Scientific Publications 2004
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2004. "Biogeochemical Transformation of Fe Minerals in a Petroleum-Contaminated Aquifer." Geochimica et Cosmochimica Acta 68(8):1791-1805. Abstract Biogeochemical Transformation of Fe Minerals in a Petroleum-Contaminated Aquifer
2004. "Influence of a Fluorobenzene Nucleobase Analogue on the Conformational Flexibility of RNA Studied by Molecular Dynamics Simulations." Nucleic Acids Research 32(21):6304-6311. doi:10.1093/nar/gkh971 Abstract No abstract is available for this article at this time.
2004. "Multiple Aromaticity and Antiaromaticity in Silicon Clusters." Chemphyschem 5(12):1885-1891. Abstract A series of silicon clusters four atoms, but with different charge states (Si42+, Si4, Si42-, and NaSi4-), are studied using photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series are interpreted in terms of aromaticity and antiaromaticity, allowing prediction of how structures of how structures of the four-atom silicon cluster change upon addition or reduction of two electrons. It is shown that Si42+ is square planar, analogous to the recently discovered aromatic A142- cluster. Upon addition of two electrons, the neutral Si4 becomes σ-antiaromatic, resulting in a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si42-: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of the doubly charged Si42-, a stabilizing cation Na+ was used to produce Si42- in the gas phase in the form of Na+ [Si42-], which was characterized experimentally using photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+ [Si42] species is highly unusual in chemistry.
2004. "Multiple Aromaticity and Antiaromaticity in Silicon Clusters." Chemphyschem 5:1885-1891. Abstract A series of silicon clusters containing four atoms but with different charge states (Si42+, Si4, Si42-, and NaSi4-) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si42+ is square-planar, analogous to the recently discovered aromatic Al42- cluster. Upon addition of two electrons, neutral Si4 becomes σ-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si42-: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si42-, a stabilizing cation (Na+) was used to produce Si42- in the gas phase in the form of Na+[Si42-], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+[Si42-] species is highly unusual.
2004. "Observation of Au2H- Impurity in Pure Gold Clusters and Implications for the Anomalous Au-Au Distances in Gold Nanowires." Journal of Chemical Physics 121(17):8231-8236. Abstract Au2H- was recognized and confirmed as a minor contamination to typical photoelectron spectra of Au2-, produced by laser vaporization of a pure Au target using an ultrahigh purity helium carrier gas. The hydrogen source was shown to be from trace H impurities present in the bulk gold target. Carefully designed experiments using H2- and D2- seeded helium carrier gas were used to study the electronic structure of Au2H- and Au2D- using photoelectron spectroscopy and density functional calculations. Well-resolved photoelectron spectra with vibrational resolution were obtained for Au2H- and Au2D-. Two isomers were observed both experimentally and theoretically.
2004. "Electronic Structure and Chemical Bonding in MOn- and MOn Clusters (M=Mo, W; n=3-5): A Photoelectron Spectroscopy and ab Initio Study." Journal of the American Chemical Society 126(49):16134-16141 . doi: 10.1021/ja046536s Abstract Photoelectron spectroscopy (PES) and ab initio calculations are combined to investigate the electronic structure of MOn- clusters (M=W, Mo; n=5). Similar PES spectra were observed between the W and Mo species. A large energy gap between the first and second PES bands was observed for MO3- and correlated with a stable closed-shell MO3 neutral cluster. The electron binding energies of MO4- increase significantly relative to those of MO3-, and there is also an abrupt spectral pattern change between MO3- and MO4-, Both MO4- and MO5- give PES features with extremely high electron binding energies (>5.0eV) due to oxygen-2p-based orbitals. The experimental results are compared with extensive density functional and ab initio [CCSD(T)] calculations, which were performed to elucidate the electronic and structural evolution for the tungsten oxide clusters. WO3 is found to be a closed-shell, nonplanar molecule with C3v symmetry. WO4 is shown to have a triplet ground state (3A2) with D2d symmetry, whereas WO5 is found to be an unusual charge-transfer complex, (O2-)WO3+. WO4 and WO5 are shown to posses W-O. and O2-. Radical characters, respectively.
2004. "Icosahedral Gold Cage Clusters: M@Au₁₂⁻ (M = V, Nb, and Ta)." Journal of Chemical Physics 121(17):8369-8374. Abstract We report the observation and characterization of a series of stable bimetallic 18-valence-electron clusters containing a highly symmetric 12-atom icosahedral Au cage with an encapsulated central heteroatom of group VB transition metals, M@Au₁₂⁻ (M = V, Nb, Ta). Electronic and structural properties of these clusters were probed by anion photoelectron spectroscopy and theoretical calculations. Characteristics of the M@Au₁₂⁻ species include their remarkably high binding energies and relatively simple spectral features, which reflect their high symmetry and stability. The adiabatic electronic binding energies of M@Au₁₂⁻ were measured to be 3.70 ± 0.03, 3.77 ± 0.03, and 3.76 ± 0.03 eV for M = V, Nb, and Ta, respectively. Comparison of density functional calculations with experimental data established the highly symmetric icosahedral structures for the 18-electron cluster anions, which may be promising building blocks for cluster-assembled nanomaterials in the form of stoichiometric [M@Au₁₂⁻]X⁺ salts.
2004. "Icosahedral Gold Cage Clusters: M@Au-12 (M=V, Nb, and Ta)." Journal of Chemical Physics 121(17):8369-8374. Abstract Gold clusters and nanoparticles have become an active research field lately because of the discovery of remarkable catalytic properties of nanogold and their potential applications in nanoelectronics, nanosensors, and as biological markers. The chemistry of gold is dominated by the strong relativistic effects and the so-called aurophilic attraction, leading to highly unusual structures for gold clusters and compounds relative to those for copper and silver. One of the most interesting findings has been the planar gold cluster anions with more than ten atoms. These unique planar cluster anions were experimentally discovered using ion mobility, interpreted on the basis of the strong relativistic effects of Au, and were further confirmed by a joint photoelectron spectroscopy (PES) and density-functional theory (DFT) study. Among other exciting discoveries in elemental gold clusters include the tetrahedral Au20 cluster and the golden fullerene Au32 cage cluster reported recently.
2004. "Competition Between Linear and Cyclic Structures in Monochromium Carbide Clusters CrCn- and CrCn (n=2-8): A Photoelectron Spectroscopy and Density Functional Study." Journal of Chemical Physics 120(19):8996-9008. Abstract Photoelectron spectroscopy (PES) is combined with density functional theory (DFT) to study the monochromium carbide clusters CrCn- and CrCn (n=2-8). Well-resolved PES spectra were obtained, yielding structural, electronic, and vibrational information about both the anionic and neutral clusters. Experimental evidence was observed for the coexistence of two isomers for CrC2-, CrC3-, CrC4- and CrC6-. Sharp and well-resolved PES spectra were observed for CrCn- (n=4,6,8), whereas broad spectra were observed for CrC5- and CrC7-. Extensive DFT calculations using the generalized gradient approximation were carried out for the ground and low-lying excited states of all the CrCn- and CrCn species, as well as coupled-cluster calculations for CrC2- and CrC2. Theoretical electron affinities and vertical detachment energies were calculated and compared with the experimental data to help the assignment of the ground states and obtain structural information. We found that CrC2- and CrC3- each possess a close-lying cyclic and linear structure, which were both populated experimentally.
2004. "Sequential Oxidation of the Cubane [4Fe-4S] Cluster from [4Fe-4S]- to [4Fe-4S]3+ in Fe4S4Ln- Complexes." Journal of the American Chemical Society 126(27):8413-8420. Abstract Gaseous Fe4Sn- (n = 4-6) clusters and synthetic analog complexes, Fe4S4Ln- (L = Cl, Br, I; n = 1-4), were produced by laser vaporization of a solid Fe/S target and electrospray from solution samples, respectively, and their electronic structures were probed by photoelectron spectroscopy. Low binding energy features derived from minority-spin Fe 3d electrons were clearly distinguished from S-derived bands. We showed that the electronic structure of the simplest Fe4 S4- cubane cluster can be described by the two-layer spin-coupling model previously developed for the [4Fe] cubane analogs. The photoelectron data revealed that each extra S atom in Fe4S5- and Fe4S6- removes two minority-spin Fe 3d electrons from the [4Fe-4S] cubane core and each halogen ligand removes one Fe 3d electron from the cubane core in the Fe4S4Ln- complexes, clearly revealing a behavior of sequential oxidation of the cubane over five formal oxidation states: [4Fe-4S]- [4Fe-4S]0 [4Fe-4S]+ [4Fe-4S]2+ [4Fe-4S]3+. The current work shows the electron-storage capability of the [4Fe-4S] cubane, contributes to the understanding of its electronic structure, and further demonstrates the robustness of the cubane as a structural unit and electron transfer center.
2004. "Chromogenic and Neurotoxic Effects of Aliphatic γ-Diketone: Computational Insights into the Molecular Structures and Mechanism." Journal of Physical Chemistry B 108(19):6098-6104. doi:10.1021/jp0312868 Abstract First principles electronic structure calculations have been performed to predict chromogenic properties of various candidate structures, including pyrrole monomers and dimers and their derivatives, of the chromophores formed from the reactions of 2,5-hexanedione (2,5-HD), a prototype of neurotoxic aliphatic y-diketones, with NH3, amino acids, and proteins. The calculated results indicate that the pyrrole monomer structures and a previously proposed dimer structure do not have an absorption in the visible region whereas a novel type of pyrrole dimer structure has absorptions in the visible region if the methyl (CH3) groups on the pyrrole rings are oxidized to CHO groups. The calculated results for the oxidized pyrrole dimer models for cross-linked proteins are consistent with all of the available experimental data for the chromogenic and neurotoxic effects of 2,5-HD. Our results strongly support the conclusion that the chromogenic effects of aliphatic y-diketones are closely related to their neurotoxic effects and further predict that both the chromogenic and neurotoxic effects are associated with the same chemical reaction process. Such a reaction process most likely starts from the formation of the pyrrole-protein adducts followed by the dimerization and further oxidation.
2004. "Anisotropic Thermal Properties of Single-Wall Carbon Nanotube Reinforced Nanoceramics." Philosophical Magazine Letters 84(2):419-423. Abstract Dense single-wall carbon nanotube (SWCNT) reinforced alumina nanocomposites have been fabricated by novel spark-plasma-sintering (SPS) technique. Anisotropic thermal properties have been found in the carbon nanotube composites. The introduction of ropes of SWCNT gives rise to a decrease of the transverse thermal diffusivity with increasing carbon nanotube content while it does not change the in-plane thermal diffusivity. This is scientifically interesting and technologically important for the development of materials for novel thermal barrier coatings.
2004. "Characterization of the High-spin Heme x in the Cytochrome b6f Complex of Oxygenic Photosynthesis ." Biochemistry 43(51):16329-16336. doi:10.1021/bi048363p Abstract X-ray structures at 3.0 -3.1 Å resolution of the cytochrome b6f complex from the cyanobacterium, Mastigocladus laminosus (1) and the green alga, Chlamydomonas reinhardtii (2) showed the presence of a unique heme, heme x, that is covalently linked by a single thioether bond to a Cys residue (Cys35) on the electrochemically negative (n) side of the cytochrome b6 polypeptide. Heme x faces the inter-monomer quinone exchange cavity. The only axial ligand associated with this heme is a H2O or OH־ that is H-bonded to the propionate of the stromal side heme bn, showing that is penta-coordinate. The spectral properties of this heme were hardly defined at the time of the structure determination. The pyridine hemochromagen redox difference spectrum for heme x covalently bound to the cytochrome b polypeptide isolated from SDS-PAGE displays a broad spectrum of low amplitude with a peak at 553 nm, similar to that of other hemes with a single thioether linkage. The binding of CO and a hydrophobic cyanide analogue, butyl isocyanide (BIC), to dithionite-reduced b6f complex perturbs and significantly shifts the redox difference visible spectrum. Together with EPR spectra displaying ‘g’ values of the oxidized complex at 6.7 and 7.4, the character of heme x is defined to be ferric high spin in a rhombic environment. In addition to a possible function in photosystem I-linked cyclic electron transport, the 5-coordinate state implies that there is at least one more function of heme x that is related to axial binding of a physiological ligand.
2004. "Aggregation, Coarsening, and Phase Transformation in ZnS NanoparticlesStudied by Molecular Dynamics Simulations." Nano Letters 4(4):713-718. doi:10.1021/nl035238a 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. Molecular dynamics simulations at 300 K in vacuum were used to study nanoparticle motion and structural changes during aggregation and coarsening of 3 nm sphalerite (ZnS) particles and atomic diffusion during the subsequent phase transformation. Interaction forces between atoms in different nanoparticles can induce translational and rotational movements of the nanoparticles, driving them to find appropriate locations and orientations for aggregation. Following aggregation, the coarsened particle adopts a near-amorphous structure that transforms rapidly to wurtzite. Atomic diffusion is faster on the surface than in the bulk. Transient episodes of very fast atomic diffusion occur locally on the surface. Diffusion plays significant roles in nanoparticle structural change, aggregation, coarsening, and surface nucleation.
2004. "Toward the Solution Synthesis of the Tetrahedral Au₂₀ Cluster." Journal of Physical Chemistry B 108(33):12259-12263. Abstract We report solution synthesis of the recently discovered tetrahedral Au₂₀ cluster coordinated with eight PPh₃ (Ph = Phenyl) ligands. The composition and molecular weight of the Au₂₀ ( PPh₃)₈ complex were confirmed by the isotopic pattern of its doubly charged cation using high resolution mass spectrometry. Collision-induced dissociation experiment showed that four PPh₃ can be easily dissociated from Au₂₀ ( PPh₃)₈²⁺, resulting in a highly stable Au₂₀ ( PPh₃)₄²⁺ molecule. This observation is consistent with the tetrahedral structure, in which the four apex sites are expected to be strongly bound to the PPh₃ ligands, and is confirmed by theoretical calculations, which predicted a highly stable Au₂₀ (PH₃)₄ complex with Au-PH₃ bond energies of ~1 eV. The current experimental and theoretical observations suggest that large quantity of ligand-stabilized tetrahedral Au₂₀ can be obtained, opening the door for exploring its anticipated novel chemical, optical, and catalytical properties.
2004. "Studies of Electronic Stopping Powers Using Time of Flight Spectrometry." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:256-262. Abstract Determination of electronic stopping powers using Time of Flight (ToF) spectrometry have been demonstrated by measuring energy loss of He, O, and Al particles based on a ToF Elastic Recoil Detection Analysis (ERDA) set-up. In transmission geometry, the energy loss of the particles in self-supported stopping foils of C, Si and SiC is measured over a continuous range of energies using the ToF spectrometer. This study emphasizes the difference of the stopping power determination with and without dependence on the Si detector calibration over a wide energy range. By calibrating the Si detector for each channel over the measured energy region, the improved approach eliminates much of the error associated with pulsed height defects and measurement uncertainties of less than 4% are achieved. Stopping powers from this study are compared with limited experimental data from the literature and SRIM (The Stopping and Range of Ions in Matter) 2000 and 2003 predictions. In general, the predicted values are in reasonable agreement with the experimental data, and an improved accuracy of SRIM 2003 over SRIM 2000 can be observed in some cases. Furthermore, Bragg’s rule is valid in SiC for O and Al over the energy region studied.
2004. "Damage Accumulation and Defect Relaxation in 4H-SiC." Physical Review. B, Condensed Matter and Materials Physics 70(12):125203, 1-7. Abstract A nonlinear dependence of damage disorder on dose is observed for both the Si and C sublattices in 4H-SiC under 2 MeV Au irradiation at 165 K. The relative disorder observed along the <4403> direction is much higher than that along the <0001> direction. Molecular dynamics (MD) simulations demonstrate that most single interstitial configurations are formed on the Si-C dimer rows that are parallel to the <0001> direction. As a result, these interstitials are shielded by the Si and C atoms on the lattice sites, which significantly reduces the contribution of these interstitials to the backscattering/reaction yield along the <0001> direction. During isochronal annealing below room temperature, the relative disorder decreases along the <0001> direction, as expected; however, the disorder is stable on the Si sublattice and increases slightly on the C sublattice along the <4403> direction due to relaxation of some metastable defects to lower energy configurations. As the annealing temperature increases, similar recovery behavior on both the Si and C sublattices along the <0001> direction indicates coupling of Si and C recovery processes; however, slightly higher recovery temperatures on the C sublattice along the <4403> direction suggests some decoupling of the Si and C recovery processes. Based on the structures and energetics of defects from MD simulations, new insights into defect configurations and relaxation processes are described.
2004. "Damage Accumulation and Amorphization in Samarium Titanate Pyrochlore." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:89-94. Abstract Damage accumulation in Sm2Ti₂O₇ single crystals irradiated with Au₂⁺ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectrometry using a 2.0 MeV He⁺ beam along the <001> channeling direction. The relative disorder on the Sm sublattice follows a nonlinear dependence on ion fluence. The nonlinear behavior is described well by a disorder accumulation model that indicates a predominant role of a defect-stimulated amorphization process. The critical dose for amorphization at 300 K is ~0.14 dpa, which is in good agreement with in-situ transmission electron microscopy results for polycrystalline Sm₂Ti₂O₇ irradiated with 600 keV Bi⁺ ions and with Gd₂Ti₂O₇ doped with 244Cm. Despite the six orders of magnitude difference in damage rates, the good agreement between the amorphization doses in Sm2Ti₂O₇ at 300 K and 244Cm-doped Gd₂Ti₂O₇ at 340 K indicates that damage accumulation at these temperatures is relatively independent of dose rate.
2004. "Electronic Stopping Powers in Silicon Carbide." Physical Review. B, Condensed Matter and Materials Physics 69(20):205201, 1-9. Abstract The stopping powers in silicon carbide (SiC) of nine different ions, ranging from Be to Au, have been determined using a time-of-flight elastic recoil detection analysis (TOF ERDA) set-up. In transmission geometry, the energy loss of ions in a self-supporting SiC film is measured over a continuous range of recoil energies, from tens to hundreds keV/nucleon, using Time-of-Flight (TOF) spectrometry. By essentially calibrating the Si detector for each channel using the TOF spectrometer, the error resulting from nominal energy calibration is eliminated. An uncertainty of less than 4% is achieved in the stopping measurements. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) code are in reasonable agreement with much of the experimental data, and the SRIM 2003 predictions are in somewhat better agreement than SRIM 2000 for most ions. There are, however, still some discrepancies between SRIM predictions and the experimental data. For Ni, I and Au ions, the predicted values from SRIM 2003 are up to 40% less than the measured values. The stopping data from this study as well as previous work suggest that both the modified Bohr and Bloch formulas are suitable for scaling the stopping number of heavy ions in SiC within the classical interaction regime, and the Bloch formula is more appropriate to use at higher energies.
2004. "Electronic Stopping Powers for Heavy Ions in Silicon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 215(1-2):48-56. Abstract The stopping powers in silicon of heavy ions, with atomic numbers ranging from 4 to 29, have been determined using a Time-of-Flight Elastic Recoil Detection Analysis (ToF ERDA) set-up. In transmission geometry, the energy loss of heavy elastic recoils in the self-supporting silicon foil of known thickness is measured over a continuous range of recoil energies using Time-of-Flight (ToF) spectrometry. By eliminating the well-known calibration problem of Si detectors for heavy ions, an accuracy of less than 4% is achieved. The stopping powers are parameterized using a sixth order polynomial and compared with the limited experimental data in the literature. In the energy regimes where experimental data exist, the present data exhibit good agreement with most data. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) are in reasonable agreement with much of the experimental data, and SRIM 2003 predictions are in somewhat better agreement than SRIM 2000. There are, however, still some discrepancies between SRIM predictions and the experimental data.
2004. "Damage Evolution on Sm and O Sublattices in Au-Implanted Samarium Titanate Pyrochlore." Journal of Applied Physics 95(5):2866-2872. Abstract Damage evolution on the Sm and O sublattices in Sm₂Ti₂O₇ single crystals irradiated with 1 MeV Au₂⁺ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectroscopy and 16O(d,p)17O nuclear reaction analysis. The damage accumulation behavior at each irradiation temperature indicates that the relative disorder on the O sublattice is higher than that on the Sm sublattice, and the relative disorder on each sublattice follows a nonlinear dependence on dose that is well described by a disorder accumulation model. While there is little difference in damage accumulation behavior on the Sm sublattice at 170 and 300 K irradiation, the rate of damage accumulation decreases dramatically at 700 K due to dynamic recovery. The critical dose for amorphization at 170 and 300 K is ~0.14 dpa, and a higher dose of ~ 0.22 dpa is observed under irradiation at 700 K. During thermal annealing in an 18O environment, a significant increase in the 18O exchange was observed between 800 and 900 K, which is just below the previously determined critical temperature, 950 K, for amorphization in Sm₂Ti₂O₇, suggesting that the mobility of O vacancies may be important in defining the critical temperature.
2004. "Effects of Implantation Temperature on Damage Accumulation in Al-Implanted 4H-SiC." Journal of Applied Physics 95(8):4012-4018. Abstract Damage accumulation in 4H-SiC under 1.1 MeV Al₂(₂⁺) irradiation is investigated as a function of dose at temperatures from 150 to 450 K. Based on Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) channeling spectra, the damage accumulation on both the Si and C sublattices have been determined, and a disorder accumulation model has been fit to the data. The model fits indicate that defect-stimulated amorphization is the primary amorphization mechanism in SiC over the temperature range investigated. The temperature dependence of the cross section for defect-stimulated amorphization and the critical dose for amorphization indicate that two different dynamic recovery processes are present, which are attributed to short-range recombination and long-range migration of point defects below and above room temperature, respectively. As the irradiation temperature approaches the critical temperature for amorphization, cluster formation has an increasing effect on disorder accumulation, and ion flux plays an important role on the nature and evolution of disorder. Dislocation loops, which are mostly formed under high ion flux, act as sinks for point defects, thereby reducing the disorder accumulation rate.
2004. "Annealing Behavior of Al-Implantation-Induced Disorder in 4H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:647-651. Abstract Single crystal 4H-SiC films were implanted at 150 K with 1.1 MeV Al₂₂⁺ and subsequently annealed at elevated temperatures. Rutherford backscattering spectrometry (RBS) results indicate that the relative Si disorder at the damage peak recovers significantly as the annealing temperature increases. However, the residual Si disorder is more resistant to high-temperature annealing in the region of the implanted Al. The maximum concentration of Al profile measured by secondary ion mass spectroscopy (SIMS) is a factor of 1000 lower than the level of the residual Si disorder at the same region. Analysis of these results indicates that the excess residual Si disorder around the implanted Al projected range cannot be accounted for by just the Al interstitials; instead, it appears that each implanted Al stabilizes or inhibits recovery for an equivalent of a few hundred Si interstitials under the current experimental conditions.
2004. "Towards Formation of Buckminsterfullerene C₆₀ in Quantum Chemical Molecular Dynamics." Journal of Chemical Physics 122:Art No. 014708. doi:10.1063/1.1825375 Abstract The abstract is currently not available at this time.
2004. "Quantum Chemical Molecular Dynamics Model Study of Fullerene Formation from Open-Ended Carbon Nanotubes." Journal of Physical Chemistry A 108(15):3182-3194. doi:10.1021/jp0373090 S1089-5639(03)07309-2 Abstract The abstract for this product is not available at this time.
2004. "Electronic Structure of Tubular Aromatic Molecules Derived from the Metallic (5,5) Armchair Single Wall Carbon Nanotube." Journal of the American Chemical Society 126(11):3597-3607. Abstract All-electron static and time-dependent DFT electronic calculations, with complete geometrical optimization, are performed on tubular molecules up to C210H20 that are finite sections of the (5,5) metallic single wall carbon nanotube with hydrogen termination at the open ends. We find pronounced C-C bond reconstruction at the tube ends; this initiates bond alternation that propagates into the tube centers. For the especially low band gap molecules C120H20, C150H20, and C180H20, alternation increases, and a second nearly isoenergic structural isomer of different alternation is found. A small residual C-C bond alternation and band gap may be present in the infinite tube. The van Hove band gap forms quickly with length, while the metallic Fermi point (at the crossing of linear bands) forms very slowly with length. There are no endlocalized states at energies near the Fermi energy. The HOMO-LUMO gap and the lowest singlet excited state, whose energies show a periodicity with length as previously calculated, are optically forbidden. However, each molecule shows an intense visible “charge transfer” transition, not present in the infinite tube, whose energy varies smoothly with length; this transition should be an identifying signature for these molecules. The static axial polarizability per unit length increases rapidly with N as the “charge transfer” transition moves into the infrared; this indicates increasing metallic character. However, the ionization potential, electron affinity, chemical hardness, and relative energetic stability all show the length periodicity seen in the HOMO-LUMO gap, in contrast to the optical “charge transfer” transition and the static axial polarizability. These periodicities, due to a one-dimensional quantum size effect as originally modeled by Coulson in 1938, nevertheless cancel in the calculated Fermi energy, which varies smoothly toward a predicted bulk work function near 3.9 eV. A detailed study of C190H20 with up to eight extra electrons or holes shows the total energy is closely fit by a simple classical charging model, as is commonly applied to metallic clusters.
