Computational Studies Of The Role Of Metal Oxides In Combustion-Generated Environmentally Persistent Free Radicals, Randall Hall
Dec 2011
Computational Studies Of The Role Of Metal Oxides In Combustion-Generated Environmentally Persistent Free Radicals, Randall Hall
Randall W. Hall
No abstract provided.
A Kink-Based Path Integral Method For Multi-Electron Systems, Randall Hall
May 2010
A Kink-Based Path Integral Method For Multi-Electron Systems, Randall Hall
Randall W. Hall
No abstract provided.
Ab Initio Study Of The Formation And Degradation Reactions Of Semiquinone And Phenoxyl Radicals, Randall W. Hall, Cheri A. Mcferrin, Barry Dellinger
Dec 2007
Ab Initio Study Of The Formation And Degradation Reactions Of Semiquinone And Phenoxyl Radicals, Randall W. Hall, Cheri A. Mcferrin, Barry Dellinger
Randall W. Hall
Calculations of the energetics of formation, stability, and reactivity of o-semiquinone, p-semiquinone, and phenoxyl radicals have been performed using B3LYP/6-31G(d,p), BHandHLYP/6-31G(d,p), BHandHLYP/6-311++G(d,p), BHandHLYP/aug-cc-pVDZ, and QCISD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p) model chemistries. Formation of these radicals from potential molecular precursors catechol, hydroquinone, and phenol is readily achieved under combustion conditions through unimolecular scission of the phenoxyl–hydrogen bond or abstraction of the phenoxyl hydrogen by a hydrogen atom or hydroxyl radical. The resulting radicals are resonance stabilized and resist decomposition and oxidation. Activation energies for the decomposition of the radicals through concerted elimination of carbon monoxide range from ≈55 to 75 kcal/mol. Activation energies for the …
Structural Characterization Of Al10O6IBu16(Μ-H)2, A High Aluminum Content Cluster: Further Studies Of Methylaluminoxane (Mao) And Related Aluminum Complexes, Feng-Jeng Wu, Larry S. Simeral, Anthony A. Mrse, Jan L. Eilertsen, Lacramioara Negureanu, Zhehong Gan, Frank R. Fronczek, Randall W. Hall, Leslie G. Butler
Dec 2007
Structural Characterization Of Al10O6IBu16(Μ-H)2, A High Aluminum Content Cluster: Further Studies Of Methylaluminoxane (Mao) And Related Aluminum Complexes, Feng-Jeng Wu, Larry S. Simeral, Anthony A. Mrse, Jan L. Eilertsen, Lacramioara Negureanu, Zhehong Gan, Frank R. Fronczek, Randall W. Hall, Leslie G. Butler
Randall W. Hall
The first structurally characterized isobutyl-containing aluminoxane compound is presented. The Al10O6iBu16(μ-H)2 (I) cluster is produced from neat octakis-isobutyltetraluminoxane (Al4O2iBu8) at 80 °C in 6−8 h followed by slow crystallization. The crystal is triclinic (space group P1̄) with the molecule lying on an inversion center. This aluminoxane contains both nearly linear, 154(2)°, aluminum-bridging hydrides and three-coordinate aluminum sites. Solid-state 27Al magic-angle spinning (MAS) NMR experiments were done at 19.6 and 40 T (833 MHz and 1.703 GHz, 1H) and at 30−35 kHz spinning speeds, leading to the determination of the Cq and η values for the two four-coordinate Al sites and …
Methyaluminoxane (Mao) Polymerization Mechanism And Kinetic Model From Ab Initio Molecular Dynamics And Electronic Structure Calculations, Lacramioara Negureanu, Randall W. Hall, Leslie G. Butler, Larry A. Simeral
Dec 2005
Methyaluminoxane (Mao) Polymerization Mechanism And Kinetic Model From Ab Initio Molecular Dynamics And Electronic Structure Calculations, Lacramioara Negureanu, Randall W. Hall, Leslie G. Butler, Larry A. Simeral
Randall W. Hall
MAO is the cocatalyst in metallocene catalytic systems, which are widely used in single-site olefin polymerization due to their high stereoselectivity. To date, the structures of the catalytically active compound or compounds in MAO have eluded researchers. Although many structural models have been proposed, none are generally accepted. In this study, aspects of the formation mechanism of MAO are addressed. Molecular dynamics simulations at the MP2 level of theory were carried out for presumed elementary steps in MAO formation via hydrolysis of trimethylaluminum (TMA). Methane production was observed, in agreement with experiment, as well as intermediate species that are consistent …
Ab Initio And 1H Nmr Study Of The Zn(Ii) Complexes Of A Nido- And A Closo-Carboranylporphyrin, Petia Bobadova-Parvanova, Yuko Oku, Anura Wickramasinghe, Randall W. Hall, Graca H. Vicente
Jun 2004
Ab Initio And 1H Nmr Study Of The Zn(Ii) Complexes Of A Nido- And A Closo-Carboranylporphyrin, Petia Bobadova-Parvanova, Yuko Oku, Anura Wickramasinghe, Randall W. Hall, Graca H. Vicente
Randall W. Hall
n ab initio study of a promising nido-carboranylporphyrin for the boron neutron capture therapy of tumors, and of its closo precursor is reported. Base-induced deboronation of neutral ZnDCP, believed to exist as a mixture of 3 stereoisomers, produces the tetraanionic ZnDCP4- as a complex mixture of isomers. 1H NMR data and ab initio calculations support these findings. The position of the axial pyridine ligand in ZnDCP4- and the orientation of the endo hydrogen atoms on the open faces of the nido-carborane cages significantly influence the total energy of the ZnDCP4- structures. It is suggested that the "cocktail" of isomers possibly …
Tools And Strategies For Processing Diffusion-Ordered 2d Nmr Spectroscopy (Dosy) Of A Broad, Featureless Resonance: An Application To Methylaluminoxane (Mao), Jan Lasse Eilertsen, Randall W. Hall, Larry S. Simeral, Leslie G. Butler
Feb 2004
Tools And Strategies For Processing Diffusion-Ordered 2d Nmr Spectroscopy (Dosy) Of A Broad, Featureless Resonance: An Application To Methylaluminoxane (Mao), Jan Lasse Eilertsen, Randall W. Hall, Larry S. Simeral, Leslie G. Butler
Randall W. Hall
DOSY has been extremely successful in many studies of molecular weight distributions, especially when the components are separable along the chemical shift axis. However, an unresolved NMR resonance yields the familiar problem of overlapping exponential decays. In a study of methylaluminoxane (MAO), a set of data processing and simulation tools were developed: read Bruker data files (Matlab); preliminary non-linear least-squares fit with f-test (Matlab); movie generation of the fits (Matlab); conversion of diffusion coefficients to molecular masses through molecular volumes (Gaussian-98); and simulation of DOSY data sets for various molecular mass distributions (Mathematica). These tools are presented here and briefly …
High-Field 19.6 T 27Al Solid-State Mas Nmr Of In Vitro Aluminated Brain Tissue, Pamela L. Bryant, Walter J. Lukiw, Zhehong Gan, Randall W. Hall, Leslie G. Butler
Sep 2003
High-Field 19.6 T 27Al Solid-State Mas Nmr Of In Vitro Aluminated Brain Tissue, Pamela L. Bryant, Walter J. Lukiw, Zhehong Gan, Randall W. Hall, Leslie G. Butler
Randall W. Hall
The combination of 27Al high-field solid-state NMR (19.6 T) with rapid spinning speeds (17.8 kHz) is used to acquire 27Al NMR spectra of total RNA human brain temporal lobe tissues exposed to 0.10 mM Al3+ (as AlCl3) and of human retinal pigment epithelial cells (ARPE-19), grown in 0.10 mM AlCl3. The spectra of these model systems show multiple Al3+ binding sites, good signal/noise ratios and apparent chemical shift dispersions. A single broad peak (−3 to 11 ppm) is seen for the aluminated ARPE-19 cells, consistent with reported solution-state NMR chemical shifts of Al-transferrin. The aluminated brain tissue has a considerably …
Structure Of Xe+N Clusters (N=3–30): Simulation And Experiment, Jose A. Gascon, Randall W. Hall, Christoph Ludewigt, Helmut Haberland
Dec 2001
Structure Of Xe+N Clusters (N=3–30): Simulation And Experiment, Jose A. Gascon, Randall W. Hall, Christoph Ludewigt, Helmut Haberland
Randall W. Hall
We present an experimental and computational study of the photoabsorption line shape of Xe+N clusters. Positively charged xenon clusters have a unique feature not seen in lighter rare gas cation clusters: there are two families of isomers whose ground states consist of neutral atoms surrounding either a linear trimer ion core or a linear tetramer ion core. Interconversion of these two isomers is possible at temperatures as low as 60 K. The combination of simulation and experiment demonstrates the existence of these two families of isomers and their manifestation in the photoabsorption spectra. Clusters present, in general, either of the …
Automated, Web-Based, Second-Chance Homework, Randall W. Hall, Leslie G. Butler, Saundra Y. Mcguire, Sean P. Mcglynn, Gary L. Lyon, Ron L. Reese, Patrick A. Limbach
Nov 2001
Automated, Web-Based, Second-Chance Homework, Randall W. Hall, Leslie G. Butler, Saundra Y. Mcguire, Sean P. Mcglynn, Gary L. Lyon, Ron L. Reese, Patrick A. Limbach
Randall W. Hall
Procedures and programs for Web-based homework administration were developed and tested. The special feature of the protocol is the option of a "second chance". When a student gives a wrong answer in the weekly homework assignment, a second version of the same question is posed to that student within 48 hours. Both the original and the second versions of the homework are Web-based and automatically graded. More than 90% of the students used the second chance at least once during the semester. More than 60% used this feature for every homework assignment. The second-chance option motivates students to study the …
Opportunities In Computational Materials Science, Randall Hall, Juana Moreno
Sep 2001
Opportunities In Computational Materials Science, Randall Hall, Juana Moreno
Randall W. Hall
The White House Materials Genome Initiative intends to double the speed with which we discover, develop, and manufacture new materials. In order for this initiative to be successful an unprecedented collaboration between computer scientists, applied mathematicians, computational scientists, and engineers with expertise in each of the aspects of the simulation-guided design of modern materials must be established. We must also take advantage of the enormous national investments in the next generation of hyperparallel, heterogeneous, multicore supercomputers to develop experimentally verified algorithms. In this talk I will describe new collaborative efforts in Louisiana towards developing a State-wise team of scientist to …
Spectra And Geometries Of Ar+ N (N = 30 - 60) [Invited], Jose Gascon, Randall W. Hall
May 2001
Spectra And Geometries Of Ar+ N (N = 30 - 60) [Invited], Jose Gascon, Randall W. Hall
Randall W. Hall
We report finite temperature simulations of AR+n (N = 30−60) using a semiempirical model Hamiltonian. We calculate the photoabsorption spectra and analyze the charge distribution in both ground and excited states. The maximum photoabsorption wavelength for this set of clusters is in good agreement with experimental data. We analyze the average position of the atoms and the finite temperature distribution of the positive charge over the cluster. We find the charge localized on three atoms in the ground state and over the first two solvation spheres in the excited states. The extent of delocalization in the excited states can be …
Structural Characterization Of Mao And Related Aluminum Complexes. 1. Solid-State 27al Nmr With Comparison To Efg Tensors From Ab Initio Molecular Orbital Calculations, Pamela L. Bryant, Chris R. Harwell, Anthony A. Mrse, Earl F. Emery, Zhehong Gan, Tod Caldwell, Arneil P. Reyes, Philip Kuhns, David W. Hoyt, Larry S. Simeral, Randall W. Hall, Leslie G. Butler
Dec 2000
Structural Characterization Of Mao And Related Aluminum Complexes. 1. Solid-State 27al Nmr With Comparison To Efg Tensors From Ab Initio Molecular Orbital Calculations, Pamela L. Bryant, Chris R. Harwell, Anthony A. Mrse, Earl F. Emery, Zhehong Gan, Tod Caldwell, Arneil P. Reyes, Philip Kuhns, David W. Hoyt, Larry S. Simeral, Randall W. Hall, Leslie G. Butler
Randall W. Hall
Experimental and ab initio molecular orbital techniques are developed for study of aluminum species with large quadrupole coupling constants to test structural models for methylaluminoxanes (MAO). The techniques are applied to nitrogen- and oxygen-containing complexes of aluminum and to solid MAO isolated from active commercial MAO preparations. (Aminato)- and (propanolato)aluminum clusters with 3-, 4-, and 6-coordinate aluminum sites are studied with three 27Al NMR techniques optimized for large 27Al quadrupole coupling constants: field-swept, frequency-stepped, and high-field MAS NMR. Four-membered (aminato)aluminum complexes with AlN4 coordination yield slightly smaller Cq values than similar AlN2C2 sites: 12.2 vs 15.8 MHz. Planar 3-coordinate AlN2C …
Ab Initio Calculation Of 81Br Nqr Transition Frequencies Of Brominated Aromatics (Flame Retardants), Chris R. Harwell, Anthony A. Mrse, Leslie G. Butler, Randall W. Hall
Sep 1999
Ab Initio Calculation Of 81Br Nqr Transition Frequencies Of Brominated Aromatics (Flame Retardants), Chris R. Harwell, Anthony A. Mrse, Leslie G. Butler, Randall W. Hall
Randall W. Hall
Brominated aromatics, used extensively as flame retardants, have been studied with 81Br nuclear quadrupole resonance (NQR) spectroscopy. NQR requires lengthy frequency searches because 81Br NQR transition frequencies in brominated aromatics are spread over a wide (40 MHz) range. We investigate the ability of ab initio calculations to narrow this search range by predicting 81Br NQR transition frequencies for a series of brominated aromatics, using restricted Hartree−Fock (RHF) and Becke's three-parameter Lee−Yang−Parr density functional theory hybrid method (B3LYP). Basis sets used are of double and triple-ζ quality with varying degrees of polarization included on bromine. Geometries are the isolated molecules, with …
Changes In The Fermi Surface At The Magnetization Reorientation Transition In Fe/Cu(100), Reginaldt H. Madjoe, Alexey N. Koveshnikov, Christopher Harwell, Randall W. Hall, Roger L. Stockbauer, Richard L. Lutz
Dec 1998
Changes In The Fermi Surface At The Magnetization Reorientation Transition In Fe/Cu(100), Reginaldt H. Madjoe, Alexey N. Koveshnikov, Christopher Harwell, Randall W. Hall, Roger L. Stockbauer, Richard L. Lutz
Randall W. Hall
We present a study of the evolution of the valence bandstructure of Fe/Cu(100) as a function of Fe coverage in the 0–12 monolayer range. Angle-resolved photoelectron spectroscopy was used to probe the valence electronic structure and to extract Fermi surface contours as a function of increasing Fe coverage. The first few monolayers show Fe-induced densities of states that are significantly different from those in thicker films. At 5 ML, distinct changes occur in the Fermi surface, where a magnetization reorientation transition has been observed previously.
Single-Crystal 27Al Nmr Of Andalusite And Calculated Electric Field Gradients: The First Complete Nmr Assignment For A 5-Coordinate Aluminum Site, Pamela L. Bryant, Chris R. Harwell, Katherine Wu, Frank R. Fronczek, Randall W. Hall, Leslie G. Butler
Dec 1998
Single-Crystal 27Al Nmr Of Andalusite And Calculated Electric Field Gradients: The First Complete Nmr Assignment For A 5-Coordinate Aluminum Site, Pamela L. Bryant, Chris R. Harwell, Katherine Wu, Frank R. Fronczek, Randall W. Hall, Leslie G. Butler
Randall W. Hall
Andalusite, Al2SiO5, contains 5- and 6-coordinate aluminum sites, and is a preeminent model for 27 Al NMR spectroscopy. We describe a combined NMR, crystallography, and theory project: single-crystal 27 Al NMR spectra at 298 K, the crystal structure of andalusite at 115 K, and electric field gradient calculations. The low-symmetry 5-coordinate site is a stiff test of the computational methods. In addition, the chemical shift tensor is measured for the 5-coordinate site. The small body of 27 Al NMR data, especially for rare 5-coordinate sites, inspires the calculation of NMR parameters. We explore the accuracy of two approaches for 27 …
Tight-Binding Simulations Of Argon Cation Clusters, Guillermo A. Morales, Jessica Faulkner, Randall W. Hall
Dec 1997
Tight-Binding Simulations Of Argon Cation Clusters, Guillermo A. Morales, Jessica Faulkner, Randall W. Hall
Randall W. Hall
A simple, semiempirical model was used to study the ground and excited stateproperties of argon cation clusters at 60 K. The model is a tight-binding Hamiltonian whose parameters are determined from atomic and diatomic properties.Monte Carlo simulations were used to calculate the average properties of these clusters. The photoabsorptionspectrum was in good agreement with previous calculations and experiments. The splitting of the photoabsorptionspectrum for clusters with greater than 14 atoms was investigated. The two excited states corresponding to the splitting arise from a 3-atom ion core, perturbed by a 4th atom, with solvation from the remaining atoms. The perturbation of …
A Non-Atom-Based Orbital Basis Set For Small Sodium Clusters, Melissa R. Prince, Randall W. Hall
Apr 1995
A Non-Atom-Based Orbital Basis Set For Small Sodium Clusters, Melissa R. Prince, Randall W. Hall
Randall W. Hall
The electronic structure of small sodium clusters is investigated using a basis set motivated by path integral Monte Carlo calculations. The goal of this study is to develop a small basis set that can be used to study large sodium clusters. Past and present path integral calculations suggest that the correlated electronic wave function is characterized by electron density located between several atoms, rather than on the atoms. This is in agreement with previous quantum chemical and density functional calculations, which find the total electron density has maxima in the same locations. A basis set used in previous quantum chemical …
Formally Exact Path Integral Monte Carlo Calculations Using Approximate Projection Operators, Randall W. Hall
Dec 1991
Formally Exact Path Integral Monte Carlo Calculations Using Approximate Projection Operators, Randall W. Hall
Randall W. Hall
A class of approximate projection operators is used to reduce the variance in path integral Monte Carlo calculations in a formally exact manner. Paths are classified according to the projection operators, allowing the identification of paths whose contribution to the variance is negligible. The approach is applied to two canonical systems: Two like‐spin electrons in a three dimensional harmonic well and the two dimensional Hubbard model, representing localized and extended electronic states, respectively. Time savings of 15–900 over straightforward Monte Carlo calculations are observed.
Determination Of The Vertical Ionization Potentials Of Small Sodium Clusters Using Path Integral Monte Carlo Calculations, Sean E. Hays, Randall W. Hall
Sep 1991
Determination Of The Vertical Ionization Potentials Of Small Sodium Clusters Using Path Integral Monte Carlo Calculations, Sean E. Hays, Randall W. Hall
Randall W. Hall
Feynman's path integral formulation of quantum mechanics, supplemented by an approximate pseudopotential to account for exchange, is used to determine the vertical ionization potentials of Na1-Na4. Results are in good agreement with both the trends and the quantitative results of earlier work, We observe that the amount of discretion necessary to obtain converged energies is greater than that needed to obtain qualitatively correct information regarding the nature of the correlated wave function.
Finite Temperature Effects In Na3+ And Na3: A Path Integral Monte Carlo Study, Randall W. Hall
Jul 1989
Finite Temperature Effects In Na3+ And Na3: A Path Integral Monte Carlo Study, Randall W. Hall
Randall W. Hall
Path integral Monte Carlo techniques are used to study Na3+ and Na3 at finite temperatures. In accord with previous classical trajectory calculations, we find that vibrational motion significantly distorts the clusters from the previously predicted zero temperature geometries, due to the flat Born-Oppenheimer potential energy surface. More importantly, these distorted clusters show significant localized electronic bonding, in contrast to the delocalized bonding found in previous studies of the zero temperature structures.
A Comparison Of Path Integral And Density Functional Techniques In A Model Two Electron System, Randall W. Hall
Jun 1989
A Comparison Of Path Integral And Density Functional Techniques In A Model Two Electron System, Randall W. Hall
Randall W. Hall
Path integral Monte Carlo and density functional techniques are used to study a two-electron system for which exact results can be obtained. The energy and single-particle density are calculated for the lowest singlet and triplet states. The results show that both methods give reasonable agreement with the exact answer. The path integral calculations also give the correlated two-electron wave functions, which compare favorable with the exact results. Finally, the exchange-correlation hole is evaluated from the path integral simulations. The study demonstrates that path integral calculations are as accurate as density functional calculations and yield additional information in the form of …
Linkage Position In Oligosaccharides By Fast Atom Bombardment Ionization, Collision Activated Dissociation, Tandem Mass Spectrometry And Molecular Modeling, Roger A. Laine, Krishna M. Pamidimukkala, Alfred D. French, Randall W. Hall, Saeed A. Abbas, Rakesh K. Jain, Khushi L. Matta
Sep 1988
Linkage Position In Oligosaccharides By Fast Atom Bombardment Ionization, Collision Activated Dissociation, Tandem Mass Spectrometry And Molecular Modeling, Roger A. Laine, Krishna M. Pamidimukkala, Alfred D. French, Randall W. Hall, Saeed A. Abbas, Rakesh K. Jain, Khushi L. Matta
Randall W. Hall
No abstract provided.
The Aperiodic Crystal Description Of Diffusion In Concentrated Polymer Solvent Systems, Randall W. Hall
Dec 1987
The Aperiodic Crystal Description Of Diffusion In Concentrated Polymer Solvent Systems, Randall W. Hall
Randall W. Hall
The aperiodic crystal picture is used to predict the solvent number fraction dependence of the mutual diffusion coefficient for two dense polymer-solvent systems. This theory, which has been applied successfully to diffusion in one component glasses, gives results that are distinct from previous free volume theories. The theory provides a way to calculate the free energy surface for a dense disordered system, from which the barrier heights to diffusion can be estimated.
Quantum Monte Carlo Study Of A Lattice Atom Model For Excitonic Fluids, Randall W. Hall, Peter G. Wolynes
May 1986
Quantum Monte Carlo Study Of A Lattice Atom Model For Excitonic Fluids, Randall W. Hall, Peter G. Wolynes
Randall W. Hall
A lattice model of the atom is used to study the excitonic transition in dense fluids. With the use of a simple Hamiltonian for both atomic and interatomic interactions, simulations were made by allowing nuclei to move and by using hard-sphere configurations for the nuclei. The hard-sphere simulations are compared with several analytical theories. Rehybridization of atomic orbitals is found to occur in the hard-sphere systems, while clustering is present in the full simulations.
G. N. Lewis' Atom And Quantum Monte Carlo Studies Of Liquids, Randall W. Hall, Peter G. Wolynes
May 1986
G. N. Lewis' Atom And Quantum Monte Carlo Studies Of Liquids, Randall W. Hall, Peter G. Wolynes
Randall W. Hall
Liquids that involve changes in electronic structure are difficult to simulate using pairwise additive forces. In this paper we use a semiempirical model of the internal structure of atoms in order to simulate simultaneously electronic and nuclear dynamics of fluids. The proposed excitonic phase of mercury is critically examined with these models.
Behavior Of An Electron In Helium Gas, John Bartholomew, Randall W. Hall, Bruce J. Berne
Jun 1985
Behavior Of An Electron In Helium Gas, John Bartholomew, Randall W. Hall, Bruce J. Berne
Randall W. Hall
A quantum-mechanical electron in dense helium gas is examined by computer simulation using realistic, soft potentials. The behavior of the system is observed as the density is increased; there are indications that the electron (at least partially) localizes and forms a bubblelike state.
Nonergodicity In Path Integral Molecular Dynamics, Randall W. Hall, B. J. Berne
Dec 1983
Nonergodicity In Path Integral Molecular Dynamics, Randall W. Hall, B. J. Berne
Randall W. Hall
Molecular dynamics can be used to evaluate the path integral representation of the density matrix. Implicit in this method is the assumption that the dynamical system is ergodic. It is shown here that this is not the case in many systems that are of interest. For example water or liquid neon can not be simulated by this method. Monte Carlo methods do not suffer from this weakness.
A Path Integral Monte Carlo Study Of Liquid Neon And The Quantum Effective Pair Potential, Devarajan Thirumalai, Randall W. Hall, Bruce J. Berne
Dec 1983
A Path Integral Monte Carlo Study Of Liquid Neon And The Quantum Effective Pair Potential, Devarajan Thirumalai, Randall W. Hall, Bruce J. Berne
Randall W. Hall
The path integral Monte Carlo (PIMC) method is used to simulate liquid neon at T=40 K. It is shown that quantum effects are not negligible and that when the quantum effective pair potential is used in a classical molecular dynamics simulation the results obtained for the radial distribution function agrees with that predicted by a full path integral Monte Carlo simulation. The validity of this procedure is further shown by comparing the results obtained by this method to experimental measurements of liquid neon at T=35 K.
