Browsing by Subject "Computational Chemistry"
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Item Computational Studies of the Electronic Structures and Mechanisms of Late Transition Metal Systems(2013-08-27) Pitts, AmandaLate transition metal species are heavily studied because of their diverse applications in industrial, synthetic, and biological processes. Transition metals can alter the thermodynamic aspects of a reaction by creating an alternative, lower-energy pathway, which is not accessible without a metal. Numerous investigations have been performed to better understand the elementary steps within these reactions. The significant increase in available computing power coupled with the further development of transition-metal friendly quantum chemical methods has assisted in making computational chemistry an important method in predicting transition-metal mechanisms. This dissertation is divided into four parts, one for each of the transition-metal systems that were studied. The first system focuses on the formation of a carbon-bromine bond from the reaction of Ni(Ar)(Br)(pic) (Ar = 2-phenylpyridine, pic = 2-picoloine) with Br2. Unlike the typical behavior of heavier group 10 metals that have a wider range of stable oxidation states, Ni was found to undergo a change multiplicity during this reaction. The mechanism proceeds through a triplet state pathway that is stabilized by a Br2-/NiIII interaction instead of the NiIV singlet state pathway. The second two systems are concerned with inter- and intramolecular carbon-hydrogen bond activation, respectively. In the second system the lifetimes of carbon-hydrogen activation of four cycloalkanes with the Cp?Rh(CO) fragments (Cp?= ?5-C5H5 or ?5-C5Me5) were calculated. The lifetimes were found to be dependent of the size of the cycloalkane reacting and the number of possible reaction paths associated with the specific cycloalkane. A intramolecular carbon-hydrogen bond activation mechanism was calculated for the RuII(SC6H3Me2-2,6-?1S)2(PMe3)3 species for the third system. The dominant pathway was predicted to be the equatorial mechanism which proceeds through a ?-bond metathesis reaction. The final transition-metal system involves the transfer of CuI from the Atox1 metal binding site to a metal binding domain on the ATP7A or ATP7B proteins. This system was found to proceed through a dissociative pathway wich each two-coordinate and three-coordinate species stabilized by adopting the optimized the S lone pair/Cu 3d ?-overlap.Item Systematic Approach for Chemical Reactivity Evaluation(Texas A&M University, 2004-09-30) Aldeeb, Abdulrehman AhmedUnder certain conditions, reactive chemicals may proceed into uncontrolled chemical reaction pathways with rapid and significant increases in temperature, pressure, and/or gas evolution. Reactive chemicals have been involved in many industrial incidents, and have harmed people, property, and the environment. Evaluation of reactive chemical hazards is critical to design and operate safer chemical plant processes. Much effort is needed for experimental techniques, mainly calorimetric analysis, to measure thermal reactivity of chemical systems. Studying all the various reaction pathways experimentally however is very expensive and time consuming. Therefore, it is essential to employ simplified screening tools and other methods to reduce the number of experiments and to identify the most energetic pathways. A systematic approach is presented for the evaluation of reactive chemical hazards. This approach is based on a combination of computational methods, correlations, and experimental thermal analysis techniques. The presented approach will help to focus the experimental work to the most hazardous reaction scenarios with a better understanding of the reactive system chemistry. Computational methods are used to predict reaction stoichiometries, thermodynamics, and kinetics, which then are used to exclude thermodynamically infeasible and non-hazardous reaction pathways. Computational methods included: (1) molecular group contribution methods, (2) computational quantum chemistry methods, and (3) correlations based on thermodynamic-energy relationships. The experimental techniques are used to evaluate the most energetic systems for more accurate thermodynamic and kinetics parameters, or to replace inadequate numerical methods. The Reactive System Screening Tool (RSST) and the Automatic Pressure Tracking Adiabatic Calorimeter (APTAC) were employed to evaluate the reactive systems experimentally. The RSST detected exothermic behavior and measured the overall liberated energy. The APTAC simulated near-adiabatic runaway scenarios for more accurate thermodynamic and kinetic parameters. The validity of this approach was investigated through the evaluation of potentially hazardous reactive systems, including decomposition of di-tert-butyl peroxide, copolymerization of styrene-acrylonitrile, and polymerization of 1,3-butadiene.Item Theoretical and Experimental Evaluation of Chemical Reactivity(2011-10-21) Wang, QingshengReactive chemicals are presented widely in the chemical and petrochemical process industry. Their chemical reactivity hazards have posed a significant challenge to the industries of manufacturing, storage and transportation. The accidents due to reactive chemicals have caused tremendous loss of properties and lives, and damages to the environment. In this research, three classes of reactive chemicals (unsaturated hydrocarbons, self-reacting chemicals, energetic materials) were evaluated through theoretical and experimental methods. Methylcyclopentadiene (MCP) and Hydroxylamine (HA) are selected as representatives of unsaturated hydrocarbons and self-reacting chemicals, respectively. Chemical reactivity of MCP, including isomerization, dimerization, and oxidation, is investigated by computational chemistry methods and empirical thermodynamic?energy correlation. Density functional and ab initio methods are used to search the initial thermal decomposition steps of HA, including unimolecular and bimolecular pathways. In addition, solvent effects are also examined using water cluster methods and Polarizable Continuum Models (PCM) for aqueous solution of HA. The thermal stability of a basic energetic material, Nitroethane, is investigated through both theoretical and experimental methods. Density functional methods are employed to explore the initial decomposition pathways, followed by developing detailed reaction networks. Experiments with a batch reactor and in situ GC are designed to analyze the distribution of reaction products and verify reaction mechanisms. Overall kinetic model is also built from calorimetric experiments using an Automated Pressure Tracking Adiabatic Calorimeter (APTAC). Finally, a general evaluation approach is developed for a wide range of reactive chemicals. An index of thermal risk is proposed as a preliminary risk assessment to screen reactive chemicals. Correlations are also developed between reactivity parameters, such as onset temperature, activation energy, and adiabatic time to maximum rate based on a limited number, 37 sets, of Differential Scanning Calorimeter (DSC) data. The research shows broad applications in developing reaction mechanisms at the molecular level. The methodology of reaction modeling in combination with molecular modeling can also be used to study other reactive chemical systems.