Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis (Ph. D.) -- California Institute of Technology, 2010. / Title from home page (viewed 03/08/2010). Advisor and committee chair names found in the thesis' metadata record in the digital repository. Includes bibliographical references.

Thesis (Ph. D.) -- California Institute of Technology, 2010. / Title from home page (viewed 02/23/2010). Advisor and committee chair names found in the Acknowledgments pages of the thesis. Includes bibliographical references.

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Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Robert Eugene Wyatt. Vita. Includes bibliographical references. Available also from UMI Company.

This thesis reports some quantum chemical calculations directed at elucidating principles useful for refining calculations of electron distribution and other properties for complex molecules. In this work calculations have been made with the valence bond and molecular orbital methods using minimum basis sets of Slater-type orbitals on the ground states of the molecules HF and HO, and on states of HF⁺ corresponding to the ionization of either a 1s electron or a 2pπ electron from fluorine in HF. Calculations have been made for molecular energies, bond lengths, force constants, dipole moments, and electron distributions as given by Mulliken population analysis. For HF, the perfect pairing model with molecule-optimized exponents yields molecular energies about 6 kcal./mole lower than the comparable molecular orbital calculations; the dipole moment calculated by the perfect pairing method is 0.3 D. closer to the experimental value (1.82 D.) than that calculated by the molecular orbital method. The HF equilibrium bond length and force constants are calculated to a reasonable degree of accuracy with the two methods, although the first ionization potentials seem to be better calculated by the molecular orbital method either by Koopman's Theorem or by taking the difference between the energies of the two states. The calculations reported in this thesis show clearly that in general free atom exponents are not reliable for calculating molecular properties, and this is important for calculations on larger molecules which most frequently use basis functions appropriate to free atoms. As part of a programme for finding ways of optimizing exponents relatively inexpensively, for use with more complex molecules, an approximation due to Lowdin, for overlap charge distributions in electron repulsion integrals, was tested. The results reported in this thesis show that the method has promise in providing a way of initially optimizing exponents prior to the actual calculation wherein all integrals are evaluated exactly. / Science, Faculty of / Chemistry, Department of / Graduate

<p>Chemical reactions are ubiquitous and very important for life and many other processes taking place on earth. In both theoretical and experimental studies of reactivity a transition state is often used to rationalise the outcome of such studies. The present thesis deals with calculations of transition states in radical cation rearrangements, and a principle of least motion study of the rearrangements in the barbaralyl cation.</p><p>In particular, alternative quadricyclane radical cation (<b>Q∙</b><b>+</b>) rearrangements are extensively studied. The rearrangement of <b>Q∙</b><b>+</b> to norbornadiene is extremely facile and is often used as a prototype for one-electron oxidations. However, electron spin resonance (ESR) experiments show that there are additional cations formed from <b>Q∙</b><b>+</b>. Two plausible paths for the rearrangement of <b>Q∙</b><b>+</b> to the 1,3,5-cycloheptatriene radical cation are located. The most favourable one is a multistep rearrangement with two shallow intermediates, which has a rate-limiting step of 16.5 kcal/mol. In addition, a special structure, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, is identified on these alternative paths; and its computed ESR parameters agree excellently with the experimental spectrum assigned to another intermediate on this path. Moreover, this cation show a homoconjugative stabilization, which is uncommon for radical cations.</p><p>The bicyclopropylidene (<b>BCP</b>) radical cation undergoes ring opening to the tetramethyleneethane radical cation upon γ-irradiation of the neutral <b>BCP</b>. This rearrangement proceeds through a stepwise mechanism for the first ring opening with a 7.3 kcal/mol activation energy, while the second ring opening has no activation energy. The dominating reaction coordinate during each ring opening is an olefinic carbon rehybridization.</p><p>The principle of least motion is based on the idea that, on passing from reactant to product, the reaction path with the least nuclear change is the most likely. By using hyperspherical coordinates to define a distance measure between conformations on a potential energy surface, a possibility to interpret reaction paths in terms of distance arises. In applying this measure to the complex rearrangements of the barbaralyl cation, a correct ordering of the conformations on this surface is found.</p>

<p>In this thesis, quantum chemical calculations are undertaken mostly in order to interpret experimental results, but also to learn about computational techniques, their performance and their limitations. In paper I, the ionization-cleavage process of alkenes is investigated and two pathways are followed, one of initial cleavage and subsequent ionization and on the opposite, the other one of initial ionization and subsequent cleavage. The calculations reveal that ionization is best described by a vertical process, which is much faster than the relaxation of the molecule to its ionized structural minimum. Further, in paper II, the core hole excited state of ammonia is investigated and found to dissociate in an ultra-fast manner nicely explained by the calculated potential energy surface showing a very low barrier for dissociation. In paper III, the static and dynamic structures of two halogenated dimethyl ether radical cations are studied in ESR experiments, and it is found that, while the chlorinated molecule remains unaffected, the fluorinated molecule undergoes a dissociation or association reaction before the measurement takes place, the resulting fragments are searched for but not identified decisively. In paper IV, the stability of Jahn-Teller distorted selectively deuteriated benzene radical cation isotopomers is investigated by ESR experiments and density functional theory calculations. The temperature dependence, between 4.2 K and 77 K, of the ESR spectra is explained. Finally, in paper V, the hydrogen inversion in aziridine and methyl and dimethyl substituted aziridines is investigated. The rate constants and kinetic isotope effects are calculated using various techniques of transition state theory and tunneling correction methods.</p>

In this thesis, quantum chemical calculations are undertaken mostly in order to interpret experimental results, but also to learn about computational techniques, their performance and their limitations. In paper I, the ionization-cleavage process of alkenes is investigated and two pathways are followed, one of initial cleavage and subsequent ionization and on the opposite, the other one of initial ionization and subsequent cleavage. The calculations reveal that ionization is best described by a vertical process, which is much faster than the relaxation of the molecule to its ionized structural minimum. Further, in paper II, the core hole excited state of ammonia is investigated and found to dissociate in an ultra-fast manner nicely explained by the calculated potential energy surface showing a very low barrier for dissociation. In paper III, the static and dynamic structures of two halogenated dimethyl ether radical cations are studied in ESR experiments, and it is found that, while the chlorinated molecule remains unaffected, the fluorinated molecule undergoes a dissociation or association reaction before the measurement takes place, the resulting fragments are searched for but not identified decisively. In paper IV, the stability of Jahn-Teller distorted selectively deuteriated benzene radical cation isotopomers is investigated by ESR experiments and density functional theory calculations. The temperature dependence, between 4.2 K and 77 K, of the ESR spectra is explained. Finally, in paper V, the hydrogen inversion in aziridine and methyl and dimethyl substituted aziridines is investigated. The rate constants and kinetic isotope effects are calculated using various techniques of transition state theory and tunneling correction methods.