 3560 2486 1642 1179 1056 642

aug-cc-pVTZ

MP2 (1) (56) (118) (127) (12) (0) (31) (262) (26) (6) (28) (1)

3468 2472 1318 1200 652 262 3608 2535 1655 1207 1063 636

CCSD(T) (3) (60) (133) (143) (15) (0) (38) 266 (25) (9) (30) (1)

3426 2449 1317 1178 638 401 3558 2469 1605 1149 1006 553

B3LYP 3455 2445 1332 1195 644 256 3554 2492 1667 1193 1066 649

MP2 (1) (56) (113) (136) (13) (0) (32) (259) (27) (6) (28) (1)

3485 2480 1329 1209 681 260 3615 2547 1675 1218 1077 645

CCSD(T) (4) (58) (128) (147) (13) (0) (45) (258) (26) (7) (29) (1)

3466 2430 1353 1196 673 274 3580 2493 1679 1181 1061 656

Intensities (km/mol) are listed in parentheses. 3130



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Table 4. Calculated Anharmonicities (cm−1) of the Various Modes of 11BH3NH3 at the DFT (B3LYP) and MP2 Levels of Theory with the aug-cc-pVDZ and aug-cc-pVTZ Basis Sets

modes (calculated modes with anharmonic correction), except for the symmetric B−H (ν2) and symmetric (ν4) BH3 deformation, for which the effect is much larger, 109 and 156 cm−1, respectively. To this end, the accurate calculation of the anharmonic frequencies can aid in the assignment of these two modes. Our best estimated anharmonic frequencies are also listed in Table 1. These were obtained from the CCSD(T)/ aug-cc-pVTZ harmonic frequencies and the MP2/aug-cc-pVTZ anharmonic corrections via the scheme described in eq 1. We also indicate the IR intensities (km/mol) computed at the MP2/aug-cc-pVTZ level of theory. In general, there is good agreement between our best estimates for the frequencies and the gas phase fundamentals observed in this study. For instance, for the ν4 mode (symmetric BH3 deformation), the calculated anharmonic frequency is within 30 cm−1 from the observed gas phase fundamental and helps resolve the discrepancy between the current measurement and the previous matrix isolation studies. The only difference in the good agreement between the observed gas phase and calculated frequencies lies in the ν2 (symmetric B−H stretch) mode, for which the difference amounts to 150 cm−1. It is worth noting that our estimated frequency for ν10 lies within 21 cm−1 of the value previously obtained in the matrix isolation study. It is instructive to assess the accuracy of our best estimates for the frequencies by examining the convergence of the calculated results with both the level of electron correlation and the size of the basis set. Given the scheme that we relied upon to determine our best estimates for the frequencies (eq 1), we partition this assessment into the convergence of two parts: (i) the harmonic frequencies and (ii) the anharmonic corrections. Table 3 shows the variation of the harmonic frequencies at the B3LYP, MP2, and CCSD(T) levels of theory with the aug-ccpVDZ and aug-cc-pVTZ basis sets. Note that the basis set dependence is not the same for all frequencies. Overall, we find that there is a small change in the harmonic frequencies for the B3LYP and MP2 levels of theory upon increasing the basis set from aug-cc-pVDZ to aug-cc-pVTZ. The maximum differences amount to ∼25 cm−1 (B3LYP) and ∼30 cm−1 (MP2), with most differences lying in the 10−15 cm−1 range. Both typical and maximum differences are larger at the CCSD(T) level, for which most typical differences are on the order of ≤40 cm−1. There are, however, some notable exceptions. For instance, upon increasing the basis set, the ν6 (torsion) and ν9 (asymmetric NH3 deformation) harmonic frequencies decrease by 127 cm−1 and 74 cm−1, whereas the ν11 (asymmetric BH3 rocking) and ν12 (asymmetric NH3 rocking) increase by 55 cm−1 and 103 cm−1, respectively. It should be noted that for all four of the above cases, the CCSD(T)/aug-cc-pVTZ results are more in line with the corresponding values at the B3LYP and MP2 levels (with the same basis set), so the observed differences can be attributed to the use of the smaller aug-ccpVDZ basis set at the CCSD(T) level. The anharmonicities, computed as (ω − ν) for the various level of theory/basis set combinations, are listed in Table 4. Overall, we observe a very small variation (≤10 cm−1) between B3LYP and MP2 with the smaller (aug-cc-pVDZ) basis set. However, this difference increases for some cases to ∼20 cm−1 with the larger aug-cc-pVTZ set, and it has a very large (82 cm−1) difference for ν2, which changes from a positive (+63 cm−1) anharmonicity at the B3LYP to a negative one (−19 cm−1) at the MP2 level. Given these nonuniform changes, it is useful to further assess the performance of the various combinations of harmonic frequencies and anharmonic

aug-cc-pVDZ

aug-cc-pVTZ

mode

B3LYP

MP2

B3LYP

MP2

ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12

156 4 48 −6 47 20 178 107 64 23 34 7

159 5 46 −4 45 25 182 100 74 23 34 4

123 63 55 −11 47 45 171 104 64 18 37 13

145 −19 56 −12 43 22 170 101 71 16 34 8

corrections in producing the most accurate predictions (via eq 1) when compared with experiment. The relative errors, νcalculated − νmeasured, of the computed/estimated anharmonic frequencies with the various levels of theory and basis set combinations with respect to the measured gas phase fundamentals are listed in Table 5. The results of Table 5 suggest that the CCSD(T) harmonic frequencies when combined with the B3LYP anharmonicities produce an overall best agreement with the measured band origins.

IV. DISCUSSION Alton et al.106 measured the vapor pressure of a number of alkylamine boranes, ammonia triborane, and ammonia borane using a Knudsen cell. Their attempt to measure the vapor pressure of ammonia borane was, however, limited to two data points “because of the small weight loss” observed for this compound and could at best state that the upper limit of the vapor pressure of ammonia borane was 1 × 10−3 Torr at 25 °C. This upper limit dovetails with our semi-empirical estimate of the vapor pressure of 1 × 10−4 Torr at 22 °C. This also fits with our observation that when the White cell and the ammonia borane sample, both of which are evacuated and connected, are sealed off from the vacuum pump, the pressure in the White cell rises to about 1 × 10−3 Torr (22 °C) over an 8 h period. This value was determined after monitoring and taking into account the background pressure rise when the ammonia borane sample was not present. This was also done over an 8 h period. With the improved resolution used to record the spectra of ammonia borane presented here, compared to that of the matrix isolation study, assignment of the fundamental bands was greatly facilitated by the ability to identify parallel (ΔK = 0) and perpendicular (ΔK = ± 1) bands. For all the vibrational bands we were able to assign to ammonia borane and fit rotational transitions, the upper state B values were found to be ∼0.58 cm−1, close to the ground state B value as determined by microwave spectroscopy.54,55 The ab initio calculations of the band intensities and their anharmonicity-corrected wavenumber positions were also invaluable in assigning the bands. In an argon matrix, the vibrational bands of ammonia borane shift by varying amounts due to dispersive, electrostatic, and inductive interactions with the argon solvent;62 but after taking this into account, the fundamental assignments of the vapor 3131



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Table 5. Relative Errors (νcalculated − νmeasured) of the Calculated Anharmonic Frequencies from the Observed (Gas Phase, ν2, ν3, ν4, ν7, ν8, ν9 and ν11) Fundamentalsa aug-cc-pVDZ

aug-cc-pVTZ

Mode

B3LYP

MP2

CCSD(T)-h B3LYP-anh

CCSD(T)-h MP2-anh

B3LYP

MP2

CCSD(T)-h B3LYP-anh

CCSD(T)-h MP2-anh

ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12

−43 130 −24 9 −9

−28 168 −17 26 4

−67 147 −20 6 −12

−70 145 −18 4 −10

−5 83 −12 28 −6

3 200 −16 43 35

6 68 9 29 23

−16 150 8 30 27

−36 −27 −33 −30 −20

8 29 −30 −2 −13

−38 −44 −70 −60 −70

−42 −37 −80 −60 −70

−35 −18 −8 −11 −13

27 40 −7 16 1

−9 −17 4 −23 −18

−8 −14 −3 −21 −15

a For CCSD(T), two entries are shown: one based on the B3LYP, the other on the MP2 anharmonicities (-anh) added to the CCSD(T) harmonic frequencies (-h). For ν1, ν5, and ν10, the previous matrix isolation values are used; ν6 is IR inactive, and ν12 has not been measured.

matrix and determined that there were small amounts of polymeric species present, and they attributed some of the small absorptions in their infrared matrix isolation spectrum to these impurity compounds. So, for example, they observed three weak bands at 3337, 3224, and 3164 cm−1 and assigned the 3337 cm−1 band as the symmetric N−H stretch without rationale. The other two bands were assumed to be from polymeric impurities. Again the corresponding band in the Raman30 spectrum (3316 cm−1) is red-shifted by 70 cm−1 compared to the argon matrix value for this band. For amino borane, which has a C2v ethylene-like structure, Gerry et al.67 assign the bands at 3451 and 3533.8 cm−1 in the vapor phase spectrum to the N−H symmetric and asymmetric stretches, respectively. Carpenter and Ault108 made infrared matrix isolation measurements of amino borane in an argon matrix and assigned bands at 3437 and 3519 cm−1 as the N−H symmetric and asymmetric stretches, respectively. Smith et al.35 assigned the two modes at 2415 and 2427 cm−1 in the matrix as belonging to the ammonia borane 11B−H and 10 B−H asymmetric stretches, respectively. Again, these agree well with our assignment for the ν8 band in the vapor phase of 2406 (11B) and 2418 cm−1 (10B). The matrix isolation results for these bands are thus blue-shifted by 9 cm−1 for both isotopomers, relative to the vapor phase results. The Raman30 measurements give the 11B−H and 10B−H asymmetric stretches at 2328 and 2375 cm−1, respectively. The Raman results for these modes are red-shifted from the vapor phase values by 78 cm−1 (11B) and 43 cm−1 (10B). The ν8 band is calculated to be centered at 2392 cm −1 ( 11 B) when anharmonicity corrections are taken into account and it is also calculated to have the largest intensity of all the fundamental bands. Indeed, the signal-to-noise for this band is the largest for all of the bands we observed (Figure 3), and the measured band center is within 0.6% of the calculated value. What is interesting about this band is the considerable difference in appearance of the ν8 bands of the two isotopomers. The ν8 10B−H asymmetric stretch rotational structure (Figure 4, top trace) is very similar to that of the ν7 band (Figure 2) with a series of strong, evenly spaced Q-branch pile ups. The ν8 11B−H asymmetric stretch rotational structure (Figure 4, bottom trace), however, is spread out, suggesting that a strong perturbation is affecting the 11B−H mode.

phase spectrum also align reasonably well with those determined from the matrix isolation infrared spectra of Smith et al.,35 although reassignment of some of the bands was required. Care, of course, needs to be exercised when comparing the band positions obtained in other environments. The spectroscopy of ammonia borane in solid form crystalline30 or polycrystalline41is affected by the strong intermolecular protonic (NH) and hydridic (BH) interactions. The fact that ammonia borane is a solid at room temperature, compared to ethane with which it is isoelectronic, is evidence of the extent of these interactions. Considerable attention3,30,59 has also been given to the dative N−B bond length in different environments and how this affects the N−B stretch wavenumber. All these issues need to be kept in mind when comparing disparate spectroscopic results for this molecule. In their argon matrix isolation spectrum of ammonia borane, Smith et al.35 observed a strong peak at 3386 cm−1 that they assigned as the asymmetric N−H stretch. In recent Raman30 spectra of the room-temperature tetragonal phase crystal form of ammonia borane (referred to hereafter simply as the Raman spectrum), a band at 3316 cm−1 was assigned as the asymmetric N−H stretch. The Raman30 and matrix isolation35 bands assigned to this mode are red-shifted by 100 cm−1 and 30 cm−1, respectively, from the perpendicular band we observe at 3417.81(6) cm−1 (Figure 2) and assign to the asymmetric N−H stretch. This difference is reasonable since in the crystal considerable hydrogen bonding forces are at work, whereas in the matrix, the ammonia borane molecules are diluted, and interactions with the argon are expected to be smaller. The shift for the corresponding asymmetric N−H stretch in ammonia tetramer, (NH3)4, in a helium droplet100 relative to that in the monomer is only (3443 − 3392 =) 51 cm−1. This difference is (3443 − 3380 =) 63 cm−1 for liquid ammonia.107 In these cases, however, only the intermolecular forces are operant and not the strong interaction of the BH3 Lewis acid moiety. For the asymmetric N−H stretch, our calculated band center with anharmonic correction is 3410 cm−1, a frequency that is close to our measured value of 3418 cm−1. Similarly, our calculations give the symmetric N−H stretch band center for the “free” molecule at 3321 cm−1, but we do not observe this band. This is, however, consistent with the fact that its predicted intensity is about an order of magnitude weaker than the asymmetric N− H stretch. Smith et al.35 monitored the vapor effusing into the 3132



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The symmetric 11B−H ν2 stretch is calculated to be centered at 2449 cm−1 and to have an intensity approximately 9 times weaker than that of the ν8 band. In the matrix isolation measurements, this band was assigned to a peak at 2340 cm−1. The small parallel band at 2299 cm−1 in the vapor phase spectrum is much weaker than the ν8 band (Figure 3), and it is 150 cm−1 to the red of its calculated anharmonic position, 2449 cm−1. This is a large (6.5%) difference, but if we assume that the band at 2299 cm−1 is ν2, then the band from the matrix isolation35 results is 41 cm−1 to the blue, which is relatively close. The fit upper state B value for this band is 0.58510(6) cm−1, which is close to that of the ground state B value of NH311BH3, 0.5843102(47) cm−1. The Raman30 band for the symmetric B−H stretch is at 2279 cm−1 which is red-shifted only 19 cm−1 from the vapor phase value. A careful examination of the high-resolution spectra of this region did not reveal any other candidate parallel bands, so we tentatively assign the ν2 B−H symmetric stretch to the parallel band at 2299 cm−1. Gerry et al.67 assign a band at 2495 cm−1 in the vapor phase spectrum of amino borane to the ν2 B−H symmetric stretch of that molecule. Unfortunately, they do not show a detailed figure of this region in their spectrum. In our vapor phase spectrum of ammonia borane, we observe a band at 2461.2 cm−1 that appears to be a parallel band and has the correct rotational spacing to be associated with amino borane; we assign this to the ν2 band of amino borane. Carpenter and Ault108 assign a band at 2499 cm−1 in their argon matrix spectrum of amino borane to the ν2 band. Again they show no detail in their published spectrum of this region. We observe a perpendicular band centered at 2564.5 cm−1 in our vapor phase ammonia borane spectrum that we assign to the ν10 asymmetric B−H stretch of amino borane. Gerry et al. also observe this band at 2564 cm−1 in the vapor phase, and Carpenter and Ault observe this band at 2568 cm−1 in the argon matrix. We mention these amino borane bands because they do grow in while recording the ammonia borane spectra. The ν 9 NH 3 11 BH 3 and NH 3 10 BH 3 asymmetric NH 3 deformations have been observed in the vapor phase at 1611 and 1612 cm−1, respectively. In the matrix isolation measurements, a moderate intensity peak at 1608 cm−1 is assigned to ν9, a red shift of only 3 cm−1. A feature in the Raman30 spectrum is at 1600 cm−1, a red shift of only 11 cm−1. So there appears to be good agreement between the different experimental results for this deformation mode. The calculated anharmonic frequency for this mode is 1608 cm−1, also very close. The vapor phase ν9 band rotational structure for both the 11B and 10 B isotopomers is similar in appearance to the ν7 and the ν8 10 B perpendicular bands, with sharp Q-branch transitions being the most intense features. The ν3 symmetric NH3 deformation, as measured in the vapor phase at 1289 and 1290 cm−1 for 11B and 10B respectively, are close to the matrix isolation35 value of 1301 cm−1, a 12 cm−1 blue shift. Smith et al.,35 however, describe the strong band at 1301 cm−1 as being an asymmetric NH3 rock. In the vapor phase, the nearest strong band is at 1289 cm−1, which is clearly a parallel band (see Figure 6 and the rotational analysis in the Results section). They35 also assign a band at 1343 cm−1 in the matrix spectrum to the NH3 symmetric deformation fundamental, but this may be the ν4 fundamental of amino borane. Gerry et al.67 provide a detailed analysis of the amino borane ν4 N−B stretch band at 1334 cm−1, and we observe this band in our ammonia borane spectrum (Figure 6). Dillen and Verhoeven59 reassigned the 1301 cm−1 matrix

isolation band to the NH3 symmetric deformation based on their scaled ab initio calculation. Our calculated anharmonic band center for this mode is 1297 cm−1. Hess et al.30 correlate the 1343 cm−1 matrix isolation band with a Raman band in the tetragonal crystal at 1357 cm−1 as an NH3 symmetric umbrella mode, which in the description we use here would be a symmetric deformation. This Raman30 band is blue-shifted by 68 cm−1 from the vapor phase measurement. In their INS spectrum of polycrystalline ammonia borane, which covers 50− 1600 cm−1, Allis et al.41 observed a peak at 1367.5 cm−1 that they assign to the NH3 symmetric deformation. In the vapor phase spectrum of ammonia borane near ν3, we see a significant perturbation where a parallel band of comparable strength appears at 1278 cm−1 about 11 cm−1 to the red of ν3. This band is too close to ν3 to have been resolved in the matrix isolation or Raman crystal spectra. We believe that it may be a band that is resonating with ν3 and borrowing intensity from it; the overtone of ν5, the N−B stretch, would have the correct symmetry to Fermi resonate with ν3, and our calculated value of 630 cm−1 for ν5 would place its overtone (2 × 630 cm−1) at about the correct position. The calculated ν12 overtone (2 × 648 cm−1) would also be close in energy to ν3 and would have an A + E symmetry. On the basis of our calculated values of 2ν5 and 2ν12 with anharmonic corrections, the 2ν12 appears to be a more logical candidate for the resonating state. The ν4 BH3 symmetric deformation fundamental is observed at 1178 cm−1 in the vapor phase spectrum, and our calculated anharmonic band center is at 1208 cm−1. Smith et al.35 assigned a strong feature at 1052 cm−1 as this mode and a very strong feature at 1186 cm−1 as the BH3 asymmetric deformation. On the basis of our ab initio calculations, the BH3 asymmetric deformation (ν10) should be very weak, while the ν4 BH3 symmetric deformation fundamental is predicted to be strong. We would reassign the matrix isolation 1186 cm−1 band as the symmetric BH3 deformation and the 1052 cm−1 band as the ν11 asymmetric BH3 rock. Dillen and Verhoeven59 do not alter the Smith et al.35 assignment for these two bands, although their calculated intensities fall in line with ours. Hess et al.30 observe two bands in the Raman spectrum that they assign as correlating with the 1186 cm−1 matrix isolation band: one is a band at 1189 cm−1 that they assign as an E symmetry BH3 scissors mode, and the other is a band at 1155 cm−1 that they assign as an A symmetry BH3 umbrella mode. Allis et al.41 assign a peak at 1154.2 cm−1 in the INS spectrum to this mode. The lowest wavenumber band that we observe and assign to the vapor phase spectrum of ammonia borane is the ν11 asymmetric BH3 rock centered at 1042 cm−1 with a calculated anharmonic band center of 1027 cm−1 for this fundamental mode. Smith et al.35 assigned a moderate intensity feature in the matrix isolation spectrum at 603 cm−1 as being the asymmetric BH3 rock, but this seems to be too far to the red to be correct. Dillen and Verhoeven59 reassigned a weak feature at 968 cm−1 to this mode; we do not observe any spectral features in the vapor phase spectrum in the vicinity of 968 cm−1, so this may have been another impurity. The ν7 band of amino borane, which appears at 1005 cm−1 in the gas phase and at 962 cm−1 in the solid phase, may be a possible explanation for the band at 968 cm−1 in the matrix isolation spectrum.35 As stated above, we believe the 1052 cm−1 band should be assigned to the asymmetric BH3 rock in that spectrum. In the Raman spectrum of the tetragonal crystal, Hess et al.30 saw a band at 1065 cm−1 that they assign as an E symmetry NBH rock, and Allis et al.41 3133



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observed a band at 1049.8 cm−1 in the INS spectrum that they describe as an E symmetry BH3 rock. If the ν10 asymmetric BH3 deformation, calculated at 1165 cm−1, is present in our spectrum, it is buried among the transitions for the ν4 band, although ν10 is predicted to be too weak to be observed anyway. In the Raman30 spectrum of the crystal, a band is observed at 1189 cm−1, and in the INS41 spectrum a band is observed at 1177.4 cm−1; both of these are described as an E symmetry BH3 scissors mode. The ν12 NH3 asymmetric rock calculated to be at 648 cm−1, but very weak, is also not observed in our spectrum. Smith et al.35 assign a very strong band at 1301 cm−1 to this mode, but this choice is almost certainly not correct. From their calculation and analysis, Dillen and Verhoeven59 suggest that this band is not observed. The INS41 spectrum of polycrystalline ammonia borane shows a band at 725.8 cm−1 that is described as an E symmetry NH3 rock. Hess et al.30 assign a peak at 727 cm−1 in the Raman spectrum of the tetragonal crystal as belonging to an E symmetry “NBH” rock; perhaps this should have read “BNH” rock.30 As stated in the Results section, we do observe a very weak absorption at 612 cm−1. It is tempting to assign this to the N−B stretch, but the signal-to-noise here is very poor, and this feature’s intensity seems to rise and fall with bands associated with amino borane rather than those of ammonia borane. More likely, that feature is the ν12 BH2 rock band of amino borane, which is estimated to be centered at 595 cm−1 based on the perturbations of the ν4 band of that molecule.67 Dillen and Verhoeven59 assigned the 603 cm−1 band in the matrix isolation spectrum to the ammonia borane N−B stretch; this may be reasonable based on their calculated band center for this mode of 553 cm−1 and our calculated value of 630 cm−1. Hess et al.30 assigned a band in the Raman spectrum at 784 cm−1 to the N−11B stretch, but this bond length shortens considerably compared to that in the vapor phase, so this would not be a useful guide. Clearly, more experimental work is needed to bring out some of the weaker bands of ammonia borane by using even longer pathlengths and, for some of the higher wavenumber bands, adopting laser techniques.

was found that many of their assigned modes came within a few tens of wavenumbers of our vapor phase assignments. Recent Raman and INS results served to confirm the reasonableness of our vapor phase assignments. The vapor phase infrared spectrum should prove useful in further understanding this complex and important molecule.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S2 give the spectrometer and sample parameters used in the experiments. Tables S3−S13 give the wavenumbers and quantum numbers of assigned ro-vibrational transitions used for fitting the band origins and rotational constants. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Address: Pacific Northwest National Laboratory P.O. Box 999 Mail Stop K8-88 Richland, WA 99352. E-mail: ta.blake@pnnl. gov. Phone: (509) 371-6131. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A portion of this research was performed using the W. R. Wiley Environmental Molecular Sciences Laboratory sponsored by the Department of Energy’s Office of Biological and Environmental Research located at the Pacific Northwest National Laboratory. PNNL is operated for the United States Department of Energy by the Battelle Memorial Institute under contract DE-AC05-76RLO 1830. S.S.X. was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Sciences, U.S. Department of Energy. Computer resources were provided by the Office of Basic Energy Sciences, US Department of Energy at the National Energy Research Scientific Computing Center, a U.S. Department of Energy Office of Science user facility at Lawrence Berkeley National Laboratory. R.L.S. and T.A.B. would like to thank Drs. Thomas Autrey and Jerome Birnbaum of the Pacific Northwest National Laboratory for supporting the initial spectroscopic temperature studies of ammonia borane through PNNL Laboratory Directed Research and Development funding. We greatly appreciate their interest in this work.

V. CONCLUSIONS We have measured for the first time the vapor phase infrared absorption spectrum of ammonia borane. On the basis of a semi-empirical calculation, we estimate the vapor pressure of this molecule at room temperature to be on the order of 10−4 Torr. Attempts at raising the temperature of the ammonia borane sample and the spectroscopic cell only increased the decomposition of the sample and the ammonia borane spectrum was quickly obscured. The best strategy for observing the infrared spectrum of this molecule is to leave the sample and cell at room temperature and increase the path length as much as possible. Of the 11 infrared active fundamental modes, seven were observed for the 11B isotopomer and four for the 10 B isotopomer. The assignment of these modes was greatly aided by the prediction of the anharmonic mode positions and their calculated intensities. It was found that the CCSD(T) harmonic frequencies when combined with the B3LYP anharmonicities gave an overall best agreement with the measured band origins. When the MP2 anharmonicities were combined with the CCSD(T) harmonic frequencies, the agreement was similar except for the ν2 band. The matrix isolation infrared spectrum of ammonia borane recorded and analyzed by Smith, Seshardi, and White35 was also helpful; it

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