Applications of the Gaussian-2 and Gaussian-3 models of theory: a structural and energetics study of selected chemical systems.

Lau Kai-Chi.Thesis (M.Phil.)--Chinese University of Hong Kong, 2001.Includes bibliographical references.Abstracts in English and Chinese.Abstract --- p.iAcknowledgements --- p.iiiTable of Contents --- p.ivChapter Chapter 1 --- Introduction --- p.1Chapter 1.1 --- The Gaussian´ؤ3 Method --- p.1Chapter 1.2 --- The G3 Method with Reduced MΦ ller-Plesset Order and Basis Set --- p.2Chapter 1.3 --- The Gaussian-3X Method --- p.2Chapter 1.4 --- The Modified G2 Method --- p.3Chapter 1.5 --- Calculation of Thermodynamical Data --- p.3Chapter 1.6 --- Remark on the Location of Transition Structures --- p.4Chapter 1.7 --- Scope of the Thesis --- p.4Chapter 1.8 --- References --- p.4Chapter Chapter 2 --- "A Gaussian-2 and Gaussian-3 Study of Alkoxide Anion Decompositions. I. H2 and CH4 Eliminations of the Methoxide, Ethoxide, i-Propoxide, and t-Butoxide Anions" --- p.6Chapter 2.1 --- Introduction --- p.6Chapter 2.2 --- Methods of Calcuations --- p.7Chapter 2.3 --- Results and Discussion --- p.8Chapter 2.3.1 --- Nature of ion-neutral complex --- p.8Chapter 2.3.2 --- Initial bond cleavage of alkoxide anions --- p.9Chapter 2.3.3 --- Dissociation of alkoxide anions --- p.10Chapter 2.4 --- Conclusions --- p.23Chapter 2.5 --- Publication Note --- p.25Chapter 2.6 --- References --- p.25Chapter Chapter 3 --- A Gaussian-2 and Gaussian-3 Study of Alkoxide Anion Decompositions. II. Alkane Eliminations of (CH3)2(C2H5)CO- and (i-Pr)(C2H5)2CO- --- p.28Chapter 3.1 --- Introduction --- p.28Chapter 3.2 --- Methods of Calculations --- p.29Chapter 3.3 --- Results and Discussion --- p.29Chapter 3.3.1 --- Initial bond cleavage of alkoxide anions --- p.30Chapter 3.3.2 --- Dissociation of alkoxde anions --- p.31Chapter 3.3.3 --- General dissociation mechanism of alkoxide anions --- p.35Chapter 3.4 --- Conclusions --- p.37Chapter 3.5 --- References --- p.37Chapter Chapter 4 --- A Gaussian-3 Study of the Photoionization and Dissociative Photoionization Channels of Dimethyl Disulfide --- p.40Chapter 4.1 --- Introduction --- p.40Chapter 4.2 --- Methods of Calculations --- p.41Chapter 4.3 --- Results and Discussion --- p.41Chapter 4.3.1 --- Bond cleavage reactions --- p.44Chapter 4.3.2 --- Dissociation channels involving transition structures --- p.45Chapter 4.4 --- Conclusions --- p.48Chapter 4.5 --- References --- p.48Chapter Chapter 5 --- A Gaussian´ؤ3 Study of the Photodissociation Channels of Propylene Sulfide --- p.50Chapter 5.1 --- Introduction --- p.50Chapter 5.2 --- Methods of Calculations --- p.51Chapter 5.3 --- Results and Discussion --- p.51Chapter 5.3.1 --- The dissociation channels involving transition structures --- p.53Chapter 5.3.2 --- The dissociations of sulfur atom --- p.56Chapter 5.4 --- Conclusions --- p.58Chapter 5.5 --- References --- p.59Chapter Chapter 6 --- Thermochemistry of Phosphorus Fluorides: A Gaussian´ؤ3 and Gaussian´ؤ3X Study --- p.60Chapter 6.1 --- Introduction --- p.60Chapter 6.2 --- Methods of Calculations --- p.62Chapter 6.3 --- Results and Discussion --- p.62Chapter 6.3.1 --- Comparison of the G3 and G3X methods --- p.62Chapter 6.3.2 --- Assessments of the experimental results --- p.65Chapter 6.4 --- Conclusions --- p.71Chapter 6.5 --- References --- p.71Chapter Chapter 7 --- Conclusions --- p.74Appendix A --- p.75Appendix B --- p.7

Theoretical Insights into the Bonding in Thorium Organometallic Complexes: A Comparison with Group IV Transition Metal Chemistry

In this thesis a detailed ab initio theoretical study of organothorium chemistry is presented. The first part is devoted to examining both the bonding in and the reaction chemistry of various substituted thorium complexes. Using the chlorine ligand as a model for the usual cyclopentadienyl groups found in these systems, we examine the bonding of hydrogen and methyl ligands to thorium. Frequent comparisons with the experimental results on similar species are made. In addition, by contrasting the bonding in the thorium complexes with the bonding in the analogous Group IVB systems, a qualitative and quantitative picture of bonding, as the atomic number of the metal becomes larger, can be obtained. The reaction chemistry is studied via two different sets of processes. In the first, the deuterium (D2) exchange reaction with a thorium-hydrogen bond is examined. Several studies have been done previously, both experimentally and theoretically, on the Group IVB exchange reactions. Hence, there is enough information to see trends and to make predictions about relative reaction rates. Also, from our investigation the effect that different types of ligands have on the activation barrier to reaction can be ascertained. In the second part of the thesis, the factors that go into stabilizing bond formation are discussed concerning both main group elements and transition metals, including actinides. In particular, the process of bond formation between hydrogen atom and the alkali metals is compared with the same process in the Group IVB-hydrogen and thorium-hydrogen saturated complexes. The main difference between the alkali metal and the transition metal bonds with hydrogen is the bond strength trends with increasing atomic number. For the alkali metals the bond energies decrease down the column, yet for the transition metals and thorium it is the reverse. The conclusion is that the shape of the mostly d in character transition metal bonding orbitals is such that better overlap can be achieved with hydrogen as the orbitals become more diffuse. In the alkali metals the bonds can be described as s—s bonds whose overlap decreases with increasing diffuseness.</p

Photoelectron Spectroscopy of Alkyl and Alkenyl Free Radicals

The first bands of the photoelectron spectra of simple alkyl and alkenyl free radicals, produced by pyrolysis of organic nitrites, are presented. Thermochemical and structural information about the corresponding carbocations in the gas phase are obtained from interpreting the spectra. In particular, adiabatic ionization potentials are combined with radical heats of formation to obtain heats of formation of the cations. The photoelectron band shapes are discussed in terms of structural changes occurring upon ionization. The focus of this work is· the study of isomeric radicals. Their photoelectron spectra allow the characterization of a series of carbocations with well-defined structures, namely those which closely resemble the radicals, even when the cations can rearrange with little or no activation energy to more stable forms. Radical thermal rearrangement and decomposition pathways are also studied. Chapter I provides a brief introduction to the field of photoelectron spectroscopy of transient species, including a compilation of photoelectron data of transient species, and an introduction to the specific work presented in this thesis. Chapter II presents the photoelectron spectra of the C4H7 radicals 1-methylallyl, 2-methylallyl, allylcarbinyl and cyclobutyl radical, which provide information about the corresponding C4H+7 carbocations in the gas phase. These cations are of longstanding interest in the cyclopropylcarbinyl, cyclobutyl and allylcarbinyl interconversions observed in solvolysis reactions. The photoelectron spectra of 1-propyl, 1-butyl, isobutyl, neopentyl and 2-butyl radicals are presented in Chapter III. The corresponding primary cations are of particular interest because they are often postulated as intermediates or transition states in gas-phase and solution isomerization processes. Chapter IV describes the application of photoelectron spectroscopy to the study of low-pressure gas-surface reactions. Organic radicals, as well as stable products, have been detected directly after exiting the catalyst bed. Allylic hydrogen abstraction by heated bismuth oxide catalyst forms gas-phase allyl and 2-methylallyl radicals from propylene and isobutylene in the presence of oxygen. Mechanistic information is obtained by varying the catalyst temperature and reactant partial pressures.</p

Studies of Molecular Precursors Used in FEBID Fabrication of Nanostructures

The adoption of nanotechnology is increasingly important in many aspects of our daily life influencing the clothes we wear and most of the electronic devices we use while also underpinning the development of drugs and medical techniques that we will need at some point in our lives. The methods by which nanoscale devices are fabricated is changing from a 'top down' etching based procedure to a 'bottom up' molecule by molecule deposition and assembly. The focus of the present research is the development, design, and analysis of new precursors for focused electron beam induced deposition (FEBID) and extreme ultraviolet nanolithography (EUVL) through a large pool of experimental and computational resources. The research is divided into two areas: gas - phase analysis of precursors (largely used for fragment and radicals' analysis, and molecular design) and surface and deposition science (physical deposition of precursors, simulation analysis of surface - molecule interactions and characterization of deposition processes to obtain optimal process parameters for molecular structures). It is necessary to collect data such as cross sections of electron - molecule interactions e.g., dissociative ionization (DI) and dissociative electron attachment (DEA) to provide accurate simulations that can be used to improve the FEBID and EUVL while understanding surface processes such as molecular absorption and diffusion to determine the structure and purity of the nanostructures formed by these methods. The objective of this thesis is to provide a gas - phase and deposition analysis of potential and widely used precursors for FEBID and EUVL at the nanoscale. To achieve this the experimental technique of velocity sliced map imaging (VsMI) was used in conjunction with theoretical tools such as density functional theory (DFT) simulations using Gaussian 16 software and evaluation of cross-section data for molecular dissociation at low electron energies of 0 - 20 eV using Quantemol-N. Results of the gas - phase analysis of negative ionic fragments formed by DEA and DI with their appearance, dissociation and ionization energies, angular distributions and kinetic energies, cross-sections for DEA fragmentation at low energy and excited states calculations at values up to 10 eV are presented. These results are used as the inputs to the models of the FEBID processes. The electronic, structural, and kinetic properties of several FEBID precursors are explored, and FEBID method used to create nanostructures using a Zeiss MeRiT SEM with GEMINI column operated at 20 kV. Analysis of the deposits was performed using EDX and atomic force microscopy (AFM) analysis as well as electron stimulated desorption (ESD) and temperature programmed desorption (TPD). Complementary simulations of the dynamics of processes at the surface were studied using MBN Explorer and surface - molecule interactions with great results in simulating the deposition process of islands and structures (results presented in Chapter 8)