Browsing by Author "Schug, John C."

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    An Alternative to Full Configuration Interaction Based on a Tensor Product Decomposition
    Senese, Frederick A.; Beattie, Christopher A.; Schug, John C.; Viers, Jimmy W.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1989)
    A new direct full variational approach exploits a tensor (Kronecker) product decompositions of the Hamiltonian. Explicit assembly and storage of the Hamiltonian matrix is avoided by using the Kronecker product structure to form matrix-vector products directly from the molecular integrals. Computation-intensive integral transformations and formula tapes are unnecessary. The wave function is expanded in terms of spin-free primitive sets rather than Slater determinants or configuration state functions and is equivalent to a full configuration interaction expansion. The approach suggests compact storage schemes and algorithms which are naturally suited to parallel and pipelined machines.
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    Bonding properties of coordinated polyhedra in molecules and crystals
    Hill, Frances Cull (Virginia Tech, 1995-12-05)
    Force constants and electron density distributions in coordination polyhedra in molecules and crystals are modeled using Hartree-Fock molecular orbital methods. Model bond-stretching force constants calculated for coordination polyhedra in a series of nitride, oxide and sulfide molecules are ~ 10-20% larger than obtained with spectroscopic methods. Well-developed correlations obtain between the force constants and minimum energy bond lengths, effective nuclear charges and polyhedral compressibilities of molecules and crystals. Model electron density distributions calculated for a large number of molecules with MOn (n = 1, 2,3,4 or 6) coordination polyhedra show that the MO bonds of a given type vary in a regular way with the value of the electron density at bond critical points, bonded radii and the curvatures of the electron density. The bonded interactions in the polyhedra are examined in terms of criteria set forth by Bader and Essen (1984) and Cremer and Kraka (1984).
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    Characterization and patterned polymer films from a novel self-assembly process
    Liu, Yanjing (Virginia Tech, 1996-05-03)
    The layer-by-layer molecular-level manipulation of ionic polymer have been utilized to fabricate ultrathin multilayer films (SAMp). In this process, monolayers of polycations and polyanions are sequentially adsorbed onto a substrate surface by alternately dipping the substrate into aqueous solutions of poly(vinylamine) backbone azo (PDYE), poly(sodium 4-styrenesulfonate) (PSS), and poly(al1ylamine hydrochloride) (PAH). The ionic attractions developed between the oppositely charged polymers promote strong interlayer adhesion and a uniform and linear multilayer deposition process. UV/Vis absorbance, contact angle, and ellipsometry measurements revealed that in all cases the bilayer deposition process was linear and highly producible from layer to layer and film thickness of up to 1 µm can be easily obtained by repeating the deposition process. The typical thickness of bilayer film depend on the solution concentration. Contact angle and UV/Vis spectroscopy measurements demonstrated that the deposition time for a full monolayer coverage of azo dye and PAH was about 20 seconds. Our results showed that the mechanical stability of these SAMp films was remarkable, and SAMp films can only be removed from the substrate by scraping. SAMp films are stable in the common organic solvents and even in the high acidic media (6M HCl aqueous solution). The conformation of these films are thermally stable at high temperature. In an attempt to develop patterned surfaces of sulfonate and thiol functionality, close-packed, well-ordered (3-mercaptopropyl)trimethoxysilane (MPS) monolayer were formed on the surfaces of single crystal silicon, quartz, and glass by allowing hydrolyzed silane to self-assemble from a dilute hydrocarbon solution. The films of MPS were irradiated with an ozone-producing UV light source results in efficient conversion of the surface-localized thiol groups to sulfonated groups, a complete photo-oxidation of the thiol surface was obtained and characterized by x-ray photoelectron spectroscopy (XPS) and contact angle measurements. Sulfonated self-assembled films can be used as good organic templates for the deposition of SAMp films and for micropatterning of organic surfaces based on our results. Such results significantly extend the application of SAMp films since the sulfonate-functionalized surface can be introduced into the surfaces of aromatic polymers, metals, ceramics, semiconductors, and plastics. So that the process of SAMp deposition can be carried out onto many different substrate surfaces. The novel self-assembly technique combined with photolithography was used to develop three different methods of micropatterning fabrication in an attempt to achieve the goal of full-color flat-panel display. The characteristic of distinguishing our methods from the existed ones is that the patterning is done first and then the vertical multilayers were built-up on the patterned areas. Moreover, in this process, SAMp films were used as active species. Scanning Electron Microscopy (SEM) was employed to confirm the patterning technique. In order to block the further growth of the second film type on the sites of first film type, several molecules with inert function groups were tried. UV/Vis absorbance and contact angle measurements revealed that dodecyltrimethylammonium bromide (DTAB) atop the PAH/PSS SAMp film could prevent further adsorption of the ionic polymers.
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    Characterization of block copolymers and polymer blends by inverse gas chromatography
    Sheehy, Daniel P. (Virginia Polytechnic Institute and State University, 1984)
    The accuracy and utility of using Inverse Gas Chromatography (IGC) to characterize the microphase structure of block copolymers, and the strength of the thermodynamic interactions between the components of polymer blends and the unlike segments of block copolymers was examined. There were three parts to the study. First, the Scott ternary solution model, which is used for the study of thermodynamic interactions in polymer blends, was extended to low molecular weight mixtures. From vapor-liquid equilibrium data in the literature, the Gibbs free energy of mixing of binary mixtures (GM ) calculated with the model were compared to experimental values. Mixtures containing ketones, aromatics hydrocarbons, chlorinated hydrocarbons, alcohols and water were studied. With the exception of mixtures containing water and low molecular weight alcohols, a fair to good correlation between theoretical and experimental values was observed. Second, the Gibbs free energy of mixing of nitrocellulose, polyvinyl chloride and poly(vinylidene fluoride) containing blends were measured with the Scott model from IGC data. For the nitrocellulose containing blend, the calculated Gibbs free energy of mixing values were large in magnitude (-2.0 to -5.0 calories/gram) and in fair agreement with the experimental heats of mixing determined from microcalorimetry measurements. For the remaining blends, the IGC data could not be distinguished from the results normally obtained for immiscible blends. The calculated GM values were small in magnitude relative to the experimental error of the quantities. Concerning the block copolymers, the relative incompatibility of the constituent blocks of perfectly alternating block copolymers of polydimethylsiloxane and bis-A-polycarbonate and styrene-isoprene-styrene triblock copolymers was reflected in the measured GM values. Overall, it was concluded that IGC is a good method for characterizing thermodynamic interaction between blend and copolymer constituents, but a severe limitation of the method is that the interactions are often too weak to measure accurately. Finally, the microphase structure of the above copolymers were studied by IGC from the retention behavior of hydrocarbon probes below the upper glass transition temperature of the copolymers. The degree of microphase separation, the size of the hard phases and the continuity of the soft phases in the copolymers characterized, and the results obtained were consistent with small angle x-ray, electron microscopy and differential scanning calorimetry data on the same materials.
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    Clarendon: The Reurbanization of a Suburban Area
    Fox, Charles Francis (Virginia Tech, 1996-02-09)
    New technologies have created a renewed interest in the places where we live and work by lessening the differences between the two. To address this issue, this thesis will consider the possibilities of returning to a suburban neighborhood that has been abandoned in recent history. Housing is introduced to a neighborhood which was predominantly commercial and retail throughout its history. As more people are brought into these miniature downtowns, the life of a neighborhood can be strengthened.
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    Determination of the molecular structure of Sulphonyl Chloride Isocyanate using microwave spectroscopy
    Jo, Oksik (Virginia Tech, 1990-12-05)
    Microwave spectroscopy was used to determine whether sulphonyl chloride isocyanate (SO₂CINCO) exists as a mixture of 71° and 110° isomers or as a single 94° isomer where the angle is the dihedral angle between the S-Cl bond and N = C bond, and whether the NCO group is linear or not. The microwave spectrum of SO₂CINCO was assigned for two isotopic species, SO₂³⁵CINCO and SO₂³⁷CINCO, by considering the Stark effect and the nuclear quadrupole splittings due to chlorine and nitrogen. Rotational constants and nuclear quadrupole coupling constants were determined from the assigned peaks. The bond angles and dihedral angles were derived from the six rotational constants of the two isotopes by imposing some constraints based on the electron diffraction study. The dihedral angle C = N-S-CI obtained in this study was 93.86 ± 0.04 ° and ∠NCO was 175.68 ± 0.02° . The results indicate that SO₂CINCO exists as the 94° form at the temperature (4° K) where the microwave spectrum was recorded and that the NCO group is not linear.
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    An empirical potential for hydrogen bond energies determination of the orientation of anthracene molecules in the unit cell by means of a refractivity method: some ab initio calculations involving acetonitrile exchange reaction
    Chen, Szu-Lin (Virginia Polytechnic Institute and State University, 1987)
    Topic I An empirical potential for calculating hydrogen bonding energies is developed for systems of the type A-H--B, where A and/or B is oxygen or nitrogen. Point charge and van der Waals interaction are included in the potential. The parameters of the potential were optimized by means of a simplex algorithm within a range of A-B distances from 2.8 A through 5.0 A. The root mean square deviation between the empirical potential and the ab initio results of 216 configurations of (H₂O)₂, (NH₃)₂ and NH₃•H₂O is 0.9 kcal/mol and 0.5 kcal/mol for 61 configurations of methanol dimers. Applications of the potential to water dimers, ammonia dimers, their mixed dimers, water oligomers and ice-h as well as the β form of the methanol crystal show that the potential yields reasonable results compared to those computed by "ab initio" methods using 6-31G* basis sets. The potential is compatible with MM2 program. It is simpler than earlier potentials in that neither dipoles nor Morse potentials are involved. It should be superior to the empirical potentials developed by Jorgensen that used STO-3G ab initio calculated results as the standards. The potential might be useful for estimation of hydrogen bond energies in a local part of a large molecule to avoid the prohibitive expense of ab initio calculation. Topic II The monoclinic anthracene crystal is used as an example to demonstrate the feasibility of optimizing the orientation of molecules in the unit cell by matching calculated and experimental refractivity ellipsoids using a simplex algorithm. The calculated refractivity ellipsoid is determined by use of an empirical formula using bond directional polarizabilities. Optimization of the molecular orientations to provide the best fit to the experimental ellipsoid starting from several assumed orientations results in fits for which the maximum deviation from the experimental molecular orientation was no more than 10 degrees. The method can be applied to other monoclinic molecular crystals directly and could be extended to other crystal systems with anisotropic optical properties. Topic III Three mechanisms (Walden inversion, addition-rearrangement-elimination and proton 1,3 shift mechanisms) of the following reaction were suggested by Jay et al. and Andrade et al. respectively. CH₃CN + C⃰N- = CH₃C⃰N + CN-. The mechanism of Walden inversion was determined to be the least likely one based on Andrade's MNDO results. Our calculations, based on 3-21G and 4-31G results, show the contrary result that the Walden inversion is the most likely mechanism among the three considered. However, solvation effects were neglected in the calculations and these effects could play a major role in the choice of mechanisms. Simple calculations based on Boltzmann distribution of precursor concentrations and the Arrhenius law show that Walden inversion predominates over Jay's addition-elimination-rearrangement mechanism even when MNDO energy levels were used. Estimated orders of magnitude for the rate ratios were determined.
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    Feynman-Dyson perturbation theory applied to model linear polyenes
    Reid, Richard D. (Virginia Polytechnic Institute and State University, 1986)
    In the work described in this thesis, the Feynman-Dyson perturbation theory, developed from quantum field theory, was employed in semiempirical calculations on trans - polyacetylene. A variety of soliton-like excited states of the molecule were studied by the PPP-UHF-RPA method. The results of this study provide useful information on the nature of these states, which are thought to account for the unique electrical conduction properties of trans - polyacetylene and similar conducting polymers. Feynman-Dyson perturbation theory was also used to extend Hartree-Fock theory by the inclusion of time-independent second-order self-energy insertions. The results of calculations on polyenes show that consideration of this approach is warranted, as the contribution of the second- order terms is significant. The computer program, written during the course of the research reported here, is discussed as well.
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    A Full Variational Calculation Based on a Tensor ProductDecomposition
    Senese, Frederick A.; Beattie, Christopher A.; Schug, John C.; Viers, Jimmy W.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1989)
    A new direct full variational approach exploits a tensor (Kronecker) product decomposition of the Hamiltonian. Explicit assembly and storage of the Hamiltonian matrix is avoided by using the Kronecker product structure to form matrix-vector products directly from the molecular integrals. Computation-intensive integral transformations and formula tapes are unnecessary. The wavefunction is expanded in terms of spin-free primitive kets rather than Staler determinants of configuration state functions, and the expansion is equivalent to a full configuration interaction expansion. The approach suggests compact storage schemes and algorithms which are naturally suited to parallel and pipelined machines.
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    Lower bounds to eigenvalues by the method of arbitrary choice without truncation
    Marmorino, Matthew G. (Virginia Tech, 1999-04-21)
    After a detailed discussion of the variation theorem for upper bound calculation of eigenvalues, many standard procedures for determining lower bounds to eigenvalues are presented with chemical applications in mind. A new lower bound method, arbitrary choice without trunctation is presented and tested on the helium atom. This method is attractive because it does not require knowledge of the eigenvalues or eigenvectors of the base problem. In application, however, it is shown that the method is disappointing for two reasons: 1) the method does not guarantee improved bounds as calculational effort is increased; and 2) the method requires some a priori information which, in general, may not be available. A possible direction for future work is pointed out in the end. An extension of a lower bound method by Calogero and Marchioro has been developed and is presented in appendix G along with comments on the effective field method in appendix H for Virginia Tech access only.
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    Monte Carlo computer simulation of sub-critical Lennard-Jones particles
    Gregory, Victor Paul (Virginia Tech, 1991-03-05)
    Cluster characteristics of the 3D Lennard-Jones, LJ, fluid are determined by Metropolis Monte Carlo computer simulations. The percolation probability and cluster distribution is calculated for several state points in the gas-liquid equilibrium region of the LJ fluid. The cluster number distribution is used to analyze the distribution of clusters above and below the percolation threshold. Using scaling theory, the critical exponent, Ï , is determined from the cluster distributions. Deviation from the scaling law is evaluated using a modified scaling law that includes a surface term. It is found that the surface term is unnecessary in the gas-like area of the phase diagram. The density profiles of large non-percolating clusters are calculated in order to study the surface structure of the clusters. The coordination number within a cluster is calculated directly in the simulation and, with the cluster energy, is used to discern the amount of "liquid-like" structure of the cluster. The radius of gyration, R g, as a function of cluster size determines the fractal dimension, D f of the non-percolating and clusters above and below the percolation threshold density. Finite size effects are briefly studied and presented for a few of points.
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    Monte Carlo computer simulation of the Lennard-Jones and Stockmayer fluid phase diagrams
    Gregory, Victor Paul (Virginia Tech, 1994-02-05)
    The isotherms of the Lennard-Jones fluid and the Stockmayer fluid are calculated by Monte Carlo computer simulation using the constant NpT ensemble. Empirical coefficients are determined for a truncated virial equation of state fitted to our data. Spinodal points are located for each temperature and fluid. For temperatures less than 0.90 of the critical temperature, we succeeded in temporarily isolating clusters during the gas to liquid transition for the LJ fluid. Density profiles are calculated for clusters at and above the spinodal pressures. The clusters above the spinodal pressure have liquid-like densities at their centers and are identified as critical condensation clusters. The clusters at the spinodal increase in size with temperature and have densities roughly half as dense as the equilibrium liquid at their centers. It is found that the results are essentially system size independent.
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    Monte Carlo simulation of aqueous dilute solutions of polyhydric alcohols
    Lilly, Arnys Clifton (Virginia Polytechnic Institute and State University, 1989)
    In order to investigate the details of hydrogen bonding and solution molecular conformation of complex alcohols in water, isobaric-isothermal Monte Carlo simulations were carried out on several systems. The solutes investigated were ethanol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol and glycerol. In addition, propane, which does not hydrogen bond but does form water hydrates, was simulated in aqueous solution. The complex alcohol-water systems are very nonideal in their behavior as a function of solute concentration down to very dilute solutions. The water model employed was TIP4P water¹ and the intermolecular potentials employed are of the Jorgensen type² in which the interactions between the molecules are represented by interaction sites usually located on nuclei. The interactions are represented by a sum of Coulomb and Lennard-Jones terms between all intermolecular pairs of sites. Intramolecular rotations in the solute are modeled by torsional potential energy functions taken from ethanol, 1-propanol and 2-propanol for C-O and C-C bond rotations. Quasi-component pair correlation functions were used to analyze the hydrogen bonding. Hydrogen bonds were classified as proton acceptor and proton donor bonds by analyzing the nearest neighbor pair correlation function between hydroxyl oxygen and hydrogen and between solvent-water hydrogen and oxygen. The results obtained for partial molar heats of solution are more negative than the experimental values by 3.0 to 14 kcal/mol. In solution, all solutes reached a contracted molecular geometry with the OH groups generally on one side of the molecule. There is a tendency for the solute OH groups to hydrogen bond with water, with more proton acceptor bonds than proton donor bonds. The water-solute binding energies correlate with experimental measurements of the water-binding properties of the solute. 1. Jorgensen, W.L. et al, J. Chem. Phys., 79, 926 (1983). 2. Jorgensen, W.L., J. Phys Chem., 87, 5304 (1983).
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    Morphology Development and Fracture Properties of Toughened Epoxy Thermosets
    Kwon, Ojin (Virginia Tech, 1998-06-08)
    The phase separation process of a rubber modified epoxy system during cure was analyzed by a model developed on the basis of a thermodynamic description of binary mixture and constitutive equations for nucleation and growth rates. As epoxy resins are cured, rubber molecules are precipitated from the epoxy matrix to a non-equilibrium composition due to the decrease in the configurational entropy and the increase in the viscosity with conversion. If phase separation takes place in a metastable region, this model can monitor the changes of rubber compositions in both phases as well as the changes in the number and size of rubber particles upon conversion of polymerization. The particle size distribution at the completion of phase separation was also calculated. The effect of cure temperature on the final morphologies of a rubber modified epoxy system was discussed. The computed particle size distributions for piperidine and diaminodiphenyl sulfone cured systems showed good agreements with experimentally measured values. Depending on the activation energy for viscous flow of the epoxy matrix relative to that for the polymerization, the particle size distribution may show bimodal or unimodal distribution. The size of rubber rich phase increases to a maximum and then decreases with an increase in cure temperature. However, due to limitations of temperature range to probe in an actual experiment, one may observe only either decreasing or increasing particle size as cure temperature increases. The number of rubber particles per unit volume increases for the DGEBA/DDS/ETBN system as cure temperature increases in the temperature range of 30 °C to 220 °C. Fracture toughness of cured DGEBA/DDS/ETBN system was analyzed in terms of morphologies generated by the temperature variation. Since the volume fraction of rubber particles did not change with cure temperature, the critical stress intensity factor did not vary significantly with cure temperature as expected. However, increases in cure temperature produced smaller but more numerous particles. The critical stress intensity factor normalized by the number density of particles exhibited dependence on the radius of particles to the third power. On the other hand, the critical stress intensity factor normalized by the radius of particles showed a linear dependence with respect to the number density of particles.
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    Solution and solid state NMR studies of fluorine tagging reagents
    Spratt, Michael Phillip (Virginia Polytechnic Institute and State University, 1985)
    A series of studies are presented in which fluorine tagging reagents are used to analyze complex mixtures for compounds containing active hydrogen functional groups (e.g., hydroxyl, amine, thiol, and carboxylic groups). The existence of these derivatized functional groups is determined by utilizing a number of solution and solid-state nuclear magnetic resonance (NMR) techniques. In solution NMR studies p-fluorobenzoyl chloride was the fluorine tagging reagent of choice because of a large ¹⁹F chemical shift range for the different derivatized substrates (~10 ppm) and generally good reaction yields. Various classes of sterol and amino acid p-fluorobenzoyl derivatives were characterized on the basis of their ¹⁹F NMR isotropic chemical shifts. The presence (or absence) of hydroxyl, amine, and carboxylic acid functional groups in coal extract and pyrolysis products was also determined. The versatility of the p-fluorobenzoyl chloride as the fluorine tagging reagent in ¹⁹F NMR was enhanced by: a) enriching the carbonyl carbon of the acid chloride with labeled ¹³C isotope, thus synthesizing a dual ¹⁹F and ¹³C NMR sensitive reagent and b) using the reagent in conjunction with LC-NMR. The extension to either technique added another dimension to the NMR spectral data obtained from the p-fluorobenzoyl tagging reagent in solution NMR. Finally, preliminary data is presented illustrating how fluorine tagging reagents may be used to study functional groups (and atoms present in the immediate proximity of the group) existing on solid material utilizing solid-state NMR. Functional groups on the solid material are tagged with a fluorinated reagent. The sample is then analyzed using solid-state NMR with cross-polarization (CP), magic angle spinning (MAS), and high-power proton decoupling. The ¹⁹F dipolar coupling interactions, created by the presence of the fluorine tag, attenuate signals for these nuclei in the immediate proximity of the tagged site. A series of 1-adamantanol, steroid, and silica gel fluorinated derivatives show that the effects of the ¹⁹F dipolar interactions were modulated by complex anisotropic molecular motions (i.e., solid system with little motion, exhibiting greater signal attenuations due to ¹⁹F dipolar coupling).
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    Structure and microwave spectrum of the 2-cyano-2-propyl radical
    Claytor, Robinson C. P. (Virginia Polytechnic Institute and State University, 1988)
    The rotational spectra of the 2-cyano-2-propyl and d⁶-2·cyano-2-propyl radicals were observed using a Stark modulated spectrometer. The radicals were generated in the gas phase by UV irradiation of sublimed azoisobisbutyronitrile. They were detectable in the cell for approximately one hour. Thirty-three transitions were assigned for (CH₃)₂CCN and twenty-one for (CD₃)₂CCN. The rigid rotor rotational constants determined by calculation of the hypothetical unsplit rotational transitions are A=8276.7, B=3919.7, C=2751.5Mhz for (CH₃)₂CCN and A=6241.3, B=3490.7, C=2372.6Mhz for (CD₃)₂CCN. A program to calculate the fine splittings and hyperfine splittings due to the ¹⁴N nucleus and six protons was written. The spin rotation constants determined for the two species were Eaa=-69.9, Ebb=-36.1, Ecc=2.7Mhz and Eaa=-55.4, Ebb=-32.6 Mhz for (CH₃)₂CCN and (CD₃)₂CCN respectively. The hyperfine coupling constants for ¹⁴N are identical for both isotopic species and were found to be Taa=-17.2, Tbb=-17.1 and Tcc=34.4 Mhz. The proton and deuteron hyperfine splittings were not resolved. The structural parameters determined from an fit of the moments are rCN=1.18A, rCC=1.42A, rCMe=1.50A and CCMe=119.3°. The C₄N skeletal framework was found to be planar.
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    Studies of liquid phase intermolecular interactions utilizing ¹H and ¹³C dynamic nuclear polarization and nuclear magnetic resonance techniques
    Sun, Ziqi (Virginia Tech, 1996-09-10)
    Liquid phase ¹³C DNP experimental data were collected in a flow transfer system for different organic molecules, such as acetone, acetaldehyde, diethyl malonate, ethyl acetoacetate, diphenylmethane, and triphenylmethane. These molecules represent a wide range of functional groups with different acidities of the respective carbon-hydrogen bonds. The ¹³C DNP results demonstrated that the scalar dominated enhancement is sensitive to the acidity of carbon-hydrogen bonds as well as to the correlation times of the sample molecules. A hydrogen bonding spin polarization model is used, for the first time, to interpret the scalar components induced by the nitroxide free radical at the carbon sites of the acidic carbon-hydrogen bonds. Three aromatic molecules: nitrobenzene, 1, 2-dichlorobenzene, and toluene, are studied by the solution ¹³C DNP technique. The scalar components for the ring carbons are sensitive to the electronic environment of these carbon sites. A spin delocaliztion model is used, for the first time, to explain the scalar contribitions for the ¹³C DNP enhancements of the ring carbons. Both ¹H and ¹³C DNP experiments are performed for the Taxol/TEMPO (2, 2, 6, 6-tetramethyl-1-piperidinyloxy) system. The different ¹H enhancements for the hydrogens in the two acetyl groups indicate the different accessibility of these groups to the free radical. The ¹³C DNP results for the skeleton carbons of Taxol show the different accessibility of these carbon sites to the free radical. The solution ¹³C DNP result of adamantane indicates that the DNP enhancements and thus the correlation times for the two different carbon sites are very close under the high free radical concentration. The ¹³C DNP study of C₇₀ empty cage fullerene suggests that the endcap carbons are more accessible than those at the center of the cage, and that the scalar coupling between the cage carbons and the free radical is very weak.
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    A study of the mean-square displacement amplitudes of T and O atoms in framework silicates and aluminosilicates: evidence for rigid TO bonds, order, disorder, twinning and stacking faults in crystals
    Downs, Robert T. (Virginia Tech, 1989-12-05)
    The mean-square displacement amplitudes (MSDA) of the tetrahedral cations Si and Al are compared to the MSDA of their coordinated O atoms in framework silica polymorphs and aluminosilicate structures. Criteria are established which indicate order, structural disorder or substitutional disorder in a framework crystal.
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    A tensor product decomposition of the many-electron Hamiltonian
    Senese, Frederick A. (Virginia Polytechnic Institute and State University, 1989)
    A new direct full variational approach is described. The approach exploits a tensor (Kronecker) product construction of the many-electron Hamiltonian and has a number of computational advantages. Explicit assembly and storage of the Hamiltonian matrix is avoided by using the Kronecker product structure to form matrix-vector products directly from the molecular integrals. Computation-intensive integral transformations and formula tapes are unnecessary. The wavefunction is expanded in terms of spin-free primitive kets rather than Slater determinants or configuration state functions and is equivalent to a full configuration interaction expansion. The approach suggests compact storage schemes and algorithms which are naturally suited to parallel and pipelined machines. Sample calculations for small two- and four-electron systems are presented. The preliminary ground state potential energy surface of the hydrogen molecule dimer is computed by the tensor product method using a small basis set.
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    Zero Quantum Nuclear Magnetic Resonance experiments utilizing a toroid cell and coil
    Bebout, William Roach (Virginia Tech, 1989-04-15)
    Over the past ten to fifteen years the area of Nuclear Magnetic Resonance (NMR) Spectroscopy has seen tremendous growth. For example, in conjunction with multiple quantum NMR, molecular structural mapping of a compound can be easily performed in a two dimensional (2D) experiment. However, only two kinds of detector coils have been typically used in NMR studies. These are the solenoid coil and the Helmholtz coil. The solenoid coil was very popular with the permanent and electromagnet NMR instruments. With the advent of the superconducting magnets the popularity shifted to the Helmholtz coil, which remains the most common coil today for superconducting magnets. The Helmholtz coil, however, has been shown to have lower sensitivity than the solenoid coil. Hoult (1) has pointed out that potentially the Helmholtz coil represents a loss of signal-to-noise (S/N) by a factor of three in comparison to the solenoid coil. Since Hoult's original work, alternate methods for optimizing S/N have been explored. One of these has been the suggestion of toroid shaped resonators for NMR studies (2). A potential advantage of a toroid cell and coil is the confinement of the B₁ field to the torus region.It has been suggested that the toroid has a potential (S/N) advantage of 3.9 - 4.5 in comparison to the conventional Helmholtz cell (3). Since Zero Quantum (ZQ) experiments are independent of B₀ homogeneity, 2D ZQ experiments provide a convenient method of comparing the toroid and Helmholtz coils. In these zero quantum studies, the toroid and Helmholtz probes will be characterized in terms of several factors 1) B1 homogeneity, 2) B₀ homogeneity, 3) flip angle dependence, and 4) sensitivity. Finally, the two probes will be contrasted using spectral analysis in the spin-spin mapping of a simple molecule (nâ butanol) and a complex molecular system (taxol).