Browsing by Author "Viers, Jimmy W."

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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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    Analysis of the microwave spectrum of the bis-trifluoromethyl nitroxide
    White, Richard Michael (Virginia Polytechnic Institute and State University, 1986)
    The microwave spectrum of the stable free radical bis-trifluoromethyl nitroxide was analyzed in the 20 to 26.5 GHz region. 39 transitions have been assigned. The rigid rotor rotational constants determined by calculation of the hypothetical unsplit rotational transitions are A= 2256.94 MHz, B= 1056.08 MHz, and C=971.51 MHz. A program to calculate the fine and hyperfine splitting patterns was written which reproduced the experimentally observed splittings. The spin-rotation coupling constants are Eaa = -3597.7 MHz, Ebb= -1121.4 MHz, and Eee = 29.8 MHz. The magnitude of Eee indicates that the unpaired electron is in an orbital extending perpendicularly to the molecular plane. The determined magnetic hyperfme coupling constants are Taa= -183.74 MHz, Tbb= -177.77 MHz, Tee= 361.51 MHz, and aF= 22.8 MHz. The spin density on the fluorine atom is 11.9% which is estimated from the value of Tee· The value of the Fermi coupling constant aF indicates that the unpaired electron occupies an unhybridized p orbital. An insufficient number of unsplit rotational transitions were assigned to allow a centrifugal distortion analysis to be performed. Since isotopic substitution was not feasible an r0 structure was determined by doing a least squares fit of the three moments of inertia. The following parameters were not allowed to vary during the least squares fit: reF= 1.32A,rcN= 1.441A, F - C - F = 109.8° . The calculated structure gave a N-0 bond length of 1.212A ,a C-N-0 angle of 116.9° a C-N-C ·angle of 126.2° , and an angle which measures the deviation of the 0 atom from the CNC plane of 0° .
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    The application of thin film planar waveguides as a refractive index detector for microscale high performance liquid chromatography
    Kang, Lee (Virginia Tech, 1991-01-15)
    Thin film planar waveguides were originally introduced in microwave engineering. The spectroscopist began to use such waveguides as tools to solve chemical characterization problems since Harrick and Fahrenfort introduced the Attenuated Total'Reflection (ATR) in the early 1960's. Today, planar waveguides are playing an important and ever-increasing role in modem chemistry. In this thesis, a novel design for a refractometer involving the application of a thin film planar waveguide and coupling prism was demonstrated. This device shows the feasibility of refractive index measurements in a flowing stream. Therefore, an online detector for High Performance Liquid Chromatography (HPLC) was chosen as the vehicle to test out the concepts. The research works were devoted to studies of waveguide properties and flow dynamics in a chromatographic situation. It was found that microscale detection is possible. The sensitivity can be enhanced by using the highest propagation mode as the probe, and by selecting a proper refractive index liquid as the solvent carrier. A description of the investigation and the various factors involved in designing and optimizing a planar waveguide for refractive index detection is included. The results provide guidelines for the device as a realistic analytical detector.
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    Bond length and bonded radii variations in nitride molecules and crystals
    Buterakos, Lewis A. (Virginia Tech, 1990-08-09)
    Molecular Orbital calculations on 31 H3:e_mxm+N:e hydronitride molecules containing 3-, 4-, and 6-coordinate X-cations from rows 1-4 of the periodic table yield minimum energy bond lengths, Rt(XN), which reproduce observed bond lengths, Ro(XN), in crystalline nitrides to within o.O₃A, on average. A linear regression analysis of In[Rt(XN)] vs. In(p) with p = air, where a is the Pauling bond strength and r is the row number of the X -cation in the periodic table, gives the equation R(XN) = 1.47p-O.2\ which is shown to reproduce the observed XN bond lengths of Baur (1987) to within o.o9A, on average. This equation is statistically identical to the equation R( XN) 1.49p-O.22, derived from a linear regression analysis ofln[Ro{XN)] vs. In(p), and is similar in form to those obtained for the oxides (R(XO) = 1.39p-O.22) and the sulfides (R(XS) = 1.83p-O.21).
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    Chelate Assisted, Pressurized, Liquid Extraction for the Removal of Adsorbed Metal Contaminants From Soils
    Marshall, Karen L. (Virginia Tech, 2000-03-03)
    Chelate Assisted, Pressurized, Liquid Extraction (CAPLE) has been developed in our laboratory as an efficient, separation-based, extraction methodology for heavy metals in soils. Unlike current extraction methods used in environmental determination of contaminated soils, CAPLE is able to selectively remove adsorbed metals from the soil matrix without requiring the total destruction of the sample. By not fracturing the soil matrix particles, as with hot acid digestion methods, geologically bound metals are not liberated in the CAPLE process. This unique feature of CAPLE allows us to quantify levels of contaminant metals and correlate them to anthropological activity in the area. CAPLE requires the use of a modified supercritical fluid extractor for operation with water at sub-critical levels. The extraction of the sorbed metals is facilitated by the use of a chelating agent. Metal determinations are performed by atomic absorption (FAAS or GFAAS) or ICP emission spectrometry. CAPLE has been subjected to a variety of experimental conditions in order to elucidate the strengths and possible weaknesses of the extraction technique. The uses of the chelating agents (type and concentration) have been optimized. Possible release of metals from the resulting ionic strength of the chelating solutions have been shown not to be a factor. Both pressure and temperature effects have been studied and adjusted for optimal conditions. The majority of the research lies in the application of CAPLE to a variety of soil conditions. The effect on particle size of the soil and soil coating (humic acid and iron oxides) has been studied. In all soil systems and coatings studied, CAPLE could be optimized to completely remove chemisorbed metals. Tests of CAPLE on Cu-sludge amended soils provided excellent agreement with traditional methods of soil analysis. Not only was good agreement obtained between the recoveries of the methods, but CAPLE was also found to be much faster, more environmentally friendly, and much less prone to sample loss or sample contamination compared to traditional soil extraction methods. A final portion of this work involves a rigorous statistical analysis of CAPLE to a sequential extraction method. Since a Standard Reference Material (SRM) has not been provided for chemisorbed metals onto soils, a comparative analysis was chosen to validate the technique. Using the Cu-amended soils, CAPLE was found to effectively liberate all chemisorbed metals as compared to the sequential extraction technique. There was no statistical difference in recovery between the two extraction methods. CAPLE is shown in this work to be a viable extraction method for analyzing contaminant metals in soils. It is a rapid and efficient technique. Unlike traditional digestion methods, it is able to differentiate anthropological metals from geologically occurring metals. Its ease of use, coupled with simplicity of instrumental design and analytical reagents make it an attractive extraction technique for environmental analysis.
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    Chromatographic Properties of Silica-Based Monolithic HPLC Columns
    Smith, Jennifer Houston (Virginia Tech, 2002-09-23)
    Silica-based monolithic HPLC columns contain a novel chromatographic support in which the traditional particulate packing has been replaced with a single, continuous network (monolith) of porous silica. The main advantage of such a network is decreased backpressure due to macropores (2 μM) throughout the network. This allows high flow rates, and hence fast analyses that are unattainable with traditional particulate columns. The Chromolith SpeedROD™ (EM Science, Gibbstown NJ) is a commercially available silica-based monolithic column. This work investigated the chromatographic properties of the 50x4.60 mm (ODS) SpeedROD™. Data fit to the van Deemter equation (mean square error=0.834) indicated that the van Deemter model was valid for monolithic columns. An effective particle size of 4 μM for the SpeedROD™ column was assigned by comparing the minimum of van Deemter curves with a series of particulate columns having various particle diameters. Separation Impedance (E), an empirically derived measure of column performance, was calculated as an alternate method of evaluating column efficiency. Data collected using this model confirmed monolithic columns behaves as a (more efficient) 3 μM column. A series of experiments were designed to compare the effects of mobile phase strength and mobile phase viscosity between the SpeedROD™ column and a particulate column. The results indicated both solvent strength and viscosity have effects on the monolithic column at the optimum linear velocity. A fast (90 s) HPLC method was developed using the SpeedROD™ column and a seven-component test mixture with a large range of hydrophobicities. The precision for both retention time and peak area was measured at high linear velocities (8 mL/min) and the percent relative standard deviation (RSD) calculated. Column to column reproducibility (n=6) was measured. The overall percent RSD ranged from 0.25% to 4.56% for retention time and from 1.08% to 6.77% for peak area. Run to run reproducibility (n=15) was measured for all six columns. Averages ranged for retention time from 0.89% to 5.09% RSD and for peak area from 4.65% to 6.18% RSD. Applications for the SpeedROD™ column with various sample types were developed and discussed. These methods demonstrated the effectiveness of the SpeedROD™ at fast flow rates.
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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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    Effect of column temperature on elution order in gas chromatography
    Heng, Kien Chhiev (Virginia Tech, 1994-07-14)
    This study concerns the effect of column temperature on selectivity in gas chromatography (GC). Reversal of peak elution order at different temperatures is observed when selectivity is sufficiently changed by changing the temperature. A study of homologous functional groups and the elution order of a test mixture was made at several temperatures, by keeping constant operating chromatographic parameters such as column head pressure, split ratio, sample concentration, injector and detector temperature. The homologous series studied were esters, carboxylic acids, ketones and alkanes. The stationary phases used were Carbowax (Supelcowax 10), and 5% diphenyl and 95% dimethyl silicone ( HP Ultra 2). The identification of each compound was confirmed by two methods : comparing retention times, and spiking the sample. The results of an investigation for accurately determining the dead time is also presented. The experimental results of using 10% methane in helium and the results of a theoretical model based on Poiseuille's equation were studied and compared. It is concluded that dead time calculations are a major source of error for the calculations in this work. In addition, a derived thermodynamic equation based on the Van't Hoff equation was obtained and used to calculate the molar enthalpy and entropy of compounds over a range of temperatures. The experimental thermodynamic values were compared with the theoretical values obtained from the Clausius- Clapeyron equation. From the data obtained, a graph of In k' (retention factor) versus 1/ temperature for a homologous series, known as a Van't Hoff plot, predicts a reversal of elution order where an intersection of two lines occur. Selectivity of the stationary phase has changed solely due to temperature effects. This can only occur when different solute/solvent interactions are present. This work is an attempt to predict when peak inversion should occur based on the Van Hoff's plot. Experimental result are presented that confirm the prediction.
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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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    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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    Gaseous diffusion in liquids
    Combs, Roger J. (Virginia Polytechnic Institute and State University, 1986)
    Diffusivity of nonreactive gases in liquids provides a means of interpreting structure in the liquid state. Structural models of the liquid state include Hildebrand's condensed gas model and Eyring's pseudo-lattice model. The former model predicts a linear dependence of diffusivity with temperature while the latter model predicts linear dependence of log(D) versus 1/T. The limited temperature dependent diffusivity data to date with a typical precision of ± 5% do not permit distinguishing which temperature dependence is more linear. However, the present investigation shows that diffusivities of one gas solute in two nonpolar liquids indirectly supports a linear diffusivity temperature dependence by a Graham's law like relation. At a fixed temperature this relation equates relative diffusivities to the square root of the inverse molecular weights of the respective liquids. Diffusion of gases into nonpolar liquids have previously been measured by two techniques: (1) a pseudo-steady state technique developed by Hildebrand with diffusion through multiple capillaries and (2) a method by Walkley with diffusion through an open tube. Each of these methods requires prior knowledge of solubility of the gas in the liquid. An apparatus is constructed which combines these methods into a single experiment. Simultaneous solution of the two equations which describe the combined experiment yields both the solubility and diffusion coefficient. Diffusivities and solubilities of nitrogen, argon and oxygen into liquids of carbon tetrachloride and benzene as well as oxygen into water have been studied. The results compare favorably with the Literature. The diffusion cell for this technique consists of a capillary disk, which is flooded with liquid. Gas is admitted into the space over the open solvent. With temperature and pressure constant, volume uptake of the gas in the solvent is monitored. Time-volume uptake data is evaluated by the two diffusion equations. Although the experiment is conceptually easy, a small gas volume change over a prolonged period of time poses problems in data collection and experiment control. The data collection and control is simplified by dedicating a Microcomputer Interfaced Data Acquisition System (MIDAS) to the experiment.
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    Improving Off-line and On-line Supercritical Fluid Extraction Techniques by Elevating the Post-Restrictor Pressure
    Stone, Mark Adam (Virginia Tech, 2001-02-08)
    The high flow rate that results as fluid decompresses through the restrictor is arguably the single greatest problem with supercritical fluid extraction techniques. As a result of these high flow rates, solvent trapping is not efficient in many cases, and the more complicated sorbent trapping technique must be used. In addition, loss of the collection solvent may occur during the process making it difficult to work with small volumes, which are desirable from the standpoint of sensitivity, cost, and environmental concerns. Similarly, these high decompressed flows have made it difficult to directly interface supercritical fluid extraction methods with separation techniques. This is unfortunate as supercritical extractions are ideal for on-line coupling in other respects, such as the fact that the fluid becomes gaseous upon depressurization and that supercritical fluids generally extract less contaminant material. In this thesis we have shown that, by elevating the post-restrictor pressure, the decompressed flow rate can be reduced, and these problems can be minimized, considerably. Off-line trapping becomes much simpler when working at elevated pressures as the need for sorbent trapping is virtually eliminated and solvent trapping may be conducted with much less difficulty. Elevated post-restrictor pressures were found to be very beneficial for on-line work as well. SFE/GC was carried out with complete transfer of the extraction effluent to a capillary GC column, which has not previously been demonstrated. Likewise the use of an open-tubular column interface, maintained at moderately elevated pressures, allowed SFE/HPLC to be conducted with quantitative analyte transfer, even in the presence of 10% modifier. In all cases - but especially for the on-line methods - more practical extraction parameters were possible, including extraction vessel volume, extraction flow rate, and dynamic extraction time. Another benefit of elevating the post-restrictor pressure is that higher extraction flow rates will generally be possible. The benefits of this were not evaluated in the research presented here, however, the effect that the extraction flow rate can have on extraction time is considered, from a theoretical standpoint, in Chapter five.
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    Investigation into the fundamental principles of fiber optic evanescent sensors
    Petersen, James Vincent (Virginia Tech, 1990)
    This investigation was concerned with the development of a fiber optic evanescent sensor. Such a sensor is based upon the lateral regions of the waveguide and their interaction with a chemical environment. The basic principles and concepts involved in the construction of a fiber optic spectroscopic evanescent field sensor are developed. From this fundamental knowledge a series of sensors were constructed to characterize their responses. First, the response to simple refractive index changes and angle launch conditions were explored. This demonstrated the light interaction with the measurement environment. What followed was the use of these sensors to make spectrochemical absorption measurements suitable for a calibration curve of 1.25 x 10⁻² to 1.22 x 10⁻⁹ M concentrations for various dye systems. As a consequence of the spectrochemical studies the unique interaction between the chemical environment and the silica surface was observed. This response is based upon the chemical reactivity of the silica surface and the chemical environment. This prompted the investigation of the ion exchange characteristics of the silica waveguide surface and the chemical environment. In this investigation the exchange selectivities of the alkali, alkali earth and lanthanide metals were determined. Finally, a series of innovative fiber optic sensors base upon interferometric and refractometric measurements were investigated. These designs were based upon the fabrication characteristics of glass and plastic fiber optic waveguides.
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    Local Correlation: Implementation and Application to Molecular Response Properties
    Russ, Nicholas Joel (Virginia Tech, 2006-04-17)
    One of the most promising methods for surmounting the high-degree polynomial scaling wall associated with electron correlating wave function methods is the local correlation technique of Pulay and Saebø. They have proposed using a set of localized occupied and virtual orbitals free of the canonical constraint commonly employed in quantum chemistry, resulting in a method that scales linearly (in the asymptotic limit) with molecular size. Pulay and Saeb$oslash; first applied their methods to configuration interaction and later to M$oslash;ller-Plesset perturbation theory. Werner et al. have have extended the local correlation scheme of Pulay and Saeb$oslash; to coupled-cluster theory. One of the pitfalls of the local correlation methods developed by Pulay and Saeb$oslash; is the dependence of domain selection on the molecular geometry. In other words, as the geometry changes the domain structure of the local correlation calculation can change also, leading to discontinuities in the potential energy surface. We have examined the size of these discontinuities for the homolytic bond cleavage of fluoromethane and the heterolytic bond dissociation of singlet ketene and propadienone. Properties such as polarizabilities and optical rotation are realized through linear response theory, where the Hamiltonian is subject to an external perturbation and the wave function is allowed to respond to the applied perturbation. Within the context of local correlation it is necessary to understand how the domain structure alters in response to an applied perturbation. We have proposed using solutions to the CPHF equations (coupled-perturbed Hartree-Fock) in order to predict the correlation response to an applied perturbation. We have applied this technique to the calculation of polarizabilities, with very favorable results, and also to optical rotation, with mixed results.
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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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    The microwave spectra and molecular structures of N-fluoroformyliminosulfur difluoride and pentafluorosulfanyliminosulfur diflouride
    Bailey, Sharon Rose (Virginia Tech, 1979-12-07)
    The investigation of the rotational spectra of fluoroformyliminosulfur difluoride, SF2NCOF, and pentafluorosulfanyliminosulfur difluoride, SF2NSF5 , was undertaken to determine, from the experimental moments Q.f inertia, the number and type of positional isomers and the bonding parameters. SF2NCOF and SF2NSF5 are the first iminosulfur difluorides (RN=SF2) to be investigated by microwave spectroscopy. The iminosulfur difluorides are characterized by a nitrogen-sulfur double bond with nonbonding pairs of electrons in an sp2 hybridized orbital on the nitrogen atom and in an sp3 hybridized orbital on the sulfur atom. This bonding scheme is expected to impart a unique charge distribution in the vicinity of the SF2 fluorines. In SF2NCOF the nitrogen-carbon bond may acquire some partial double bond character which can be described through resonance involving the S, N, C, and 0 atoms. The extent of this resonance would be expected to affect the length of the NC bond and to influence the height of the barrier to rotation about this bond. For SF2NCOF and SF2NSF51 the question of cis-trans conformation about the nitrogen-sulfur double bond will be considered. Additionally, several rotameric forms of SF2NCOF which can arise from rotation about the nitrogen-carbon axis and which cannot be definitely precluded by presumptive reasoning will also be examined.
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    Positronium formation in organic liquids and studies of micelle, liposome and polymer systems by position annihilation technique
    Djermouni, Belacem (Virginia Tech, 1979-12-05)
    The positron when formed as a result of nuclear decay process (β+) can interact with an electron resulting in a positronium formation (Ps). In the first part of this thesis the formation of positronium in benzene solutions containing various halogenated compounds was studied in the presence and absence of C₆F₆ additives. The observed I₂ values, which are indicative of the number of thermalized positronium atoms formed, showed a good correlation with the dissociative electron attachment parameters of the solute species. The formation and the reactions of Ps were found to be very sensitive to structural changes and phase transitions. Therefore, the positron annihilation technique was applied for studying systems such as micelles, liposomes and polymers which have found widespread applications in today's chemical industry. The micelle formation process was investigated in inverted micelles. A distinct cooperative effect of the solution resulting in abrupt changes in the number of thermal ortho-positronium atoms was observed and studied as a function of temperature, counter ion, length and structure of the hydrocarbon chain in the cationic and anionic parts of the surfactant molecules. The results are being discussed in terms of existing models. The thermotropic transition, which is an important parameter for all membranes and liposomes, was studied in sonicated dioctadecyldimethylam-monium chloride versicles (DODAC). The results obtained provide important information on fluidities and allow further assessments of cationic surfactant vesicles as membrane mimetic agents. The third application deals with the study of styrene-butadiene-styrene block copolymers obtained by casting in toluene, carbon tetra-chloride, ethyl acetate and methyl ethyl ketone. The temperature studies show the appearance of two distinct changes in the positron annihilation parameter λ₂ at -70°C and +85°C. They were attributed to the onset of motion of butadiene and styrene respectively. In addition two other abrupt changes in λ₂ were observed at -14°C and +10°C. The change at -14°C was observed in all films whereas the second at +10°C occurs only in films casted in poor solvents (ethyl acetate and methyl ethyl ketone). Their presence is discussed in terms of phase separation and morphological changes.
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    Pseudo-Molecular Ion Formation by Aromatic Acids in Negative Ionization Mode Electrospray Ionization Mass Spectrometry
    Schug, Kevin Albert (Virginia Tech, 2002-10-02)
    Pseudo-molecular ion formation is an artifact common to most analyses performed by electrospray ionization mass spectrometry. These species are non-covalent complexes formed between an analyte of interest and any other components (such as mobile phase, additives, and impurities) present in the ionized sample band. Published literature addresses pseudo-molecular ion formation in routine analyses as well as in complicated molecular recognition processes. The majority of these works are directed towards the formation of complexes in the positive ionization mode. Consequently, investigation of pseudo-molecular ion formation in the negative ionization mode is a logical extension of work in this area. Experiments presented here detail the work performed on elucidation of factors controlling ionization efficiency of aromatic acid pseudo-molecular ions by electrospray ionization in the negative ionization mode. Sets of tested acidic analytes, including ibuprofen derivatives and benzoic acid derivatives, were analyzed in the presence of various solution systems by flow injection analysis to determine the effect of pH, concentration, injection volume, and instrumental parameters on dominant ion forms observed in the mass spectra. These ion forms correspond to a deprotonated molecular ion ([M-H]-), a hydrogen-bound dimer ion ([2M-H]-), and a sodium-bridged dimer ion ([2M-2H+Na]-). Report of the latter ion form is unique to this work. Response of these ion forms were found to vary greatly with changing solution parameters, particularly in the presence of common LC-MS modifiers, such as triethylamine, acetic acid, formic acid, and ammonium formate. Results point to the formation of the sodium-bridged dimer ion during gas-phase processes following the release of ions from disintegrated droplets. Ab initio theoretical calculations and correlations with calculated solution phenomena (such as pKa and log P) were used to elucidate structural arrangements and dominant factors controlling pseudo-molecular ion formation by aromatic acids in the negative ionization mode.
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    Purely ionic and molecular orbital modelings of the bonding in mineral crystal structures
    Lindsay, Curtis George (Virginia Polytechnic Institute and State University, 1988)
    The modified electron gas (MEG) model has been used to generate ionic model CaCO₃ crystals in the calcite, aragonite, diopside, and perovskite structure types. For calcite and aragonite, the model predicts shorter CO bonds and larger bulk moduli than observed. Modeling of the thermochemistry of CaCO₃ does not reproduce the observed thermochemistry even qualitatively. The model predicts that the hypothetical diopside structure type is the most stable form CaCO₃ among the four structure types. These discrepancies may illustrate the significance of CO bond covalency in determining the physico-chemical properties of CaCO₃. The MEG model has also been used to generate model alkali halide crystals in the sphalerite, rocksalt, and CsCl types in an exploration of the reliability of the radius ratio rule. The MEG model predicts the correct cation coordination numbers for 13 of 16 alkali halides, whereas the radius ratio rule predicts the correct coordination numbers in at most 9 of the same 16 alkali halides. Analyses of the model crystal structures suggests that energy minimization is more important than packing efficiency in determining the most stable structures for ionic crystals. The molecular orbital (MO) model has been used to determine minimum-energy geometries and electron density distributions in sulfate hydroxyacid molecules. These molecules have been used to model the bonding in sulfate crystals. SO bond lengths calculated for H₂SO₄ and H₂S₂O₇ correlate linearly with fractional s-characters of the bonds, as in sulfate crystals. With increasing S coordination number, the bonded radii of S and O, as determined from electron density maps, increase at the same rate, contrary to the common assumption of constant anion H₂S₂O₇ shows a relatively large change in energy as its SOS angle is deformed from its minimum-energy value (125.6°) to l80°, in conformity with the small variation among observed SOS angles. In contrast, SiOSi and POP angles show relatively wide variations in crystals and molecules. This suggests that polysulfates may be less amenable than polysilicates or polyphosphates to polymorphism or glass formation. Other properties of H₂SO₄ are also calculated and compared with experimental observations and previous calculations.
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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.