Scientific Publications 2005
2005. "Relaxation Nuclear Magnetic Resonance Imaging Investigation of Heterogeneous Aging in a Hydroxy-Terminated Polybutadiene-Based Elastomer ." Macromolecules 38(26):10694-10701. Abstract Relaxation nuclear magnetic resonance imaging (R-NMRI) was employed to investigate the effects of thermo-oxidative aging in a hydroxy-terminated polybutadiene (HTPB) based elastomer. A series of three-dimensional (3D) Hahn-echo weighted single point images (SPI) of the elastomer were utilized to generate a 3D parameter map of the aged material. NMR spin-spin relaxation times (T2) were measured for each voxel producing a 3D NMR parameter (T2) map of the aged polymer. These T2 maps reveal a dramatic reduction of local polymer mobility near the aging surface with the degree of T2 heterogeneity varying as a function of aging. Using correlations between NMR T2 and material modulus, the impact of this heterogeneous thermo-oxidative aging on the mateial properties is discussed.
2005. "Cu3C4-: A New Sandwich Molecule with Two Revolving C22- Units ." Journal of Physical Chemistry A 109(4):562-570. Abstract A combined photoelectron spectroscopy (PES) and ab initio study was carried out on a novel copper carbide cluster in the gas phase: Cu3C4-. It was generated in a laser vaporization cluster source and appeared to exhibit enhanced stability among the Cu3Cn- series. Its PES spectra were obtained at several photon energies, showing numerous well-resolved bands. Extensive ab initio calculations were performed on Cu3C4- and two isomers were identified: a C2 structure (1A) with a Cu33+ triangular group sandwiched by two C22- units; and a linear CuCCCuCCCu structure (D-h, 1-g+). Comparison of ab initio PES spectra with experimental data showed that the sandwich Cu3C4- cluster was solely responsible for the observed spectra and the linear isomer was not present, suggesting that the C2 structure is the global minimum in accordance with CCSD(T)/6-311+G* predictions. Interestingly, a relatively low barrier (0.4-0.6 kcal/mol) was found for the internal rotation of the C22- units in the sandwich Cu3C4-. To test different levels of theory in describing the CumCn- systems and lay foundations for the validity of the theoretical methods, extensive calculations at a variety of levels were also carried out on a simpler copper carbide species CuC2-, where two isomers were found close in energy: a linear one (C-v, 1-+) and a triangular one (C2v, 1A1). The calculated electronic transitions for CuC2- were also compared with the PES data, in which both isomers were present.
2005. "Photoelectron Spectroscopy and Ab Initio Study of the Doubly-Antiaromatic B-6(2-) Dianion in the LiB6- Cluster." Journal of Chemical Physics 122(5):Art. No. 054313 . Abstract A metal-boron mixed cluster, LiB6-, was produced and characterized by photoelectron spectroscopy and ab initio calculations. A number of electronic transitions were observed and used to compare with theoretical calculations. An extensive search for the global minimum of LiB6- was carried out via an ab initio genetic algorithm technique. The pyramidal C2v (1A1) molecule was found to be the most stable at all levels of theory. The nearest low-lying isomer was found to be a triplet C2 (3B) structure, 9.2 kcal/mol higher in energy at the CCSD(T)/6-311+G(2df)//B3LYP/6-311+G* level. Comparison of calculated detachment transitions from LiB6- and the experimental photoelectron spectra confirmed the C2v pyramidal global minimum structure. Natural population calculation revealed that LiB6- is a charge-transfer complex, Li+B62-, in which Li+ and B62- interact in a primarily ionic manner. Analyses of the molecular orbitals and chemical bonding of B62- showed that the planar cluster is two-fold (- and -) antiaromatic, which can be viewed as the fusion of two aromatic B3- units.
2005. "Effects of Reduction Temperature and Metal-support Interactions on the Catalytic Activity of Pt/γ-Al2O3 and Pt/TiO2 for the Oxidation of CO in the Presence and Absence of H2." Journal of Physical Chemistry B 109(49):23430-23443. doi:10.1021/jp054888v Abstract TiO2- and -Al2O3-supported Pt catalysts were characterized by HRTEM, XPS, EXAFS, and in-situ FTIR after activation at various conditions and their catalytic properties were examined for the oxidation of CO in the absence and presence of H2 (PROX). When -Al2O3 was used as the support, the catalytic, electronic, and structural properties of the Pt particles formed were not affected substantially by the pretreatment conditions. In contrast, the surface properties and catalytic activity of Pt/TiO2 were strongly influenced by the pretreatment conditions. In this case, an increase in the reduction temperature led to higher electron density on Pt, altering its chemisorptive properties, weakening the Pt-CO bonds, and increasing its activity for the oxidation of CO. The in-situ FTIR data suggest that both the terminal and bridging CO species adsorbed on fully reduced Pt are active for this reaction. The high activity of Pt/TiO2 for the oxidation of CO can also be attributed to the ability of TiO2 to provide or stabilize highly reactive oxygen species at the metal-support interface. However, such species appear to be more reactive towards H2 than CO. Consequently, Pt/TiO2 shows substantially lower selectivities towards CO oxidation under PROX conditions than Pt/-Al2O3.
2005. "Erratum: Accurate Heats of Formation and Acidities for H₃PO₄, H₂SO₄, and H₂CO₃ from ab initio Electronic Structure Calculations." International Journal of Quantum Chemistry 104(3):379-380. Abstract Atomization energies and heats of formation at 0 and 298K for H₂CO₃, HCO₃-, H₂SO₄, HSO₄, H₃PO₄, H₂PO₄- were calculated by using coupled cluster theory including noniterative, quasi-perturbative triple excitations calculations with large basis sets. Optimized geometries and harmonic vibrational frequencies were calculated at the MP2/aug-cc-pVTZ level. Atomization energies were obtained by extrapolating CCSD(T)) valence energies to the complete basis set limit (CBS) by using the aug-cc-pV(N×d)Z (N ₌ D, T, Q) basis sets where the×d corresponds to the inclusion of tight d functions for the 2nd row atoms P and S. In order to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, a core/valence correction, a Douglas-Kroll-Hess scalar relativistic correction, and a first order atomic spin-orbit correction were included. The calculated heats of formation of the neutral molecules at 0K are ΔHf0(H₂CO₃) = -275.2, ΔHf0(H₂SO₄)₌ -167.9, and ΔHf0(H₃PO₄) = -268.8 kcal/mol. The only experimental value is ΔHf0(H₂SO₄)₌172.4 ± 2, 4 kcal/mol more negative. The calculated gas acidities (ΔG) at 298 K are 332.2, 322.2, and 304.6 kcal/mol for H₂CO₃, H₃PO₄, and H₂SO₄ respectively. The calculated values for H₃PO₄ and H₂SO₄ are in good agreement with the respective experimental values of 323.0 ±4.9 and 302.3 ± 2.6 kcal/mol. Solution acidities are computed using the fully polarizable continuum model. Using an electron density contour value of 0.001 a.u. to define the cavity for the neutral molecules and of 0.0022 a.u. for the anions, we calculate pKa(H₂CO₃) = 6.3, pKa(H₃PO₄) = 2.5, and pKa(HNO₃) = -2.4 which are within one pKa of the respective experimented values of 6.4, 2.1, and -1.4. We predict the pKa of H₂SO₄ in aqueous solution to be in the range of ₋6 to ₋8, significantly lower than the estimated experimental values in the range of ₋3.
2005. "The Stability of 9Cr-ODS Oxide Particles Under Heavy-Ion Irradiation ." Nuclear Science and Engineering 151(3):305-312. Abstract An oxide-dispersion-strengthened (ODS) martensitic steel 9Cr-ODS was irradiated with 5-MeV Ni ions at 500°C at a dose rate of 1.4 × 10-3 dpa/s to doses of 5, 50, and 150 dpa. The ODS steel has been designed for use in higher-temperature energy systems. However, the radiation effects are not fully characterized, particularly to high doses. Dense dislocations, precipitates, and yttrium-titanium oxide particles dominated the microstructure of 9Cr-ODS for both the unirradiated and irradiated cases with no dislocation loops observed. No voids were detected for doses up to 150 dpa. The average size of the oxide particles, whose size is approximately described by a lognormal distribution, slightly decreased with dose from ~12 nm for the unirradiated case to ~9 nm at 150 dpa. The decrease in size follows a square root of dose dependency, indicating the effect is radiation induced. The decrease in size is not expected to have a detrimental effect on high-temperature strength, even to extremely high dose.
2005. "Preparation and in situ Characterization of Surfaces Using Soft-Landing in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer." Analytical Chemistry 77(11):3452-3460. Abstract Mass-selected peptide ions produced by electrospray ionization were deposited onto fluorinated self-assembled monolayer surfaces (FSAM) surfaces by soft-landing using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying interactions of large ions with surfaces. Analysis of the modified surface was performed in situ by combining 2 keV Cs+ secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR-SIMS). Regardless of the initial charge state of the precursor ion, the SIMS mass spectra included singly-protonated peptide fragment ions and peaks characteristic of the surfaces in all cases. In some experiments multiply-protonated peptide ions and [M+Au]+ ions were also observed upon SIMS analysis of modified surfaces. For comparison with the in situ analysis of the modified surfaces, ex situ analysis of some of the modified surfaces was performed by 25 kV Ga+ time of flight – secondary ion mass spectrometry (ToF-SIMS). The ex situ analysis demonstrated that a significant number of soft-landed peptide ions remain charged on the surface even when exposed to air for several hours after deposition. Charge retention of soft-landed ions dramatically increases the ion yields obtained during SIMS analysis very sensitive detection of deposited material at less than 1% of monolayer coverage. Accumulation of charged species on the surface undergoes saturation due to Coulomb repulsion between charges at close to 30% coverage. We estimated that close to 1 ng of peptide could be deposited on the spot area of 4 mm2 of the FSAM surface without reaching saturation.
2005. "Mediating molecular recognition by methionine oxidation: Conformational switching by oxidation of methionine in the carboxyl-terminal domain of calmodulin." Biochemistry 44(27):9486-96. doi:10.1021/bi0504963 Abstract Oxidation of Met144/145 in calmodulin (CaM) functions as a sensor of oxidative stress, functioning to induce the nonproductive association of oxidized CaM (CaMox) with the plasma membrane Ca-ATPase (PMCA) and other target proteins to down-regulate cellular metabolism. To understand the mechanism underlying the stabilization of the PMCA in the inhibited state, we have engineered a CaM mutant (CaM-L7) that permits the site-specific oxidation of Met144 and Met145 and used circular dichroism (CD) and NMR spectroscopy to identify changes in the molecular interactions between CaM and the CaM-binding sequence of the PMCA that are modulated by methionine oxidation. Neither site-directed mutagenesis nor oxidation of Met144 and Met145 results in large changes in the CD spectra or the positions of cross-peaks in the NMR spectra for calcium-activated CaM, indicating that there are no gross structural changes that substantially alter the backbone fold. Nevertheless, there are significant tertiary structural changes and greater conformational heterogeneity that are apparent in the heteronuclear NMR spectra, where the positions and line-widths of selected residues are shifted and broadened. Similar localized structural changes are apparent upon association with the CaM-binding sequence C28W; however, upon complex formation the heteronuclear NMR spectra indicate a homogeneous binding of CaMox to C28W. Oxidation results in a general trend towards a random coil for residues in the C-terminal helix, as indicated by the changes in the 1H, 13C, and 13CO, with a loss of helicity for M144 and M145. Both main chain and side chain methyl group chemical shift changes indicate that, in the complex with C28W, the largest structural changes nclude residues that form the C-terminal hydrophobic pocket and that comprise the intermolecular interface with C28W. Smaller changes in the central linker region and N-terminal domain were also observed, and in the N-terminal domain involved residues that contact the C28W peptide. Using 13C-filtered, 13C-edited NMR experiments, dramatic changes in intermolecular contacts between residues in the C-terminal domain of CaM-L7 and C28W were observed upon oxidation of M144 and M145, with essentially a complete loss of contacts between C28W and M144 and M145. We propose that the inability of CaM to fully activate the PMCA after methionine oxidation originates in a reduced helical propensity in the immediate vicinity of M144 and M145, which results primarily from a global rearrangement of the tertiary structure of the C-terminal globular domain that substantially alters the interaction of this domain with the CaM binding domain of the PMCA.
2005. "ZnO nanoclusters: Synthesis and photoluminescence." Applied Physics Letters 87:241917. doi:10.1063/1.2147715 Abstract ZnO nanoclusters were prepared and deposited at room temperature using a newly developed cluster source. The nanoclusters act as a building block for the cluster films deposited on various substrates. The cluster films were characterized by transmission electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and photoluminescence. We prepared monodispersed crystalline ZnO nanoclusters of ~7 nm diameter. These clusters have a significant blueshift of ~125 meV (compared to the results published so far) within the ultraviolet region at room temperature. No PL in our samples was observed in the visible region, which implies negligible defect formation in ZnO nanocluster films.
2005. "1H, 13C, and 15N resonance assignments for Escherichia coli ytfP, a member of the broadly conserved UPF0131 protein domain family." Journal of Biomolecular NMR 33(3):197. Abstract Protein ytfP from Escherichia coli (Swiss-Prot ID: YTFP_ECOLI; NESG target ID: ER111; Wunderlich et al., 2004) is a 113-residue member of the UPF0131 protein family (Pfam ID: PF03674) of unknown function. This domain family is found in organisms from all three kingdoms, archaea, eubacteria and eukaryotes. Using triple resonance NMR techniques, we have determined 97% of backbone and 91% of side chain 1H, 13C, and 15N resonance assignments. The chemical shift and 3J(HN–Ha) scalar coupling data reveal a mixed a/b topology,β∞ββ∞βββ. BMRB deposit with Accession No. 6448. Reference: Wunderlich et al. (2004) Proteins, 56, 181–187.
2005. "Electrochemical Corrosion Behavior of Low Carbon I-Beam Steels In Simulated Yucca Mountain Repository Environment." Corrosion 61(4):381-391. Abstract The electrochemical corrosion behavior of low carbon steel was examined in a simulated Yucca Mountain (YM) ground water by varying the electrolyte concentration and temperature under aerated and deaerated conditions. The results show that in deaerated conditions, the corrosion rate is low in the order of 0.6 to 4.5mpy, between 25 to 85oC, respectively. However, in aerated conditions the measured rates were expectedly very high, in the order of 3-55mpy in the above mentioned temperature levels. The rates initially increased up to 45oC, and a decreasing trend was observed with further increase in temperature from 65 to 85oC. The maximum corrosion rate was occurred at 45oC (54.5mpy). The low corrosion rates observed in all deaerated conditions, and in aerated solutions at higher temperatures were due to the preferential adsorption of Mg-species on the steel surface, as identified by XPS analyses. The results also indicate possible localized corrosion behavior of carbon steel in aerated conditions up to 45oC.
2005. "Adsorption and Reaction of CO and CO₂ on Oxidized and Reduced SrTiO₃ (100) Surfaces." Journal of Physical Chemistry B 109(20):10327-10331. Abstract The adsorption and reaction of CO and CO2 on oxidized and reduced SrTiO3(100) surfaces have been studied using temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). XPS results indicate that the fully oxidized and vacuum annealed SrTiO3(100) surfaces are nearly defect-free with predominantly Ti4+ ions whereas the sputter-reduced surfaces contain substantial amounts of defects (~ 40% reduced Ti sites). Both CO and CO2 are found to adsorb weakly on the oxidized SrTiO3(100) surfaces. The desorption activation energies of CO and CO2 from the oxidized SrTiO3(100) surfaces are 38 and 32 kJmol-1, respectively, following adsorption at 110 K. On sputter-reduced surfaces, enhanced reactivity of CO and CO2 is observed due to the presence of oxygen vacancy sites, causing dissociative adsorption of these molecules. Our studies indicate that CO and CO2 molecules exhibit relatively weaker binding to SrTiO3(100) compared with TiO2(110) and TiO2(100) surfaces. These differences in reactivity can be attributed primarily to the influence of the Sr cations on the electronic structure of the mixed oxide of SrTiO3.
2005. "Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia." Applied Physics Letters 86(13):131906-131909. Abstract Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.