Scientific Publications 2007
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2007. "Molecular Mechanism for H₂ Release from BH₃NH₃, Including the Catalytic Role of the Lewis Acid BH₃." Journal of Physical Chemistry A 111(4):679-690. doi:10.1021/jp066175y Abstract Electronic structure calculations using various methods, up to the coupled-cluster CCSD(T) level, in conjunction with the aug-cc-pVnZ basis sets with n ) D, T, and Q, extrapolated to the complete basis set limit, show that the borane molecule (BH₃) can act as an efficient bifunctional acid-base catalyst in the H₂ elimination reactions of XHnYHn systems (X, Y ) C, B, N). Such a catalyst is needed as the generation of H₂ from isoelectronic ethane and borane amine compounds proceeds with an energy barrier much higher than that of the X-Y bond energy. The asymptotic energy barrier for H₂ release is reduced from 36.4 kcal/mol in BH₃NH₃ to 6.0 kcal/mol with the presence of BH₃ relative to the molecular asymptote. The NH₃ molecule can also participate in a similar catalytic process but induces a smaller reduction of the energy barrier. The kinetics of these processes was analyzed by both transition-state and RRKM theory. The catalytic effect of BH₃ has also been probed by an analysis of the electronic densities of the transition structures using the atom-in-molecule (AIM) and electron localization function (ELF) approaches.
2007. "Computational Study of the Release of H₂ from Ammonia Borane Dimer (BH₃NH₃)₂ and Its Ion Pair Isomers." Journal of Physical Chemistry A 111(36):8844-8856. doi:10.1021/jp0732007 Abstract High-level electronic structure calculations have been used to map out the relevant portions of the potential energy surfaces for the release of H₂ from dimers of ammonia borane, BH₃NH₃ (AB). Using the correlationconsistent aug-cc-pVTZ basis set at the second-order perturbation MP2 level, geometries of stationary points were optimized. Relative energies were computed at these points using coupled-cluster CCSD(T) theory with the correlation-consistent basis sets at least up to the aug-cc-pVTZ level and in some cases extrapolated to the complete basis set limit. The results show that there are a number of possible dimers involving different types of hydrogen-bonded interactions. The most stable gaseous phase (AB)₂ dimer results from a head-totail cyclic conformation and is stabilized by 14.0 kcal/mol with respect to two AB monomers. (AB)₂ can generate one or two H₂ molecules via several direct pathways with energy barriers ranging from 44 to 50 kcal/mol. The diammoniate of diborane ion pair isomer, [BH₄ -][NH₃BH₂NH₃ +] (DADB), is 10.6 kcal/mol less stable than (AB)₂ and can be formed from two AB monomers by overcoming an energy barrier of ~26 kcal/mol. DADB can also be generated from successive additions of two NH₃ molecules to B₂H sub 6 and from condensation of AB with separated BH₃ and NH₃ molecules. The pathway for H₂ elimination from DADB is characterized by a smaller energy barrier of 20.1 kcal/mol. The alternative ion pair [NH₄ +][BH₃NH₂BH₃ -] is calculated to be 16.4 kcal/mol above (AB)₂ and undergoes H₂ release with an energy barrier of 17.7 kcal/ mol. H₂ elimination from both ion pair isomers yields the chain BH₃NH₂BH₂NH₃ as product. Our results suggest that the neutral dimer will play a minor role in the release of H₂ from ammonia borane, with a dominant role from the ion pairs as observed experimentally in ionic liquids and the solid state.
2007. "Ammonia Triborane: Theoretical Study of the Mechanism of Hydrogen Release." Journal of Physical Chemistry C 111(26):9603-9613. doi:10.1021/jp0714062 Abstract High-level electronic structure calculations have been used to predict the thermodynamic stability of ammonia triborane B₃H₇NH₃ and the molecular mechanism of H₂ elimination from various isomeric forms in the gas phase. Geometries of stationary points were optimized at the second-order perturbation theory MP2 level, and total energies were computed at the coupled-cluster CCSD(T) theory with the aug-cc-pVnZ (n=D, T, Q) basis sets and extrapolated to the complete basis set limit. Heats of formation for the structures considered in the gas phase were evaluated at both 0 and 298 K. The lowest-energy process for H₂ release from the most stable isomer of B₃H₇NH₃ is a 1,3-elimination characterized by an energy barrier of 28.9 kcal/mol. Although the barrier height for H₂ release from B₃H₇NH₃ is slightly smaller than the B-N bond cleavage energy of 30.7 kcal/mol yielding B₃H₇ + NH₃, the calculated rate coefficients predict that bond cleavage is faster than H₂ release by 3 orders of magnitude at 298 K and 1 atm. We predict the heat of formation for the most stable isomer of B₃H₇ to be ∆H sub f (0 K) = 37.1 plus or minus 0.8 kcal/mol and ∆H sub f (298 K) = 32.5 plus or minus 0.8 kcal/mol, and for the most stable isomer of B₃H₇NH₃ to be ∆H sub f (0 K) = 0.4 plus or minus 1.0 kcal/mol and ∆H sub f (298 K) = -7.1 plus or minus 1.0 kcal/mol.
2007. "Bimetallic and Ternary Alloys for Improved Oxygen Reduction Catalysis ." Topics in Catalysis 46(3-4):276-284. doi:10.1007/s11244-007-9001-z Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Using a combination of density functional theory (DFT) calculations and an array of experimental techniques including in situ X-ray absorption spectroscopy, we identified, synthesized, and tested successfully a new class of electrocatalysts for the oxygen reduction reaction (ORR) that were based on monolayers of Pt deposited on different late transition metals (Au, Pd, Ir, Rh, or Ru), of which the Pd-supported Pt monolayer had the highest ORR activity. The amount of Pt used was further decreased by replacing part of the Pt monolayer with a third late transition metal (Au, Pd, Ir, Rh, Ru, Re, or Os). Several of these mixed Pt monolayers deposited on Pd single crystal or on carbon-supported Pd nanoparticles exhibited up to a 20-fold increase in ORR activity on a Pt-mass basis when compared with conventional all-Pt electrocatalysts. DFT calculations showed that their superior activity originated from the interaction between the Pt monolayer and the Pd substrate and from a reduced OH coverage on Pt sites, the result of enhanced destabilization of Pt–OH induced by the oxygenated third metal. This new class of electrocatalysts promises to alleviate the major problems of existing fuel cell technology by simultaneously decreasing materials cost and enhancing performance.
2007. "A Unique Coplanar Multi-center Bonding Network in DoublyAcetylide-bridged Binuclear Zirconocene Complexes: A DensityFunctional Theory Study." Journal of Organometallic Chemistry 692(21):4760-4767. doi:10.1016/j.jorganchem.2007.07.019 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. A unique p-conjugative interaction pattern was experimentally revealed in the doubly acetylide-bridged binuclear group 4 metallocene complexes, which was involved in C–C coupling/cleavage reactions of acetylides and r-alkynyl migrations. To elucidate how this multicenter bonding network affects the structural and reaction properties of these complexes, density functional theory (DFT) calculations and molecular orbital (MO) analysis were carried out on the electronic structure and r-alkynyl migration mechanisms of the doubly acetylide-bridged binuclear Zr complexes, (L2Zr)2(l-C„CH)2 (L = Cp,Cl). The B3LYP calculations suggested that the doubly [r,p] acetylide-bridged complex C2h-(L2Zr)2(l-C„CH)2 was produced by the reaction of L2Zr(C„CH)2 with L2Zr through a C2v- (L2Zr)2(l-C„CH)2 intermediate followed by an isomerization process. In particular, the isomerization of C2h- or C2v-(L2Zr)2(l- C„CH)2 is almost thermoneutral through a low barrier of 15.3–17.0 kcal/mol. The MO Walsh diagram revealed that the two isomers have a very similar six-center-six-electron bonding network. The coplanar p-conjunctive interaction by the electron donating and backdonating interactions between the metal centers and acetylide ligands significantly stabilizes the doubly acetylide-bridged binuclear group 4 metallocene complexes and the isomerization transition state.
2007. "Probing the Structural Effects on the Intrinsic Electronic and Redox Poperties of [2Fe-2S](+) Custers, a Broken-Symmetry Density Functional Theory Study ." Theoretical Chemistry Accounts 117(2):275-281. Abstract Abstract is currently not available at this time for viewing.
