Browsing by Subject "Molecular orbitals"
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Item A systematic study of basis set superposition error in the interaction energy of two hydrogen molecules(Texas Tech University, 1998-12) Chang, Daniel Ta-JenNot availableItem Perturbations of the homo level of oxo-bridged chromium(III) dimers(Texas Tech University, 1992-12) Tekut, Thomas FrancisNot availableItem Pseudopericyclic reactions(Texas Tech University, 1996-05) Wagenseller, EugeneA novel class of reactions is investigated using ab initio methodology. This class of reactions was first defined by Lemal as pseudopericyclic. However, due to ambiguities with his original model the concept remained unexplored for several years. This study uses a model which Is free of the problems associated with Lemal's model. Pseudopericyclic reactions differ from the more typical pericyclic reactions. The distinguishing features are that electrostatic factors play a larger role in determining the activation barrier and the concept of allowed and forbidden do not apply since there is not a loop of interacting orbitals. The orbital topology about the loop dictates that the transition state for these reactions have a marked preference for planarity. Pseudopericyclic reactions may. in principle, have extremely low activation energies.Item Sigma bond activation of the hydrogen molecule by cooperative interaction with a boron cation, a lithium anion, and a beryllium atom(Texas Tech University, 1999-05) Sharp, Stephanie BaxterSigma bond activation, making a sigma bond more reactive, has been one of tht' most extensively studied areas of chemistry for the past decade. Sigma bonds are the strongest bonds and therefore reactions that involve the breaking of such bonds have large activation energies. Without a doubt, the three most popular sigma bonds are H-H. C-H, and C-C which have been studied extensively in the chemical and physical sciences, and are the driving force behind many industrial processes. Activation of a sigma bond requires a reduction in the energy necessary to break the bond, and historically this has involved an interaction with a transition metal. The selective activation of alkanes has been identified as one of the Holy Grails of current chemical investigation [1].Item Theoretical and experimental studies on the reactivities of conjugated ketenes(Texas Tech University, 1998-08) Ham, SihyunThe reactivity of imidoylketene was examined using ab initio molecular orbital theory. MP4(SDQ)/6-31G*//MP2/6-31G* calculations on the conformations of imidoylketene as well as transition states for several of its reactions show parallels between the reactivity of imidoylketene and its oxygen analog formylketene. All reactions proceed via concerted, planar (or nearly so) transition structures regardless of the number of electrons involved. Calculated activation energies are remarkably lower than those for a pericyclic process, as expected from the case of formylketene. The reactions are interpreted in light of their pseudopericyclic orbital topology. N-Propylacetacetimidoylketene was produced by the solution pyrolysis of t-butyl N-propyl 3-amino-2-butenoate. Selectivities of acetimdoylketene toward various polar reagents were measured for the first time in a series of competitive trapping reactions. Significant steric and electronic discriminations of this ketone were observed, suggesting further synthetically useful applications. These experimental reactivity trends indirectly provide support for the planar, pseudopericyclic transition structures predicted by ab initio calculations. The mechanism of the reactions of nitrosoketene to form cyclic nitrones (which leads stereoselective synthesis of a-amino acids) was investigated using ab initio molecular orbital theory (MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE). The direct [3+2] cycloadditions of nitrosoketene with ketones are calculated to be favored over the alternative [4+2] pathway via concerted, asynchronous, pseudopericyclic transition states. The detailed conformations and the reactivity of nitrosoketene toward sterically and electronically different ketones render useful information of the synthetic route for the biologically important reactions. Transition structures for a series of eight cheletropic decarbonylations were optimized at the MP4(SDQ)/D95**//MP2/6-3lG* + ZPE level. Dramatic differences in activation energies and in exothermicities are discussed in terms of the molecular orbital topology. A fundamental question regarding pseudopericyclic orbital overlap is addressed, specifically, how many and what type of orbital orthogonahties in the reaction sites are needed for a reaction to be pseudopericyclic. Generalizations regarding the characteristics of the pseudopericyclic reactions are made to provide a better understanding of the "allowedness" and "favoredness" of the orbital topologies.