Vapor Phase Infrared Spectroscopy and Anharmonic ab initio Fundamental Frequencies of Ammonia Borane

Robert L. Sams, Sotiris S. Xantheas, Thomas A. BlakePacific Northwest National Laboratory

P. O. Box 999, MS K8-88Richland, WA 99352

(PNNL is operated for the US Department of Energy by the Battelle Memorial Institute under contract DE-AC05-76RLO 1830.)

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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.

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Ammonia Borane: NH3BH3

NH3BH3 for hydrogen storage: 190 g H2/kg NH3BH3

nNH3BH3 (NH2BH2)n + (n - 1) H2

(NH2BH2)n (NHBH)n + H2

2(NHBH)n (NHB – NBH)x + H2

(NHBH)n BN + H2

Karkamkar, Ardahl, Autrey. 2007. “Recent Developments on Hydrogen Release from Ammonia Borane.” Aldrich Chemical: Material Matters 2(2):6-9.

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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.

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Prior Work: Microwave

Thorne, Suenram, Lovas. 1983. “Microwave Spectrum, Torsional Barrier, and Structure of BH3NH3.” J. Chem. Phys. 78:167-171.-wave spectrum of nine vapor phase isotopic species.1 meter static, Stark modulated cell, 35-45 C,30-130 GHz.

Ethane-like structure, rs and r0 structure parameters determined.Dipole moment 5.126(17) D.

Torsional barrier about B – N bond, V3 = 716(3) cm-1 for 11BH3ND2H and V3 = 702(3) cm-1 for 11BD2HNH3.

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Prior Work: Infrared Matrix Isolation

Smith, Seshadri, White. 1973. “Infrared Spectra of Matrix Isolated BH3NH3, BD3ND3, and BH3ND3.” J. Molec. Spectrosc. 45:327-337.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 C3v symmetry: five A1, one A2 (torsion, IR inactive), six E fundamentals.Some low wavenumber assignments subsequently called into question.

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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.

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Range (cm-1) 5000 – 1800 2000 – 400 1388 – 980 2500 – 500

Beamsplitter KBr KBr KBr KBr

Detector InSb HgCdTe HgCdTe Extrinsic

Source Tungsten lamp Globar Globar Globar

Resolution (cm-1) 0.05 0.112 0.0035 0.05

No. of Scans 256 512 64 256

Apodization Boxcar Norton Med. Boxcar Boxcar

Zerofill 2 2 2 2

Aperture (mm) 3.15 5.00 2.00 3.15

Scan Vel. (kHz) 40 40 40 40

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9

Ammonia borane powder

Tube and valve on undersideof White cell

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Computational Details:

MP2 and CCSD(T) geometry optimizations withaug-cc-pVTZ basis setHarmonic frequencies at MP2 and CCSD(T) levelsFull anharmonic calculations at MP2 levelAdd MP2 anharmonicities to CCSD(T) harmonic frequencies

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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, 014108 (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, 10332 (1996)

JM Bowman. J. Chem. Phys. 68, 608 (1978)S Carter, SJ Culik and JM Bowman, J. Chem. Phys. 107, 10458 (1997)www.emory.edu/CHEMISTRY/faculty/bowman/multimode

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).

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A test case: Ammonia ClustersMN Slipchenko, BG Sartakov, and AF Vilesov, SS Xantheas, J. Phys. Chem. 111, 7460 (2007)

34053403.034493433.3

32933316.51

34063399.2

Calc.bExp.aCluster

32333256.524

34493444.63(NH3)3

32073251.024

33283309.833263317.81

34233435.134313451.434203453.834423454.83(NH3)2

31953216.124

33243335.81

34593443.13NH3

aIR Spectroscopy inside He droplets bMP2/aug-cc-pVDZ anharmonic calculations

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Mode Sym.Mode

DescriptionMatrix

IsolationGas PhaseThis Work

CCSD(T)MP2 anh

Calc.Intensitykm/mole

1 A1 Sym. NH str. 3337 3321 4

2 A1 Sym. BH str. 2340 2298.8 2449 58

3 A1 Sym. NH3 def. 1301 1281.8 1297 128

4 A1 Sym. BH3 def. 1052 1176.5 1208 147

5 A1 BN str 603 610? 630 13

6 A2 Torsion inactive inactive 252 0

7 E Asym. NH str. 3386 3419.2 3410 90

8 E Asym. BH str. 2415 2409.9 2392 516

9 E Asym. NH3 def. 1608 1613.8 1608 52

10 E Asym. BH3 def. 1186 1165 14

11 E Asym. BH3 rock 968 1042.4 1027 58

12 E Asym. NH3 rock 648 2

Ammonia Borane (11B) Fundamentals (cm-1)

Vapor Pressure of Ammonia Borane @ 22 C:

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/atm)mStrength(cBandPath(cm)

tm)(760Torr/a)Area(cmBand(Torr)p 2

1

3BH3NH

Using the 8 band strength of Dillen and Verhoeven,1325 cm-2/atm, gives pNH3BH3

= 0.00012 Torr at 22 C.

Using the 8 band strength of this work,2123 cm-2/atm, gives pNH3BH3

= 0.00007 Torr at 22 C.

Dillen, Verhoeven. 2003. “The End of a 30-Year-Old Controversy? A ComputationalStudy of the B-N Stretching of BH3NH3 in the Solid State.”J. Phys. Chem. 107:2570-2577.

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 RQ0 or Q-branch positions.Vapor pressure of ammonia borane at 22 C is estimated to be on the order of 0.07 – 0.120 mTorr

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THE END

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Mode Sym.Mode

Description Gas Phase Solid Calc.

1 A1 Sym. NH stretch 3451 3333 3416

2 A1 Sym. BH stretch 2495 2445 2530

3 A1 Sym. NH2 bend 1625 1613 1663

4 A1 B–N stretch 1337 1342 1345

5 A1 Sym. BH2 bend 1225 1130 1192

6 A2 Torsion (763) 738 789

7 B1 NH2 out of plane wag 1005 962 963

8 B1 BH2 out of plane wag 670 690 621

9 B2 Asym. NH stretch 3534 3460 3399

10 B2 Asym. BH stretch 2564 2525 2583

11 B2 Asym. NH2 rock 1131 1105 1061

12 B2 Asym. BH2 rock (595) – 704

Aminoborane (11B) Fundamentals (cm-1)

Gerry, Lewis-Bevan,Merer, Westwood. 1985. “The Infrared Spectrum of Gaseous AminoboraneNH2=BH2: Location of the Fundamentals and Rotational Structure in the 41

0 Band.”J. Molec. Spectrosc. 110:153-163.

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, 014108 (2005)

ijk 1

3

jk (qi) jk (qi)

2qi

ki(q j) ki(q ji)

2q j

ij(qk ) ij(qk )

2qk

ijkk ij(qk ) ij(qk ) 2ij(0)

qk2

iikk 1

2

ii(qk ) ii(qk ) 2ii(0)

qk2

kk (qi) kk (qi) 2kk (0)

qi2

Computational Cost Grid-based methods

requires availability of E

trivially parallelizable

number of (ab-initio, force field) points on a grid (typically Ngrid ~ 8):

Higher energy derivatives methods requires availability of (analytic) second derivatives of E

easily parallelizable

number of second derivative of E evaluations: (2•Nmode+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).

N points N mode N grid 1

2N mode (N mode 1) N grid

2 1

3N mode (N mode 1) (N mode 2) N grid

3 ...

diagonal 2-mode correlations 3-mode correlations