EMSL: Nancy J. Hess's Publications

Hess NJ, GK Schenter, MR Hartman, LL Daemen, TE Proffen, SM Kathmann, CJ Mundy, MA Hartl, DJ Heldebrant, AC Stowe, and T Autrey. 2009. "Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from 15 to 340 K." Journal of Physical Chemistry A 113(9):5723-5735. doi:10.1021/jp900839c Abstract The structural behavior of perdeuterated, 11B-enriched ammonia borane, ND311BD3, was investigated by neutron powder diffraction measurements collected over the temperature range from 15 to 340 K and by molecular dynamics simulation. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the rotation of the B-N bond parallel to the c-axis. The structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group that is problematic to extract from the metrics provided by Rietveld analysis of the NPD data alone but is evident in the molecular dynamics simulation and other spectroscopic evidence. This study highlights the valued added by complimentary experimental approaches and coupled computational studies.

Kathmann SM, VM Parvanov, GK Schenter, AC Stowe, LL Daemen, MA Hartl, JC Linehan, NJ Hess, AJ Karkamkar, and T Autrey. 2009. "Experimental and Computational Studies on Collective Hydrogen Dynamics in Ammonia Borane: Incoherent Inelastic Neutron Scattering." Journal of Chemical Physics 130(2):article no. 024507. doi:10.1063/1.3042270 Abstract Incoherent inelastic neutron scattering can be used as a sensitive probe of the vibrational dynamics in chemical hydrogen storage materials. Thermal neutron energy loss measurements at 10K are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics of pure and deuterated ammonia borane (NH3BH3, NH3BD3, and ND3BH3). A harmonic vibrational analysis on NH3BH3 clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50 to 500 cm-1) and some assignment of modes to simple motion is possible, however, it is found that the lowest modes (below 250 cm-1) are dominated by collective motion. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Cho HM, WJ Shaw, VM Parvanov, GK Schenter, AJ Karkamkar, NJ Hess, CJ Mundy, SM Kathmann, JA Sears, AS Lipton, PD Ellis, and T Autrey. 2008. "Molecular Structure and Dynamics in the Low Temperature (Orthorhombic) Phase of NH3BH3." Journal of Physical Chemistry A 112(18):4277-4283. doi:10.1021/jp711696 Abstract Variable temperature 2H NMR experiments on the orthorhombic phase of selectively deuterated NH3BH3 spanning the static to fast exchange limits of the borane and amine motions are reported. New values of the electric field gradient (EFG) tensor parameters have been obtained from the static 2H spectra of Vzz = 5.509(±0.275)×1014 statvolt/cm2 and ! = 0.00±0.05 for the borane hydrogens and Vzz = 9.615(±0.481)×1014 statvolt/cm2 and ! = 0.00±0.05 for the amine hydrogens. The molecular symmetry inferred from the observation of equal EFG tensors for both the boron and amine hydrogens is in sharp contrast with the Cs symmetry derived from diffraction studies. The origin of the apparent discrepancy has been investigated using molecular dynamics methods in combination with electronic structure calculations of NMR parameters, bond lengths, and bond angles. The computation of parameters from a statistical ensemble rather than from a single set of atomic Cartesian coordinates gives values that are in close quantitative agreement with the 2H NMR electric field gradient tensor measurements and are more consistent with the molecular symmetry revealed by the NMR spectra. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.

Heldebrant DJ, AJ Karkamkar, NJ Hess, ME Bowden, SD Rassat, F Zheng, KG Rappe, and T Autrey. 2008. "The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane." Chemistry of Materials 20(16):5332-5336. doi:10.1021/cm801253u Abstract The solid-state decomposition of ammonia borane (AB) alone and in the presence of chemical additives was investigated by a series of experimental methods to develop an approach for reducing the induction period for hydrogen release. Gas chromatography techniques were used to measure the yield of hydrogen as a function of time under isothermal conditions between 70 and 90 °C, and the polyaminoborane (PAB) products produced from hydrogen loss from AB show significant cross linking by 11B NMR spectroscopy. Raman microscopy was used to follow the transformation of crystalline AB to amorphous AB with the subsequent formation of the diammoniate of diborane (DADB). A gas burette was used to monitor the time-dependent release of hydrogen from AB in the presence of chemical additives. The combination of these approaches provides insight into the mechanism of hydrogen release from solid AB. The release of molecular hydrogen is described by a process involving sequential induction (disruption of dihydrogen bonds), nucleation (formation of DADB), and growth (hydrogen release through dehydrocoupling). Addition of DADB or ammonium chloride to neat AB significantly reduces the induction time for hydrogen release. The authors wish to acknowledge support from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. This work was performed as part of the Center of Excellence in Chemical Hydrogen Storage and in collaboration with the International Partnership for the Hydrogen Economy. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle.

Hess NJ, MR Hartman, C Brown, E Mamontov, AJ Karkamkar, DJ Heldebrant, LL Daemen, and T Autrey. 2008. "Quasielastic neutron scattering of -NH3 and -BH3 rotational dynamics in orthorhombic ammonia borane." Chemical Physics Letters 459(1-6):85-88. doi:10.1016/j.cplett.2008.04.130 Abstract Neutrons scattering techniques are ideally suited to directly probe H in materials due to the large incoherent scattering cross-section of hydrogen atom, and have been invaluable in providing direct insight into the local fluctuations and large amplitude motions in AB. Dihydrogen bonding may have a significant affect on materials to be used to store hydrogen for fuel-cell powered applications. We have noticed a trend of low temperature release of H2 in materials composed of hydridic and protonic hydrogen. This phenomenon has caught our attention and motivated our interest to gain more insight into dihydrogen bonding interactions in AB. We present results from a thorough Quasielastic Neutron Scattering (QENS) investigation of diffusive hydrogen motion in NH311BH3 and ND311BH3 to obtain (1) a direct measure of the rotational energy barriers the protonated species and (2) a confirmation of the 3-site jump model for rotational motion. The amplitude of the energy barrier of rotation of BH3 and NH3 determined by QENS are compared to those determined for BD3 and ND3 determined by 2H NMR studies.

Hess NJ, ME Bowden, VM Parvanov, CJ Mundy, SM Kathmann, GK Schenter, and T Autrey. 2008. "Spectroscopic Studies of the Phase Transition in Ammonia Borane: Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K." Journal of Chemical Physics 128(3):Art. No. 034508. doi:10.1063/1.2820768 Abstract Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 77 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However some of the modes in the orthorhombic phase appeared to merge 10 to 12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. To our knowledge this is first reported vibrational spectra in the BH and NH stretching region of single crystal NH3BH3 in the orthorhombic phase.

Hess NJ, O Qafoku, Y Xia, DA Moore, and AR Felmy. 2008. "Thermodynamic Model for Solubility of TcO2•xH2O in Aqueous Oxalate System." Journal of Solution Chemistry 37(11):1471-1487. doi:10.1007/s10953-008-9328-5 Abstract The solubility of hydrous technetium (IV) oxide (TcO2•xH2O) was studied across a broad range of pH values extending from 1.5 to 10 and in oxalate concentrations from dilute (10-6 m) to complete saturation with respect to sodium bioxalate at lower pH values and to saturation with respect to sodium oxalate at higher pH values. The solubility was measured at very long equilibriation times (i.e. as long a 1000+ days). The thermodynamic modeling results show that the dominant species in solution must have at least one more hydroxyl moiety present in the complex (e.g. TcO(OH)Ox- versus TcOOx(aq)) than proposed by previous investigators. The inclusion of the single previously unidentified species TcO(OH)Ox- in our aqueous thermodynamic model explains a wider range of observed solubility data for TcO2•xH2O in the presence of oxalate and over a broad range of pH values. Inclusion of this species is also supported by the recently proposed thermodynamic data for the TcO(OH)+ hydrolysis species which shows an enhanced stability for this species at pH values as low as one.

Parvanov VM, GK Schenter, NJ Hess, LL Daemen, MA Hartl, AC Stowe, DM Camaioni, and T Autrey. 2008. "Materials for Hydrogen Storage: Structure and Dynamics of Borane Ammonia Complex." Dalton Transactions (33):4514-4522. doi:10.1039/b718138h Abstract The activation energies for rotations in low temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated, 11B- enriched ammonia borane 11BD3ND3 sample was synthesized and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported NMR measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH3 rotation, Ea = 12.7 kJ/mol, largely depends on the molecular conformational torsion in the solid state geometry. The barrier for independent BH3 rotation, Ea = 38.3 kJ/mol, results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of Ea = 31.1 kJ/mol was calculated for rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH3 group involves strongly correlated rotation of the NH3 end of the molecule. This observation suggests that the barrier from previously reported measurement of BH3 rotation, corresponds to H3B—NH3 correlated rotation. Support for this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences Division is gratefully acknowledged. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Rai D, DA Moore, NJ Hess, KM Rosso, L Rao, and SM Heald. 2007. "Chromium(III) Hydroxide Solubility in the Aqueous K+-H+-OH--CO2- HCO3-- CO32--H2O System: A Thermodynamic Model." Journal of Solution Chemistry 36(10):1261-1285. doi:10.1007/s10953-007-9179-5 Abstract Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3-13), at two different fixed partial pressures of CO2(gas) (0.003 or 0.03 atm.), and as functions of K2CO3 concentrations (0.01 to 5.8 m) in the presence of 0.01 M KOH and KHCO3 concentrations (0.001 to 0.826 m) at room temperature (22 ± 2°C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH) (CO3)22-) and Cr(OH)4CO33-] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO2(gas) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log K0 values of reactions involving these species [{Cr(OH)3(am) + 2CO2(gas) = Cr(OH)(CO3)22- + 2H+}and {Cr(OH)3(am) + OH- + CO32- = Cr(OH)4(CO3)3-}] were found to be –(19.07  0.41), -(4.19  0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.

Brown C, T Jacques, NJ Hess, LL Daemen, E Mamontov, JC Linehan, AC Stowe, and T Autrey. 2006. "Dynamics of Ammonia Borane Using Neutron Scattering." Physica B Condensed Matter 385(Pt.1 ):266-268. Abstract We have used both the backscattering (HFBS) and time-of-flight (DCS) neutron spectrometers to investigate the proton dynamics in ammonia borane, a compound of intense interest as a model for 'chemical hydrogen storage' materials. Results indicate that the deposition of ammonia borane on a mesoporous silicate results in longer proton residence times and lower energy barriers for proton motion compared to bulk ammonia borane. The reduced activation energy for proton motions may partly explain the improved thermolysis and lowering the activation barrier for the loss of the first equivalent of H2. In addition, the phonon density of states for neat ammonia borane compares well with other spectroscopic results, with the intense peak at 22 meV assigned to the librational NH3 and BH3 modes, whereas ammonia borane on MCM-41 displays a broad, featureless spectrum indicating a poorly crystalline material.