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Browsing by Subject "Thermochemistry"

Now showing 1 - 3 of 3
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    Development and testing of a protocol for computational prediction of 1H and 13C NMR chemical shifts and thermochemistry and reaction analysis of benzyne formation and trapping
    (2013-01) Marell, Daniel Joshua
    Elucidating structures of novel compounds and investigation of new reactions are two tasks that experimental organic chemists address on a frequent basis. The pursuit of these objectives can be rigorous and time-consuming. Of the methods employed in elucidating the structure of novel compounds, nuclear magnetic resonance (NMR) is by far the most widely applied. Investigation into new reactions may require any number of techniques to understand the reaction scope, kinetics, optimal conditions, mechanisms, etc. In both cases, the use of computational methods is well-suited to augment the experimentalist's data to guide and understand the system being investigated. A protocol for facilitating computational prediction of NMR chemical shifts was developed. Application to a set of natural products previously evaluated against computed NMR shifts, showed improved accuracy, through analysis of the corrected mean-absolute error (CMAE). The protocol was further employed successfully to aid in analysis of experimental spectra for compounds synthesized by collaborators where multiple diastereomers were possible. Graphing templates were also created to allow for rapid inspection of possible structures without more in-depth statistical analysis. Thermodynamic and mechanistic analysis on the formation and reaction of benzyne was also performed. Thermodynamic restrictions on the ring-size of fused benzynocycloalkanes were investigated. Additionally, analysis of the energetics and transition state geometries for small-molecule trapping (both intra and intermolecular) of benzyne are discussed.
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    Electronic structure theory and multi-structural statistical thermodynamics for computational chemical kinetics.
    (2012-08) Papajak, Ewa
    This thesis involves the development and application of methods for accurate computational thermochemistry. It consists of two parts. The first part focuses on the accuracy of the electronic structure methods. In particular, various augmentation schemes for one-electron basis sets are presented and tested for density functional theory (DFT) calculations and for wave function theory (WFT) calculations. The relationship between diffuse basis functions and basis set superposition error is discussed. For WFT, we also compare the efficiency of conventional one-electron basis-sets to that of newly developed explicitly correlated methods. Various ways of approaching the complete basis set limit of WFT calculations are explained, and recommendations are made for the best ways of achieving balance between the basis set size, higher-order correlation, and relativistic corrections. Applications of this work include computation of barrier heights, reaction and bond energies, electron affinities, ionization potentials, and noncovalent interactions. The second part of this thesis focuses on the problem of incorporating multistructural effects and anharmonicity effects in the torsional modes into partition function calculations, especially by using a new multi-structural torsion (MS-T) method. Applications of the MS-T method include partition functions of molecules and radicals important for combustion research. These partition functions are used to obtain thermodynamic functions that are the most reliable results available to date for these molecules. The multi-structural approach is also applied to two kinetics problems: • the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical • the 1,5-hydrogen shift isomerization of the 1-butoxyl radical In both cases multi-structural effects play an important role in the final results.
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    The oxidation of zinc vapor and non-stoichiometric ceria by water and carbon dioxide to produce hydrogen and carbon monoxide.
    (2012-06) Venstrom, Luke J.
    Experimental studies of two pathways for solar thermochemical metal oxide cycles to split water and carbon dioxide are presented. The heterogeneous oxidation of Zn(g) is investigated in Part I, and the oxidation of porous ceria is investigated in Part II. The heterogeneous oxidation of Zn(g) is proposed as an improved approach for rapid and complete oxidation of Zn. Reaction rates are measured gravimetrically in a quartz tube flow reactor at atmospheric pressure for conditions in which Zn is the limiting reactant, at temperatures between 800 and 1150 K, and for Zn(g), H2O(g), and CO2 partial pressures between 10-5 and 0.25 atm. The rate of Zn(g) oxidation by CO2 is between 0.3×10-8 and 6.5×10-6 mol cm-2 s-1, permitting conversions of Zn to ZnO greater than 84% in one second. The rate of Zn(g) oxidation by H2O is between 0.8×10-7 and 1.5×10-5-2 s-1 permitting conversions greater than ~80% in one second. A finite volume based numerical model decouples mass transfer and surface kinetics from the reaction rate data. The CO2-splitting kinetics are second-order, proportional to the Zn(g) and CO2 concentrations. The kinetic parameter is expressed in Arrhenius form, and the activation energy and pre-exponential factor are 44±3 kJ mol-1 and 92±6 mol m-2 s-1 atm-2, respectively. When expressed in second-order form, the apparent activation energy and pre-exponential factor of H2O-splitting are -110 kJ mol-1 and 1.8×10-5 mol m-2 s-1 atm-2 between 800 and 1050 K. At 1100 K, the activation energy becomes positive. A precursor mechanism, where the apparent activation energy is the sum of the heat of adsorption of H2O and the activation energy of the rate-limiting kinetic step is postulated to explain this behavior. The benefit of completely converting Zn via the heterogeneous oxidation of Zn(g) is an increase in the Zn/ZnO cycle efficiency from ~6% for polydisperse aerosol reactors, which have been limited to Zn conversions of 20% for reaction times on the order of a minute, to 27% and 31% for H2O- and CO2-splitting, respectively. In Part II, the effect of material morphology on the reduction and oxidation of ceria is investigated. The oxidation by H2O and CO2 of three-dimensionally ordered macroporous ceria (3DOM CeO2), which features an interconnected, ordered pore network, solid feature sizes between 80 and 200 nm, and a moderate specific surface area of 10 m2 g-1, is compared to the oxidation of non-ordered mesoporous ceria and sintered, low porosity ceria at 1100 K in 6 isothermal chemical cycles. The 3DOM CeO2 increases the maximum H2 and CO production rates over the low porosity CeO2 by 125 and 260%, and increases the maximum H2 and CO production rates over the non-ordered mesoporous CeO2 by 75 and 175%. 3DOM CeO2, non-ordered macroporous ceria (NOM CeO2), and aggregates of ceria nanoparticles are also cyclically reduced at ~1500 K under pO2 = 10-5 atm and oxidized at ~1100 K by 25 mol% CO2. The 3DOM and NOM CeO2 retain an interconnected, disordered pore network and achieve maximum CO production rates of 6.4 and 4.0 mL min-1 g-1, respectively, an order of magnitude increase over the ~0.1 mL min-1 g-1 rate of CO production of the sintered ceria nanoparticles and low porosity ceria. The present study demonstrates the importance of engineering ceria with interconnected porosity and solid feature sizes on the order of hundreds of nm.