Scientific Publications 2010
2010. "Micro-FTIR Study of Soot Chemical Composition – Evidence of Aliphatic Hydrocarbons on Nascent Soot Surfaces." Physical Chemistry Chemical Physics. PCCP 12(20):5206-5218. doi:10.1039/b924344e Abstract Previous studies suggest that soot formed in premixed flat flames can contain a substantial amount of aliphatic compounds. Presence of these compounds may affect the kinetics of soot mass growth and oxidation in a way that is currently not understood. Using an infrared spectrometer coupled to a microscope (micro-FTIR), we examined the composition of soot sampled from a set of ethylene-argon-oxygen flames recently characterized (Abid, A.D. et al. Combust. Flame, 2008, 154, 775-788), all with an equivalence ratio 0 = 2.07 but varying in maximum flame temperatures. Soot was sampled at three distances above the burner surface using a probe sampling technique and deposited on silicon nitride thin film substrates using a cascade impactor. Spectra were taken and analyses performed for samples collected on the lowest five impactor stages with the cut-off sizes of D50 = 10, 18, 32, 56 and 100 nm. The micro-FTIR spectra revealed the presence of aliphatic C-H, aromatic C-H and various oxygenated functional groups, including carbonyl (C=O), C-O-C and C-OH groups. Spectral analyses were made to examine variations of these functional groups with flame temperature, sampling position and particle size. Results indicate that increases in flame temperature leads to higher contents of non-aromatic functionalities. Functional group concentration was found to be ordered as follows: [C=O] < [C-O] < [aliphatic C-H]. Aliphatic C-H was found to exist in significant quantities, with very little oxygenated groups present. The ratio of these chemical functionalities to aromatic C-H remains constant for particle sizes spanning 10-100 nm. The results confirm a previous experimental finding: a significant amount of aliphatic compounds is present in nascent soot formed in the flames studied, especially towards larger distances above the burner surface.
2010. "Coverage Effects on the Palladium-Catalyzed Synthesis of Vinyl Acetate: Comparison between Theory and Experiment." Journal of the American Chemical Society 132(7):2202-2207. doi:10.1021/ja907061m Abstract The high adsorbate coverages that form on the surfaces of many heterogeneous catalysts under steady-state conditions can significantly lower the activation energies for reactions that involve the coupling of two adsorbed intermediates while increasing those which result in adsorbate bond-breaking reactions. The influence of the surface coverage on the kinetics of metal-catalyzed reactions is often ignored in theoretical and even in some ultrahigh vacuum experimental studies. Herein, first principle density functional theoretical calculations are combined with experimental surface titration studies carried out over well-defined Pd(111) surfaces to explicitly examine the influence of coverage on the acetoxylation of ethylene to form vinyl acetate over Pd. The activation energies calculated for elementary steps in the Samanos and Moiseev pathways for vinyl acetate synthesis carried out on acetate-saturated palladium surfaces reveal that the reaction proceeds via the Samanos mechanism which is consistent with experimental results carried out on acetate-saturated Pd(111) surfaces. The rate-limiting step involves a β-hydride elimination from the adsorbed acetoxyethyl intermediate, which proceeds with an apparent calculated activation barrier of 53 kJ/mol which is in very good agreement with the experimental barrier of 55 ± 4 kJ/mol determined from kinetic measurements.
2010. "Analysis of Biostimulated Microbial Communities from Two Field Experiments Reveals Temporal and Spatial Differences in Proteome Profiles." Environmental Science & Technology 44(23):8897-8903. doi:10.1021/es101029f Abstract Stimulated by acetate-amendment field experiments conducted in 2007 and 2008, anaerobic microbial populations in the aquifer at the Rifle Integrated Field Research Challenge site in Colorado reduced mobile U(VI) to insoluble U(IV). During this period, planktonic biomass was sampled at various time points and used to quantitatively evaluate proteomes, both spatially and temporally to study the dynamics of the microbial community proteome dynamics in relationship to geochemical measurements. As there were no comprehensive genome sequence data available at the time, we systematically evaluated different organisms to generate a "pseudo-metagenome" for proteomics analyses. Proteomics results supported the dominance of Geobacteraceae during biostimulation and revealed a shift from iron reduction to sulfate reduction, evidenced by changes in community membership. Because U(VI) is reduced at a lower rate during sulfate reduction, detecting this shift is important to maintaining the maximum rate of U(VI) reduction. In addition, the comparison of proteome measurements made at the end of the 2007 field experiment to the 2008 field experiment revealed a modified community structure. Importantly, the failure of a community to rebound following the cessation of biostimulation needs to be included in long-term remediation strategies.
2010. "Physicochemical/Thermodynamic Framework for the Interpretation of Peptide Tandem Mass Spectra." Journal of Physical Chemistry C 114(12):5360-5366. doi:10.1021/jp905049d Abstract The interpretation of tandem mass spectra of peptides from collision-induced dissociation is discussed from the perspective of a multinomial data analysis problem and from the perspective of statistical mechanics. Both approaches use the same statistical likelihood function, and it is shown that this statistical likelihood is equivalent to an information theory entropy when scaled appropriately. The likelihood function provides a physically and chemically principled way to incorporate intensity information into the interpretation of tandem mass spectra and enables the use of predictive modeling and simulation to inform the peptide identification process.
2010. "Structure and Oxidation State of Hematite Surfaces Reacted with Aqueous Fe(II) at Acidic and Neutral pH." Geochimica et Cosmochimica Acta 74(5):1498-1512. doi:10.1016/j.gca.2009.12.018 Abstract Structural changes and surface oxidation state were examined following the reaction of hematite (001), (012), and (110) with aqueous Fe(II). X-ray reflectivity measurements indicated that Fe(II) induces changes in the structure of all three surfaces under both acidic (pH 3) and neutral (pH 7) conditions. The structural changes were generally independent of pH although the extent of surface transformation varied slightly between acidic and neutral conditions; no systematic trends with pH were observed. Induced changes on the (110) and (012) surfaces include the addition or removal of partial surface layers consistent with either growth or dissolution. In contrast, a <1 nm thick, discontinuous film formed on the (001) surface that appears to be epitaxial yet is not a perfect extension of the underlying hematite lattice, being either structurally defective, compositionally distinct, or nanoscale in size and highly relaxed. Resonant anomalous X-ray reflectivity measurements determined that the surface concentration of Fe(II) present after reaction at pH 7 was below the detection limit of approximately 0.5 to 1 mol/m2 on all surfaces. These observations are consistent with Fe(II) oxidative adsorption, whereby adsorbed Fe(II) is oxidized by structural Fe(III) in the hematite lattice, with the extent of this reaction controlled by surface structure at the atomic scale. The observed surface transformations at pH 3 show that Fe(II) oxidatively adsorbs on hematite surfaces at pH values where little net adsorption occurs, based on historical macroscopic Fe(II) adsorption behavior on fine-grained hematite powders. This suggests that Fe(II) plays a catalytic role, in which an electron from an adsorbed Fe(II) migrates to and reduces a lattice Fe(III) cation elsewhere, which subsequently desorbs in a scenario with zero net reduction and zero net adsorption. Given the general pH-independence and substantial mass transfer involved, this electron and atom exchange processes appears to be a significant subsystem within macroscopic pH-dependent Fe(II) adsorption.
2010. "Structure and Dynamics of the Hydration Shells of the Zn2+ Ion from ab initio Molecular Dynamics and Combined ab initio and Classical Molecular Dynamics Simulations." Journal of Chemical Physics 132(19):Article 194502. doi:16248, 16249, 56674 Abstract Ab initio molecular dynamics (AIMD) simulations of the hydration shells surrounding the Zn2+ ion are reported for temperatures near 300oC. Simulations using a combined ab initio and classical molecular dynamics (AIMD/MM) approach are also carried out. Both simulations are done with 64 solvating water molecules (~15 ps). The hydration structure predicted from both simulations is found to agree very well with known results from X-ray data. The 1st hydration shell contains six water molecules in an octahedral structure with the hydrogen atoms oriented away from the Zn2+ ion. The six waters in the 1st shell are located at an average distance of 2.44Å. A 2nd hydration shell is observed at 4.59Å. Beyond these shells, the bonding pattern substantially returns to the tetrahedral structure of bulk water. No exchanges are seen between the 1st and 2nd hydrations shells, however many water transfers between the 2nd and outer hydrations shells are observed to occur on a picosecond (ps) time scale via dissociative and associative mechanisms. In general, it is found that the AIMD and AIMD/MM simulations give nearly identical results for structural parameters, EXAFS spectra, and exchange dynamics. These results suggest that AIMD/MM can be used to extend the particle scale and time scale of AIMD simulations of highly charged ions in solution.
2010. "Vertical Ionization Potentials of Nucleobases in a Fully Solvated DNA Environment." Journal of Physical Chemistry B 114(17):5886-5894. Abstract Calculations of the direct ionization potentials (DIP) of DNA nucleobases in the fully solvated DNA helix are reported. The results show an unexpected large shift of roughly 3.2-3.3 eV compared to the corresponding gas-phase IP values. The DIP shift is nearly the same for all the four DNA bases and appears to vary slowly with the stacking and H-bonding interactions of the nucleobases. We demonstrate that the large shift in the DIP of bases is due to the electric potential around the DNA resulting from the long range solvent structure created by the negative phosphate groups and positive counterions of the DNA helical structure. Thermal fluctuations in the fluid can result in DIP changes of roughly 1ev on a picosecond time scale. The model used in this work is based on a QM/MM approach in which the base (or clusters of bases) are chosen as the QM system and calculated using a high-level quantum chemistry method. The remaining DNA fragment and the species in solution are included in an exact molecular mechanics (MM) model. The expected high accuracy of the QM/MM model is defended in terms of the essentially Columbic nature of the interactions of the solvent (the MM region) with isolated base in the quantum region. For the test anion, Cl-, the QM/MM approach yields the 3.4 eV (gas-phase) to 9.3 eV (aqueous solution) shift of the ionization energy in agreement with experimental values (3.6 and 9.6 eV). The localization of the electronic excitation inside the QM region is supported by current experimental and theoretical evidence indicating that the HOMO of the nucleotide is localized on the base rather than the sugar or the phosphate backbone. Our calculations performed in the native DNA environment support this localization. The QM/MM model presented in this work provides an important simplification to the difficult problem of incorporating a detailed structural model of the physiological conditions into investigations of the electronic processes in DNA.
2010. "Epitaxial Growth and Properties of Doped Transition Metal and Complex Oxide Films." Advanced Materials 22(2):219-248. doi:10.1002/adma.200901867 Abstract The detailed science and technology of crystalline oxide film growth using vacuum methods is reviewed and discussed with an eye toward gaining fundamental insights into the relationships between growth process and parameters, film and interface structure and composition, and electronic, magnetic and photochemical properties. The topic is approached first from a comparative point of view based on the most widely used growth methods, and then on the basis of specific material systems that have generated very high levels of interest. Emphasis is placed on the wide diversity of structural, electronic, optical and magnetic properties exhibited by oxides, and the fascinating results that this diversity of properties can produce when combined with the degrees of freedom afforded by heteroepitaxy.
2010. "Instability, intermixing and electronic structure at the epitaxialLaAlO3/SrTiO3(001) heterojunction." Surface Science Reports 65(10-12):317-352. doi:10.1016/j.surfrep.2010.09.001 Abstract The question of stability against diffusional mixing at the prototypical LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using onaxis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as potential energy gradients and valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO3 or SrTiO3, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Significantly, we find it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface – electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO3 below the interface as being the cause of the observed conductivity.
2010. "Compositional Tuning of Ultrathin Surface Oxides on Metal and Alloy Substrates Using Photons: Dynamic Simulations and Experiments." Physical Review. B, Condensed Matter 81(8):Art. No. 085406. doi:10.1103/PhysRevB.81.085406 Abstract Oxide synthesis with controlled functional properties is desirable for a plethora of applications but is elusive due to oxide growth kinetics. Here, we report on the ability to modify the structure and composition of ultra-thin oxides grown on Ni-Al alloy surfaces at room temperature utilizing photon illumination. Atomistic simulations that take into account dynamic charge transfer predict that the electric field produced across an oxide film in photon-assisted synthesis overcomes the activation energy barrier for ionic migration, leading to enhanced oxidation kinetics and oxygen incorporation into the oxide, enabling us to control the oxide composition at atomic length scales. Experiments (near-edge x-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy) indicate that the native oxide on 5%Ni-Al alloy is primarily composed of aluminum oxide with no nickel oxide whereas the photon-assisted oxide comprises of both aluminum oxide and nickel oxide. The ability to tune the composition at the atomic scale of the ultra-thin oxide films leads to excellent passivity as verified from polarization experiments.
2010. "Mechanistic Studies on Room Temperature Photoexcitation Effects on Passivity Breakdown of Ultrathin Surface Oxide Films Formed on Ternary Al-5%Cu-5%Ni Alloys." Journal of Physical Chemistry C 114(41):17788–17795. doi:10.1021/jp106297c Abstract How the composition of nanoscale surface oxides formed on complex metal alloys affects their passive state is a problem of great interest both from scientific and technological perspectives. In this work, we report on experimental studies regarding the problem concerning the role of room temperature photo-excitation on altering composition of ultra-thin oxides formed on ternary Al-Cu-Ni thin film alloy surface and its influence in passivity breakdown in aqueous media. Extensive studies have been carried out on pure copper, Al-5%Cu and Al-5%Cu-5%Ni alloy thin films to investigate mechanistic role of alloying elements as well as their oxide formation characteristics on the resulting passive state. The differences in nanoscale oxide composition formed has a remarkable influence on the passivity breakdown of the oxide films as verified from electrochemical measurements and possible mechanisms leading to the observations are discussed in detail.
2010. "Structure and Dynamics of N, N-diethyl-N-methylammonium Triflate Ionic Liquid, Neat and with Water, from Molecular Dynamics Simulations." Journal of Physical Chemistry A 114(48):12764-12774. doi:10.1021/jp108189z Abstract We investigated by means of molecular dynamics simulations the properties (structure, thermodynamics, ion transport, and dynamics) of the protic ionic liquid N,N-diethyl-N-methyl-ammonium triflate (dema:Tfl) and of selected aqueous mixtures of dema:Tfl. This ionic liquid, a good candidate for a water-free proton exchange membrane, is shown to exhibit high ion mobility and conductivity. For bulk melts in the temperature range of 303-453K, both liquid densities and enthalpies of vaporization are found to decrease roughly linearly with increasing temperature. The radial distribution functions reveal a significant long-range structural correlation. The ammonium cations [dema]+ are found to diffuse slightly faster than the triflate anions [Tfl]-, and both types of ions exhibit enhanced mobility at higher temperatures, leading to higher ionic conductivity of these ionic liquids. Analysis of the dynamics of ion pairing clearly points to the existence of long-lived contact ion pairs in this ionic liquid. We also examined the effects of water on the ionic properties of dema:Tfl-water mixtures. From the structural analysis it was found that water molecules tend to replace counter ions in the coordination shell and hydrogen bond to both ions, thus weakening their mutual association. As water concentration increases, water molecules start to connect with each other and eventually form a large network that percolates through the system. It is also found that water has a strong influence on the ion dynamics in the mixtures. As the concentration of water increases, both translational and rotational motion of [dema]+ and [Tfl]- are significantly enhanced. As a result, higher ionic conductivity is observed with increased hydration level. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2010. "Towards P-Type Conductivity in SnO2 Nanocrystals through Li Doping." Nanotechnology 21(3):035708. doi:10.1088/0957-4484/21/3/035708 Abstract This paper examines electrical transport properties and Li doping in SnO2 synthesized by the sol–gel method. Solid-state 7Li-NMR lineshapes reveal that Li ions occupy two distinct sites with differing dynamic mobilities. The chemical exchange rate between the two sites is, however, too slow for detection on the NMR timescale. Compressed nanoparticulate films of this doped semiconductor exhibit a positive Seebeck coefficient implying a p-type conductivity. A variable-temperature direct current conductivity, over a 25–350 ◦C temperature range, follows an Efros–Shklovskii variable range hopping (ES-VRH) conduction mechanism (ln(ρ) versus T −1/2) at temperatures below 100 ◦C with a crossover to 2D Mott variable range hopping (M-VRH) (ln(ρ) versus T −1/3) conduction at temperatures above 250 ◦C. In a transition region between these two limiting behaviors, the dc resistivity exhibits an anomalous temperature-independent plateau. We suggest that its origin may lie in a carrier inversion phenomenon wherein the majority carriers switch from holes to electrons due to Li ion expulsion from the crystalline core and creation of oxygen vacancies generated by loss of oxygen at elevated temperatures.
2010. "trans-K3[TcO2(CN)4]." Acta Crystallographica. Section E 66(8):i61 - i62. doi:10.1107/S1600536810028205 Abstract The dioxotetracyanotechnetate anion, [TcO2(CN)4]3-, of the title complex has octahedral symmetry. The technetium is located on a center of inversion and is bound by two oxygen atoms and four cyano ligands. The Tc═O bond distance of 1.7721 (12) Å is consistent with double bond character. The potassium cations [located on special (1/2,0,1) and general positions] reside in octahedral or tetrahedral environments; interionic K···O and K···N interactions occur in the 2.7877 (19)-2.8598 (15) Å range.
2010. "Kinetic lattice Monte Carlo simulations of interdiffusion in strained silicongermanium alloys." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 28(1):C1G18. doi:10.1116/1.3294704 Abstract Point-defect-mediated diffusion processes are investigated in strained SiGe alloys using kinetic lattice Monte Carlo *KLMC* simulation technique. The KLMC simulator incorporates an augmented lattice domain and includes defect structures, atomistic hopping mechanisms, and the stress dependence of transition rates obtained from density functional theory calculation results. Vacancy-mediated interdiffusion in strained SiGe alloys is analyzed, and the stress effect caused by the induced strain of germanium is quantified separately from that due to germanium-vacancy binding. The results indicate that both effects have substantial impact on interdiffusion. © 2010 American Vacuum Society.
2010. "Homogeneous Ni catalysts for H2 Oxidation and Production: An Assessment of Theoretical Methods, from Density Functional Theory to Post Hartree-Fock Correlated Wave-Function Theory ." Journal of Physical Chemistry A 114(48):12716-12724. doi:10.1021/jp106800n Abstract A systematic assessment of theoretical methods applicable to the accurate characterization of catalytic cycles of homogeneous catalysts for H2 oxidation and evolution is reported. For these catalysts, H2 bond breaking or formation involve di-hydrogen, di-hydride, hydride-proton, and di-proton complexes. The key elementary steps have heterolytic character. In the context of Density Functional Theory (DFT) we investigated the use of functionals in the generalized gradient approximation (GGA) as well as hybrid functionals. We compared the results with wavefunction theories based on perturbation theory (MP2 and MP4) and on coupled-cluster expansions (CCSD and CCSD(T)). Our findings suggest that DFT results based on Perdew functionals are in semi-quantitative agreement with the CCSD(T) results, with deviations of a few kcal/mol only. On the other hand, the B3LYP functional is not even in qualitative agreement with CCSD[T]. Surprisingly the MP2 results are found to be extremely poor, a finding that we attribute to the limited treatment in MP2 theory of dynamic electron correlation effects in Ni(0) oxidation state. This material is based upon work supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
2010. "The Different Impacts of SO2 and SO3 on Cu/Zeolite SCR Catalysts." Catalysis Today 151(3-4):266-270. doi:10.1016/j.cattod.2010.01.013 Abstract The different impacts of SO2 and SO3 on Cu/zeolite SCR catalysts were investigated by SCR performance tests and multiple characterization techniques including temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS). The results indicate that a larger amount of highly dispersed CuSO4 formed in the zeolite catalysts (Z-CuSO4) upon SO3 poisoning, explaining the much more significant deactivation of the Cu/zeolite catalysts that were exposed to SO3 compared to poisoning by SO2. This paper provides the first demonstration that active sites of Cu/zeolite SCR catalysts involved in the storage and removal of sulfur can react with SO2 and SO3 in very different ways. In particular, the significant differences in the extent of sulfur uptake account for the considerably different impacts of SO2 and SO3 poisoning on the performance of Cu/zeolite SCR catalysts.
2010. "Self-energy and excitonic effects in the electronic and optical properties of TiO₂ crystalline phases." Physical Review. B, Condensed Matter and Materials Physics 82(4):045207. doi:10.1103/PhysRevB.82.045207 Abstract We present a unified ab initio study of electronic and optical properties of TiO₂ rutile and anatase phases with a combination of density-functional theory and many-body perturbation-theory techniques. The consistent treatment of exchange and correlation, with the inclusion of many-body one-particle and two-particles effects in self-energy and electron-hole interaction, produces a high-quality description of electronic and optical properties, giving, for some quantities, the first available estimation for this compound. In particular, we give a quantitative estimate of the electronic and direct optical gaps, clarifying their role with respect to previous measurements obtained by various experimental techniques. We obtain a description for both electronic gap and optical spectra that is consistent with experiments by analyzing the role of different contributions to the experimental optical gap and relating them to the level of theory used in our calculations. We also show the spatial properties of excitons in the two crystalline phases, highlighting the localization character of different optical transitions. This paper aims at understanding and firmly establishing electro-optical bulk properties, yet to be clarified, of this material of fundamental and technological interest for green energy applications.
2010. "Comprehensive Solid-State NMR Characterization of Electronic Structure in Ditechnetium Heptoxide ." Journal of the American Chemical Society 132(38):13138-13140. Abstract A relativistic density functional description of the electronic structure of Tc2O7 has been evaluated by comparison with solid state 99Tc and 17O NMR spectroscopic data (the former isotope a weak beta-emitter). Every site in the molecule can be populated by a nucleus with favorable NMR characteristics, providing the rare opportunity to obtain a comprehensive set of chemical shift and electric field gradient tensors for a small molecular transition metal oxide. NMR parameters were computed for the central molecule of a (Tc2O7)17 cluster, using standard ZORA optimized all-electron QZ4P basis sets for the central molecule and DZ basis sets for surrounding atoms. The magnitudes of the predicted tensor principal values appear to be uniformly larger than observed experimentally, but discrepancies were within the accuracy of the approximation methods used. The convergence of calculated and measured NMR data suggests that the theoretical analysis has validity for the quantitative understanding of structural, magnetic, and chemical properties of Tc(VII) oxides. The William R. Wiley Environmental Molecular Sciences Laboratory is a U.S. Department of Energy (DOE) national scientific user facility located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
2010. "Probing the Oxygen Environment in UO22+ by Solid-State O-17 Nuclear Magnetic Resonance Spectroscopy and Relativistic Density Functional Calculations." Journal of Chemical Physics 132(8):084501. doi:10.1063/1.3308499 Abstract A combined theoretical and solid-state O-17 NMR study of the electronic structure of the uranyl ion UO22+ in (NH4)4UO2(CO3)3 and rutherfordine UO2CO3 is presented, the former representing a system with a hydrogen-bonding environment around the uranyl oxygens, and the latter exemplifying a uranyl environment without hydrogens. A fully relativistic ab initio treatment reveals unique features of the U-O covalent bond, including the finding of O-17 chemical shift anisotropies that are among the largest ever reported (>1200 ppm). Computational results for the oxygen electric field gradient tensor are found to be consistently larger in magnitude than experimental solid-state O-17 NMR measurements in a 7.05 T magnetic field indicate. A modified version of the Solomon theory of the two-spin echo amplitude for a spin-5/2 nucleus is developed and applied to the analysis of the O-17 echo signal of UO22+. The William R. Wiley environmental Molecular Sciences Laboratory is a US Department of Energy national scientific user facility located at Pacific Northwest National Laboratory (PNNL) in Richland, Washington. PNNL is operated by Battelle for the US Department of Energy.
2010. "LiMnPO4 Nanoplate Grown via Solid-State Reaction in Molten Hydrocarbon for Li-ion Battery Cathode." Nano Letters 10(8):2799–2805. doi:10.1021/nl1007085 Abstract Electrochemically active LiMnPO4 nanoplates have been synthesized via novel single step solid state reaction in molten hydrocarbon. The LiMnPO4 prepared show unique porous nanoplate shape ~50nm in thickness with highly preferred crystallographic orientation. The reversible cycling of carbon coated LiMnPO4 show flat potential at 4.1 V vs. Li with specific capacity reaching up to 168mAh/g and excellent cycling performance using only galvanostatic charging / discharging mode.
2010. "High efficiency and low roll-off blue phosphorescent organic light-emitting devices using mixed host architecture." Applied Physics Letters 97:033304. doi:10.1063/1.3464969 Abstract We report high efficiency and low roll-off for blue electrophosphorescent organic light emitting devices (OLEDs) based on a mixed host layer architecture. The devices were fabricated using a mixed layer of di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC), a hole transport material, and 2,8-bis(diphenylphosphoryl)dibenzothiophene (PO15), an electron transport material, as the host layer doped with the blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2’]picolinate (FIrpic). Using a mixed layer as the host allowed us to achieve high power efficiency (59 lm/W at 100 cd/m2), low turn-on voltage (2.7 V for >10 cd/m2), and low roll-off in these devices.
2010. "Phosphate Removal by Anion Binding on Functionalized Nanoporous Sorbents." Environmental Science & Technology 44(8):3073-3078. doi:10.1021/es100787m Abstract Phosphate was captured from aqueous solutions by cationic metal-EDA complexes anchored inside mesoporous silica MCM-41 supports (Cu(II)-EDA-SAMMS and Fe(III)-EDA-SAMMS). Fe-EDA-SAMMS was more effective at capturing phosphate than the Cu-EDA-SAMMS and was further studied for matrix effects (e.g., pH, ionic strength, and competing anions) and sorption performance (e.g., capacity and rate). The adsorption of phosphate was highly pH dependent; it increased with increasing pH from 1.0 to 6.5, and decreased above pH 6.5. The adsorption was affected by high ionic strength (0.1 M of NaCl). In the presence of 1000-fold molar excess of chloride and nitrate anions, phosphate removal by Fe-EDA-SAMMS was not affected. Slight, moderate and large impacts were seen with bicarbonate, sulfate and citrate anions, respectively. The phosphate adsorption data on Fe-EDA-SAMMS agreed well with the Langmuir model with the estimated maximum capacity of 43.3 mg/g. The material displayed rapid sorption rate (99% of phosphate removal within 1 min) and lowering the phosphate content to ~ 10 µg/L of phosphorus, which is lower than the EPA’s established freshwater contaminant level for phosphorous (20 µg/L).
2010. "Synthesis, Structure and Dehydrogenation of Magnesium Amidoborane Monoammoniate." Chemical Communications 46(31):5752-5754. Abstract Magnesium amidoborane monoammoniate (Mg(NH2BH3)2 NH3) which crystallizes in a monoclinic structure (space group P21/a) has been synthesized by reacting MgNH with NH3BH3. Dihydrogen bonds are established between coordinated NH3 and BH3 of [NH2BH3] in the structure, promoting stoichiometric conversion of NH3 to H2. T. A. and A. K. acknowledge support from US DoE EERE CHS CoE. The high-field NMR experiments described here were carried out in the Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.
2010. "Characterization and Reactivity of Iron Nanoparticles Prepared with Added Cu, Pd, and Ni." Environmental Science & Technology 44(13):5079-5085. Abstract The presence of a secondary metal on iron particles affects redox reactivity in engineered remediation systems. However, the structural characteristics of the metal additives and mechanism responsible for changes in reactivity have not been fully elucidated. Here, we synthesized iron nanoparticles with Cu, Pd, and Ni content ranging from 0-2 mol% via a solution deposition process (SDP), hydrogen reduction process (HRP), or hydrogen reduction of ferrihydrite coprecipitated with the metal cations (HRCO). Results from solid-state characterization show that the synthetic methods produced similar iron core/magnetite shell particles but produced substantial differences in terms of the distribution of the metal additive. In SDP, the metal additives were heterogeneous distributed on the surface of the particles. The metal additives were clearly discernable in TEM images as spherical nanoparticles (2-4 nm) on the HRP and HRCO particles. In addition, we hypothesize that the metal additive is also present as solute within the iron core of the HRCO particles. Kinetic batch experiments of carbon tetrachloride (CT) degradation were performed to quantitatively compare the redox reactivity of the particles. Overall, metal additives resulted in enhanced overall pseudo-first order rate constants of CT degradation (kO,CT) compared to that of the iron nanoparticles. For the bimetallic iron nanoparticles prepared by SDP and HRP, kO,CT increased with the concentration of metal additives. The values of chloroform yield (YCF) were independent of the identity and amount of metal additives. However, both kO,CT and YCF of the HRCO iron particles were significantly greater. Results suggest that it is the distribution of the metal additives that most strongly impacts reactivity and product distribution. For example, for materials with ca. 0.9 50 mol% Ni, reactivity and YCF varied substantially (HRCO>SDP>HRP), and HRCO-NiFe resulted in the lowest final chloroform concentration because the chloroform was rapidly dechlorinated. In addition, sequential spike experiments for long-term reactivity demonstrated that the presence of the metal additives facilitated reduction by enabling greater utilization of Fe0.
2010. "Electron Affinities, Fluoride Affinities, and Heats of Formation of the Second RowTransition Metal Hexafluorides: MF6 (M = Mo, Tc, Ru, Rh, Pd, Ag)." Journal of Physical Chemistry A 114(28):7571-7582. doi:10.1021/jp1022949 Abstract High-level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the second row transition metal hexafluorides. The electron affinities, heats of formation, first (MF6 f MF5 + F) and average M-F bond dissociation energies, and fluoride affinities of MF6 (MF6 + Ff MF7 -) and MF5 (MF5 + F- f MF6-) were calculated. The electron affinities are higher than those of the corresponding third row hexafluorides, making them stronger one-electron oxidizers. The calculated electron affinities, in good agreement with the available experimental values, are 4.23 eV for MoF6, 5.89 eV for TcF6, 7.01 eV for RuF6, 6.80 eV for RhF6, 7.95 eV for PdF6, and 8.89 eV for AgF6. The corresponding pentafluorides are also very strong Lewis acids, although their acidities on the pF- scale are about one unit lower than those of the third row pentafluorides. The performance of a wide range of DFT exchange-correlation functionals was benchmarked by comparing them to our more accurate CCSD(T) results.
2010. "Prediction of Reliable Metal-PH₃ Bond Energies for Ni, Pd, and Pt in the 0 and +2 Oxidation States." Inorganic Chemistry 49(12):5546-5553. doi:10.1021/ic1004853 Abstract Phosphine-based catalysts play an important role in many metal-catalyzed carbon-carbon bond formation reactions yet reliable values of their bond energies are not available. We have been studying homogeneous catalysts consisting of a phosphine bonded to a Pt, Pd, or Ni. High level electronic structure calculations at the CCSD(T)/complete basis set level were used to predict the M-PH₃ bond energy (BE) for the 0 and +2 oxidation states for M=Ni, Pd, and Pt. The calculated bond energies can then be used, for example, in the design of new catalyst systems. A wide range of exchange-correlation functionals were also evaluated to assess the performance of density functional theory (DFT) for these important bond energies. None of the DFT functionals were able to predict all of the M-PH3 bond energies to within 5 kcal/mol, and the best functionals were generalized gradient approximation functionals in contrast to the usual hybrid functionals often employed for main group thermochemistry.
2010. "Third Row Transition Metal Hexafluorides, Extraordinary Oxidizers, and Lewis Acids: Electron Affinities, Fluoride Affinities, and Heats of Formation of WF₆, ReF₆, OsF₆, IrF₆, PtF₆, and AuF₆." Inorganic Chemistry 49(3):1056-1070. doi:10.1021/ic901967h Abstract High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the third row transitionmetal hexafluorides. The electron affinities, heats of formation, first (MF₆ → MF₅ + F) and average M-F bond dissociation energies, and fluoride affinities of MF₆ (MF₆ + F⁻→ MF₇ ⁻) and MF₅ (MF₅ + F⁻→ MF₆ ⁻) were calculated. The electron affinities which are a direct measure for the oxidizer strength increase monotonically from WF₆ to AuF₆, with PtF₆ and AuF₆ being extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values, and are as follows: WF₆ (3.15 eV), ReF₆ (4.58 eV), OsF₆ (5.92 eV), IrF₆ (5.99 eV), PtF₆ (7.09 eV), and AuF₆ (8.20 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated, and only three gave satisfactory results. The corresponding pentafluorides are extremely strong Lewis acids, with OsF₅, IrF₅, PtF₅, and AuF₅ significantly exceeding the acidity of SbF₅. The optimized geometries of the corresponding MF₇⁻ anions for W through Ir are classical MF₇⁻ anions with M-F bonds; however, for PtF₇⁻ and AuF₇⁻ non-classical anions were found with a very weak external F-F bond between an MF₆⁻ fragment and a fluorine atom. These two anions are text book examples for “superhalogens” and can serve as F atom sources under very mild conditions, explaining the ability of PtF₆ to convert NF₃ to NF₄⁺, ClF₅ to ClF₆⁺, and Xe to XeF⁺ and why Bartlett failed to observe XePtF₆ as the reaction product of the PtF₆/Xe reaction.
2010. "A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis- 1,3,5-hexatriene." Journal of Molecular Spectroscopy 262(1):49-60. doi:10.1016/j.jms.2010.05.002 Abstract Assignments of the vibrational fundamentals of cis- and trans-1,3,5-hexatriene are reevaluated with new infrared and Raman spectra and with quantum chemical predictions of intensities and anharmonic frequencies. The rotational structure is analyzed in the high-resolution (0.0013-0.0018 cm -1) infrared spectra of three C-type bands of the trans isomer and two C-type bands of the cis isomer. The bands for the trans isomer are at 1010.96 cm-1 (v14), 900.908 cm-1 (v16), and 683.46 cm-1 (v17). Ground state (GS) rotational constants have been fitted to the combined ground state combination differences (GSCDs) for the three bands of the trans isomer. The bands for the cis isomer are at 907.70 cm-1 (v33) and 587.89 cm-1 (v35). GS rotational constants have been fitted to the combined GSCDs for the two bands of the cis isomer and compared with those obtained from microwave spectroscopy. Small inertial defects in the GSs confirm that both molecules are planar. Upper state rotational constants were fitted for all five bands.