Vapor Phase Infrared Spectroscopy and Anharmonic ab initio Fundamental Frequencies of Ammonia Borane Robert L. Sams, Sotiris S. Xantheas, Thomas A. Blake Pacific Northwest National Laboratory P. O. Box 999, MS K8-88 Richland, WA (PNNL is operated for the US Department of Energy by the Battelle Memorial Institute under contract DE-AC05-76RLO 1830.) 1

Acknowledgements: Thanks to Dr. Jerry Birnbaum and Dr. Thomas Autrey of PNNL for their interest in and support of this work. The experimental work was done in the Environmental Molecular Sciences Laboratory, a national scientific user facility that is sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. High-resolution spectral analysis being performed by Prof. Joe Nibler and students at Oregon State University. 2

Ammonia Borane: NH 3 BH 3 NH 3 BH 3 for hydrogen storage: 190 g H 2 /kg NH 3 BH 3 nNH 3 BH 3 (NH 2 BH 2 ) n + (n - 1) H 2 (NH 2 BH 2 ) n (NHBH) n + H 2 2(NHBH) n (NHB – NBH) x + H 2 (NHBH) n BN + H 2 Karkamkar, Ardahl, Autrey “Recent Developments on Hydrogen Release from Ammonia Borane.” Aldrich Chemical: Material Matters 2(2):    

Objectives: Under what experimental conditions is an adequate amount of ammonia borane vapor produced so that its infrared absorption spectrum can be recorded. Measure vapor phase fundamental band centers of ammonia borane at modest resolution. Use ab initio quantum chemistry techniques to calculate the structure and the fundamental band centers (with full anharmonic corrections) of ammonia borane. 4

Prior Work: Microwave Thorne, Suenram, Lovas “Microwave Spectrum, Torsional Barrier, and Structure of BH 3 NH 3.” J. Chem. Phys. 78:  -wave spectrum of nine vapor phase isotopic species. 1 meter static, Stark modulated cell,  C, GHz. Ethane-like structure, r s and r 0 structure parameters determined. Dipole moment 5.126(17) D. Torsional barrier about B – N bond, V 3 = 716(3) cm -1 for 11 BH 3 ND 2 H and V 3 = 702(3) cm -1 for 11 BD 2 HNH 3. 5

Prior Work: Infrared Matrix Isolation Smith, Seshadri, White “Infrared Spectra of Matrix Isolated BH 3 NH 3, BD 3 ND 3, and BH 3 ND 3.” J. Molec. Spectrosc. 45: Argon/ammonia borane (400 to 800:1) deposition on CsI window at liquid hydrogen temperature. Absorption spectrum 250 to 4000 cm -1. Assignment of eleven fundamentals based on C 3v symmetry: five A 1, one A 2 (torsion, IR inactive), six E fundamentals. Some low wavenumber assignments subsequently called into question. 6

PNNL Infrared Experiment: Data recorded using Bruker IFS 120HR spectrometer. Room temp. (22  C) sample of ammonia borane open to White cell with 68 m optical path. 7 Range (cm -1 )5000 – – – – 500 BeamsplitterKBr DetectorInSbHgCdTe Extrinsic SourceTungsten lampGlobar Resolution (cm -1 ) No. of Scans ApodizationBoxcarNorton Med.Boxcar Zerofill2222 Aperture (mm) Scan Vel. (kHz)40

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9 Ammonia borane powder Tube and valve on underside of White cell

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Computational Details: MP2 and CCSD(T) geometry optimizations with aug-cc-pVTZ basis set Harmonic frequencies at MP2 and CCSD(T) levels Full anharmonic calculations at MP2 level Add MP2 anharmonicities to CCSD(T) harmonic frequencies 15

Methods for Obtaining Anharmonic Spectra: 1.Higher Energy Derivatives Perturbative evaluation of cubic force constants to second order & semi-diagonal quartic constants to first order - V. Barone V Barone, J. Chem. Phys. 122, (2005) V Barone, J. Chem. Phys. 120, 3059 (2004) 2.Grid - Based methods (VSCF, CC-VSCF, VCI) RB Gerber & co-workers, S Carter, JM Bowman & co-workers RB Gerber and MA Ratner, Chem. Phys. Lett. 68, 195 (1979) J-O Jung and RB Gerber, J. Chem. Phys. 105, (1996) JM Bowman. J. Chem. Phys. 68, 608 (1978) S Carter, SJ Culik and JM Bowman, J. Chem. Phys. 107, (1997) 3.MCTDH (Multi Configuration Time Dependent Hartree) wavefunction propagation method - H.-D. Meyer M. H. Beck, A. Jäckle, G. A. Worth, H.-D. Meyer, Phys. Rep. 324, 1–105 (2000). 16

A test case: Ammonia Clusters MN Slipchenko, BG Sartakov, and AF Vilesov, SS Xantheas, J. Phys. Chem. 111, 7460 (2007) Calc. b Exp. a Cluster (NH 3 ) (NH 3 ) NH 3 a IR Spectroscopy inside He droplets b MP2/aug-cc-pVDZ anharmonic calculations

18 ModeSym. Mode Description Matrix Isolation Gas Phase This Work CCSD(T) MP2 anh Calc. Intensity km/mole 1 A1A1 Sym. N  H str  A1A1 Sym. B  H str A1A1 Sym. NH 3 def A1A1 Sym. BH 3 def A1A1 B  N str ? A2A2 Torsioninactive E Asym. N  H str E Asym. B  H str EAsym. NH 3 def EAsym. BH 3 def  EAsym. BH 3 rock EAsym. NH 3 rock  6482 Ammonia Borane ( 11 B) Fundamentals (cm -1 )

Vapor Pressure of Ammonia 22  C: 19 Using the 8 band strength of Dillen and Verhoeven, 1325 cm -2 /atm, gives p NH 3 BH 3 = Torr at 22  C. Using the 8 band strength of this work, 2123 cm -2 /atm, gives p NH 3 BH 3 = Torr at 22  C. Dillen, Verhoeven “The End of a 30-Year-Old Controversy? A Computational Study of the B-N Stretching of BH 3 NH 3 in the Solid State.” J. Phys. Chem. 107:

Conclusions: We were able to record the infrared spectrum of vapor phase ammonia borane for the first time. Long path length and room temperature are important for observing ammonia borane in the vapor phase. We were able to observe and assign seven of the eleven infrared active bands. The B–N stretch 5 band is very weak. Band origins assigned based on R Q 0 or Q-branch positions. Vapor pressure of ammonia borane at 22  C is estimated to be on the order of 0.07 – mTorr 20

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22 ModeSym. Mode DescriptionGas PhaseSolidCalc. 1 A1A1 Sym. N  H stretch A1A1 Sym. B  H stretch A1A1 Sym. NH 2 bend A1A1 B–N stretch A1A1 Sym. BH 2 bend A2A2 Torsion(763) B1B1 NH 2 out of plane wag B1B1 BH 2 out of plane wag B2B2 Asym. NH stretch B2B2 Asym. BH stretch B2B2 Asym. NH 2 rock B2B2 Asym. BH 2 rock(595) –704 Aminoborane ( 11 B) Fundamentals (cm -1 ) Gerry, Lewis-Bevan,Merer, Westwood “The Infrared Spectrum of Gaseous Aminoborane NH 2 =BH 2 : Location of the Fundamentals and Rotational Structure in the Band.” J. Molec. Spectrosc. 110:

Basic concepts: Higher-energy derivatives Third energy derivatives with respect to normal coordinates,  ijk, are evaluated by numerical differentiation of the analytical second derivatives,  ij, at small displacements  q according to: Only a few fourth energy derivatives are required for the calculation of the ro- vibrational energy levels. These are evaluated numerically from the second energy derivatives: V. Barone, J. Chem. Phys. 122, (2005)

Computational Cost Grid-based methods requires availability of E trivially parallelizable number of (ab-initio, force field) points on a grid (typically N grid ~ 8): Higher energy derivatives methods requires availability of (analytic) second derivatives of E easily parallelizable number of second derivative of E evaluations: (2N mode +1) spectroscopic constants in closed - form expressions (yield vibrationally averaged structures & rotational constants) D. A. Clabo Jr., W. D. Allen, R. B. Remington, Y. Yamaguchi and H. F. Schaefer III, Chem. Phys. 123, 187 (1988); W. D. Allen, Y. Yamaguchi, A. G. Császár, D. A. Clabo, Jr., R. B. Remington and H. F. Schaefer III, Chem. Phys. 145, 427 (1990). diagonal2-mode correlations3-mode correlations