Thomas Autrey

In Situ Multinuclear NMR Spectroscopic Studies of the Thermal Decomposition of Ammonia Borane in Solution

The development of condensed phase hydrogen storage materials for fuel cell powered vehicles capable of meeting the 2015 system target goals of >82 g H2 L-1 volumetric density and >90 g H2 kg-1 gravimetric density has attracted recent interest. The details of the mechanisms for hydrogen release from AB are not completely understood; however, significant progress has been made in furthering our understanding of these mechanisms. This work was funded by the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) as part of the Chemical Hydrogen Storage Center and carried out at the Pacific Northwest National Laboratory (operated by Battelle for DOE).

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

Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 88 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 10-12 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. In addition, electronic structure calculations are used to assist in the interpretation and assignment of the normal modes.

Interaction of lithium hydride and ammonia borane in THF

The two-step reaction between LiH and NH(3)BH(3) in THF leads to the production of more than 14 wt% of hydrogen at 40 degrees C.

Matrix isolation, time-resolved IR, and computational study of the photochemistry of benzoyl azide

It was shown recently on the basis of DFT calculations (N. P. Gritsan and E. A. Pritchina, Mendeleev Commun., 2001, 11, 94) that the singlet states of aroylnitrenes undergo tremendous stabilization due to an extra N–O bonding interaction. To test experimentally the multiplicity and the structure of the lowest state of benzoylnitrenes we performed a study of their photochemistry in Ar matrices at 12 K. Formation of two species was observed on irradiation of benzoyl azide (1b) and its 4-acetyl derivative (1c). One of these species has an IR spectrum, which is consistent with that of isocyanate (2b,c). The IR and UV spectra of the second intermediate are in very good agreement with the calculated spectra of the singlet species (3b,c), whose structure is intermediate between that of a carbonylnitrene and an oxazirene. We further examined the photochemistry of benzoyl azide in solution at ambient temperatures by nanosecond time-resolved IR methods and obtained additional evidence for the singlet ground state of benzoylnitrene as well as insight into its reactivity in acetonitrile, cyclohexane, and dichloromethane. The above experiments were accompanied by quantum chemical calculations which included also a thorough investigation of the parent species, formylnitrene, at different levels of theory.

Synthesis, Structure and Dehydrogenation of Magnesium Amidoborane Monoammoniate

Magnesium amidoborane monoammoniate (Mg(NH(2)BH(3))(2) x NH(3)) which crystallizes in a monoclinic structure (space group P2(1)/a) has been synthesized by reacting MgNH with NH(3)BH(3). Dihydrogen bonds are established between coordinated NH(3) and BH(3) of [NH(2)BH(3)](-) in the structure, promoting stoichiometric conversion of NH(3) to H(2).

Defining active catalyst structure and reaction pathways from ab initio molecular dynamics and operando XAFS: Dehydrogenation of dimethylaminoborane by rhodium clusters

We present the results of a detailed operando XAFS and density functional theory (DFT)-based ab initio molecular dynamics (AIMD) investigation of a proposed mechanism of the dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh(4) cluster catalyst. Our AIMD simulations reveal that previously proposed Rh structures, based on XAFS measurements, are highly fluxional, exhibiting both metal cluster and ligand isomerizations and dissociation that can only be accounted for by examining a finite temperature ensemble. It is found that a fluxional species Rh(4)(H(2)BNMe(2))(8)(2+) is fully compatible with operando XAFS measurements, suggesting that this species may be the observed catalyst resting state. On the basis of this assignment, we propose a mechanism for catalytic DMAB dehydrogenation that exhibits an energy barrier of approximately 28 kcal/mol.

The diammoniate of diborane: crystal structure and hydrogen release

[(NH(3))(2)BH(2)](+)[BH(4)](-) is formed from the room temperature decomposition of NH(4)(+)BH(4)(-), via a NH(3)BH(3) intermediate. Its crystal structure has been determined and contains disordered BH(4)(-) ions in 2 distinct sites. Hydrogen release is similar to that from NH(3)BH(3) but with faster kinetics.

Neutron powder diffraction and molecular simulation study of the structural evolution of ammonia borane from 15 to 340 K.

The structural behavior of (11)B-, (2)H-enriched ammonia borane, ND(3)(11)BD(3), over the temperature range from 15 to 340 K was investigated using a combination of neutron powder diffraction and ab initio molecular dynamics simulations. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the alignment of the B-N bond near parallel to the c-axis. The orthorhombic to tetragonal structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group. The resulting hydrogen disorder is problematic to extract from the metrics provided by Rietveld refinement but is readily apparent in molecular dynamics simulation and in difference Fourier transform maps. At the phase transition, Rietveld refinement does indicate a disruption of one of two dihydrogen bonds that link adjacent ammonia borane molecules. Metrics determined by Rietveld refinement are in excellent agreement with those determined from molecular simulation. This study highlights the valuable insights added by coupled experimental and computational studies.

Analysis of the activation and heterolytic dissociation of H2 by frustrated Lewis pairs: NH3/BX3 (X = H, F, and Cl).

We performed a computational study of H(2) activation and heterolytic dissociation promoted by prototype Lewis acid/base pairs NH(3)/BX(3) (X = H, F, and Cl) to understand the mechanism in frustrated Lewis pairs (FLPs). Although the NH(3)/BX(3) pairs form strong dative bonds, electronic structure theories make it possible to explore the potential energy surface away from the dative complex, in regions relevant to H(2) activation in FLPs. A weakly bound precursor complex, H(3)N·H(2)·BX(3), was found in which the H(2) molecule interacts side-on with B and end-on with N. The BX(3) group is pyramidal in the case of X = H, similar to the geometry of BH(5), but planar in the complexes with X = F and Cl. The latter complexes convert to ion pairs, [NH(4)(+)][BHX(3)(-)] with enthalpy changes of 7.3 and -9.4 kcal/mol, respectively. The minimum energy paths between the FLP and the product ion pair of the chloro and fluoro complexes were calculated and analyzed in great detail. At the transition state (TS), the H(2) bond is weakened and the BX(3) moiety has undergone significant pyramidal distortion. As such, the FLP is prepared to accept the incipient proton and hydride ion on the product-side. The interaction energy of the H(2) with the acid/base pair and the different contributions for the precursor and TS complex from an energy decomposition analysis expose the dominant factors affecting the reactivity. We find that structural reorganization of the precursor complex plays a significant role in the activation and that charge-transfer interactions are the dominant stabilizing force in the activated complex. The electric field clearly has a role in polarizing H(2), but its contribution to the overall interaction energy is small compared to that from the overlap of the p(N), σ(H-H), σ*(H-H), and p(B) orbitals at the TS. Our detailed analysis of the interaction of H(2) with the FLP provides insight into the important components that should be taken into account when designing related systems to activate H(2).

Kinetic and thermodynamic investigation of hydrogen release from ethane 1,2-di-amineborane

The thermodynamics and kinetics of hydrogen (H2) release from ethane 1,2-di-amineborane (EDAB, BH3NH2CH2CH2NH2BH3) were measured using Calvet and differential scanning calorimetry (DSC), pressure-composition isotherms, and volumetric gas-burette experiments. The results presented here indicate that EDAB releases ∼ 10 wt.% H2 at temperatures ranging from 100 °C to 200 °C in two moderately exothermic steps, approximately −10 ± 1 kJ mol−1 H2 and −3.8 ± 1 kJ mol−1 H2. Isothermal kinetic analysis shows that EDAB is more stable than ammonia borane (AB) at temperatures lower than 100 °C; however, the rates of hydrogen release are faster for EDAB than for AB at temperatures higher than 120 °C. In addition, no volatile impurities in the H2 released by EDAB were detected by mass spectrometry upon heating with 1 °C min−1 to 200 °C in a calorimeter.