published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Master / Master of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Perovskites, ABX₃, and their associated solid-solutions are a particularly important and attractive area of research within materials chemistry. Owing to their structural and compositional flexibility and potential physical properties they are one of the largest classes of materials currently under investigation. This thesis is concerned with the synthesis and structural characterisation of several perovskite-based materials using a combined approach of high-resolution synchrotron X-ray and neutron powder diffraction (NPD), solid-state Nuclear Magnetic Resonance (NMR) and first-principles Density Functional Theory (DFT) calculations. Initial investigations concentrated on room temperature NaNbO₃, a perovskite widely debated in the literatue. Published crystallographic data indicate NaNbO₃ possesses two crystallographically distinct Na sites in space group Pbcm. Whilst some of our materials appear in agreement with this (notably a commercially purchased sample) many of our laboratory-synthesised samples of NaNbO₃ routinely comprise of two phases, which we show to be the antiferroelectric Pbcm and polar P2₁ma polymorphs. Several different synthetic methods were utilised during this investigation and the quantity of each phase present was found to vary as a function of preparative method. ²³Na, ⁹³Nb and ¹⁷O DFT calculations were used in conjunction with experiment to aid in spectral analysis, assignment and interpretation. In addition, ab initio random structure searching (AIRSS) was utilised in an attempt to predict the most stable phases of NaNbO₃. This proved to be both successful and highly informative. A series of NaNbO₃-related solid-solutions, namely K[subscript(x)]Na[subscript(1-x)]NbO₃ (KNN), Li[subscript(x)]Na[subscript(1-x)]NbO₃ (LNN) and Na[subscript(1-x)]Sr[subscript(x/2)]□[subscript(x/2)]NbO₃ (SNN) have also been synthesised and characterised. The substitution of K⁺ , Li⁺ and Sr²⁺ cations onto the A site appears to produce the same polar P2₁ma phase initially identified in the room temperature NaNbO₃ investigation. The abrupt change in cation size in the KNN and LNN series, and the introduction of vacancies in the SNN series, is thought to result in a structural distortion which, in turn, causes the formation of the P2₁ma phase. A low temperature synchrotron X-ray powder diffraction study (12 < T < 295 K) was completed for a sample of NaNbO₁ composed of the P2₁ma polymorph (~90%) and a small quantity of the Pbcm phase (~10%). A region of phase coexistence was identified between the P2₁ma, R3c and Pbcm phases over a relatively large temperature range. Full conversion of the P2₁ma phase to the low temperature R3c phase was not possible and, consistently, the P2₁ma phase was the most abundant phase present. Factors such as structural, strain, crystallite size and morphology are thought to be crucial in determining the exact phases of NaNbO₃ produced, both at low and room temperature. The solid-solution La[subscript(1-x)]Y[subscript(x)]ScO₃ was also investigated. Compositions x = 0, 0.2, 0.4, 0.6, 0.8 and 1 were successfully synthesised and characterised. Refined high-resolution NPD data indicates that an orthorhombic structure, in space group Pbnm, was retained throughout the solid-solution. Using ⁴⁵Sc and ⁸⁹Y MAS NMR each sample was found to exhibit disorder, believed to result from both a distribution of quadrupole and chemical shifts. NMR parameters were calculated for several model Sc and Y compounds using DFT methods to determine the feasibility and accuracy of ⁴⁵Sc and ⁸⁹Y DFT calculations. These proved successful and subsequent calculations were completed for the end members LaScO₃ and YScO₃. DFT calculations were also utilised to gain insight into the disorder exhibited in the La[subscript(1-x)]Y[subscript(x)]ScO₃ solid-solution.

The work in this thesis mainly focuses on the assessment of density functional methods for computing structures and energies of organic and bioorganic molecules. Previous studies found dramatic conformational and stability changes from B3LYP to MP2 geometry optimization for some Tyr-Gly conformers. Possible reasons could be large intramolecular basis set superposition errors (BSSEs) in the MP2 calculations and the lack of dispersion in the B3LYP calculations. The fragmentation method and three kinds of rotation methods were used to investigate intramolecular BSSE. It is concluded that the rotation method cannot be used to correct intramolecular BSSE along a rotation profile. Another methodology is to employ modern density functionals. We focused on M06-L with the Tyr-Gly conformer ‘book6’. Potential energy profiles were determined by computing the energy for geometries optimized at various fixed values of a distance that controls the degree of foldedness of the structure. M06-L manifested itself as a very promising method to investigate the potential energy surface of small peptides containing aromatic residues. To predict Tyr-Gly structures, 108 potential conformers were created with a Fortran program. The geometry optimizations were done using M06-L/6-31G(d) and M05-2X/6-31+G(d). Two schemes were employed and the most stable conformers were compared to the 20 stable conformers found by B3LYP. Both schemes found 10 conformers similar to one of the B3LYP stable conformers, as well as several newly found conformers. The study of a missing B3LYP stable conformer showed that the possible reason of missing conformers may be the lack in dispersion in B3LYP theory. To study the hydration effect, we studied the conformations of neutral and zwitterionic 3-fluoro-γ-aminobutyric acid (3F-GABA) in solution using different solvation models, mainly the explicit water molecule models. Zwitterionic forms of 3F-GABA are preferred in solution. M06-2X performs better in calculating transition energy profiles than MP2.

Yim Wai-leung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 73-77). / Abstracts in English and Chinese. / THESIS COMMITTEE --- p.ii / ABSTRACT (English version) --- p.iii / ABSTRACT (Chinese version) --- p.v / ACKNOWLEDGEMENTS --- p.vi / TABLE OF CONTENTS --- p.vii / LIST OF FIGURES --- p.ix / LIST OF TABLES --- p.xi / Chapter CHAPTER 1. --- General Introduction / Chapter Section 1.1 --- Introduction --- p.1 / Chapter Section 1.2 --- Electronic Structure Calculation / Chapter 1.2.1 --- Density Functional Theory --- p.2 / Chapter 1.2.2 --- "Exchange, Correlation and the Local Density Approximation" --- p.4 / Chapter 1.2.3 --- Bloch's Theorem and Plane Wave Basis Set --- p.6 / Chapter 1.2.4 --- The Pseudopotential Approximation --- p.10 / Chapter Section 1.3 --- Molecular Dynamics / Chapter 1.3.1 --- Molecular Dynamics --- p.12 / Chapter 1.3.2 --- Nose Thermostat / Chapter 1.3.2.1 --- Introduction --- p.14 / Chapter 1.3.2.2 --- Feedback Method --- p.15 / Chapter Section 1.4 --- Case Studies / Chapter 1.4.1 --- Thermal properties of small silver clusters --- p.18 / Chapter 1.4.2 --- Thermal decomposition channels of NTO --- p.20 / Chapter CHAPTER 2. --- Ab Initio Molecular Dynamics Study on Agn (n=4-6) / Chapter Section 2.1 --- Introduction --- p.22 / Chapter Section 2.2 --- Computational Method --- p.24 / Chapter Section 2.3 --- Results and Discussion / Chapter 2.3.1 --- Ag2 --- p.26 / Chapter 2.3.2 --- Ag4 --- p.30 / Chapter 2.3.3 --- Ag5 --- p.36 / Chapter 2.3.4 --- Ag6 --- p.45 / Chapter Section 2.4 --- Summary --- p.49 / Chapter CHAPTER 3. --- Ab Initio Molecular Dynamics Study on Thermal Decomposition of NTO / Chapter Section 3.1 --- Introduction --- p.52 / Chapter Section 3.2 --- Computation Details --- p.55 / Chapter Section 3.3 --- Results and Discussion / Chapter 3.3.1 --- Comparison of the Quantum Calculations by VASP and Gaussian98 --- p.56 / Chapter 3.3.2 --- Exploring the Reaction Channels --- p.62 / Chapter 3.3.2.1 --- Hydrogen-transfer Activation --- p.62 / Chapter 3.3.2.2 --- Homolytic Cleavage of C-N02 Bond --- p.63 / Chapter 3.3.2.3 --- Nitro-nitrite Rearrangement --- p.64 / Chapter 3.3.2.4 --- Direct Ring Rupture --- p.64 / Chapter 3.3.3 --- Energetic Consideration --- p.65 / Chapter 3.3.4 --- Activation Barriers --- p.70 / Chapter 3.3.5 --- Summary --- p.72 / REFERENCES --- p.73

Cheung Chung Hong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 126-137). / Abstracts in English and Chinese. / ABSTRACT (English version) --- p.iii / ABSTRACT (Chinese version) --- p.vi / ACKNOWLEDGMENTS --- p.viii / TABLE OF CONTENTS --- p.ix / LIST OF FIGURES --- p.xiii / LIST OF TABLES --- p.xvii / Chapter CHAPTER ONE: --- INTRODUCTION --- p.1 / Chapter 1.1 --- Molecular Structure Determination --- p.1 / Chapter 1.2 --- Methods for Structure Determination --- p.1 / Chapter 1.2.1 --- Infrared (IR) Spectroscopy --- p.1 / Chapter 1.2.2 --- Electron Microscopy (EM) --- p.2 / Chapter 1.2.3 --- X-ray Diffraction (XRD) --- p.2 / Chapter 1.2.4 --- Nuclear Magnetic Resonance (NMR) Spectroscopy --- p.3 / Chapter 1.3 --- Solid State NMR Spectroscopy 一 Chemical Shielding Tensor --- p.4 / Chapter 1.4 --- Scope of This Thesis --- p.6 / Chapter CHAPTER TWO: --- PRINCIPLE AND THEORY --- p.7 / Chapter 2.1 --- Introduction --- p.7 / Chapter 2.2 --- Chemical Shielding Tensor (CST) --- p.7 / Chapter 2.2.1 --- Basis of Nuclear Magnetic Resonance --- p.7 / Chapter 2.2.2 --- Theoretical Description of CST --- p.8 / Chapter 2.3 --- CST Calculation --- p.11 / Chapter 2.3.1 --- Ramsey's Theory --- p.11 / Chapter 2.3.2 --- Gauge-including Atomic Orbitals (GIAO) Method --- p.13 / Chapter 2.4 --- Density Functional Theory (DFT) --- p.14 / Chapter 2.4.1 --- KS Theorem --- p.14 / Chapter 2.4.2 --- B3LYP - Hybrid DFT Functional --- p.16 / Chapter 2.5 --- Molecular Mechanic (MM) --- p.16 / Chapter 2.5.1 --- Basis Principle 一 Ball and Spring Model --- p.16 / Chapter 2.5.2 --- MM+ --- p.18 / Chapter 2.5.2.1 --- Bond Stretching --- p.18 / Chapter 2.5.2.2 --- Bond Bending --- p.19 / Chapter 2.5.2.3 --- Torsional Motion --- p.19 / Chapter 2.5.2.4 --- van der Waals Interaction --- p.19 / Chapter 2.5.2.5 --- Electrostatic Interaction --- p.20 / Chapter 2.5.2.6 --- Stretching-bending Interaction --- p.21 / Chapter 2.6 --- Isodesmic Reaction --- p.21 / Chapter CHAPTER THREE: --- METHODOLOGY AND EVALUATION --- p.23 / Chapter 3.1 --- Introduction --- p.23 / Chapter 3.2 --- Computational Method --- p.24 / Chapter 3.2.1 --- Evaluation of Different NMR Computation Level --- p.24 / Chapter 3.2.2 --- Computational Error 一 Round Off Error --- p.32 / Chapter 3.3 --- Local Structure Determination and Evaluation --- p.34 / Chapter 3.3.1 --- Methodology --- p.34 / Chapter 3.3.2 --- Definition of Local Structure --- p.37 / Chapter 3.3.3 --- Models used in Local Structure Determination --- p.37 / Chapter 3.3.3.1 --- Model without CST Modification (Mod-1) --- p.38 / Chapter 3.3.3.2 --- Models with CST Modification --- p.39 / Chapter 3.3.3.2.1 --- Model Using Correction Factors (Mod-2) --- p.39 / Chapter 3.3.3.2.2 --- Model Using Linear Equation (Mod-3) --- p.40 / Chapter 3.3.4 --- Evaluation - Me2P(S)C(OH)R1R2 --- p.41 / Chapter 3.4 --- Determination of Possible Conformation in Powder and Evaluation --- p.53 / Chapter 3.4.1 --- Introduction --- p.53 / Chapter 3.4.2 --- Methodology --- p.54 / Chapter 3.4.2.1 --- Preparation of Calibration Curve --- p.54 / Chapter 3.4.2.2 --- Determination of Possible Conformation Contributing to the Observed CST --- p.57 / Chapter 3.4.3 --- Evaluation --- p.59 / Chapter 3.5 --- Discussion --- p.72 / Chapter 3.5.1 --- Factors Affecting CST --- p.72 / Chapter 3.5.1.1 --- Inter-molecular Interaction --- p.72 / Chapter 3.5.1.1.1 --- O-H----S --- p.73 / Chapter 3.5.1.1.2 --- C-H----S --- p.78 / Chapter 3.5.1.1.3 --- van der Waals Forces --- p.80 / Chapter 3.5.1.2 --- Conformation --- p.81 / Chapter 3.5.2 --- Effect of Experimental Error on the Calculated Structure --- p.82 / Chapter 3.6 --- Summary --- p.84 / Chapter CHAPTER FOUR: --- APPLICATIONS --- p.86 / Chapter 4.1 --- Introduction --- p.86 / Chapter 4.2 --- Tetramethyldiphoshine Disulfide (TMPS) --- p.86 / Chapter 4.2.1 --- Local Structure --- p.87 / Chapter 4.2.2 --- Conformation --- p.92 / Chapter 4.3 --- "Carbon Compounds : 1,3,5-Trimethoxybenzene (1,3,5-TMB) and 1,4-Dimethoxybenzene (1,4-DMB)" --- p.97 / Chapter 4.3.1 --- Local Structure --- p.98 / Chapter 4.3.2 --- Conformation --- p.103 / Chapter 4.4 --- Conclusion --- p.110 / Chapter CHAPTER FIVE: --- CONCLUSIONS --- p.111 / APPENDIX --- p.114 / REFERENCES --- p.126

Hu Hong Bing. / "May 2004." / "13" in title is superscript. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

In the first part, the mechanisms for the dissociation of TPA are studied under three types of conditions. The unimolecular dissociation is initiated by the breaking of either the N-Calpha and Calpha -Cbeta bonds and leads to many complicated processes. Within the confined space inside neutral zeolite channels, the diffusion of H radicals enhances a cycle of reactions, which accounts for the experimental observation of dipropylamine and monopropylamine. In the presence of an acidic site, the dissociation of TPA goes through catalyzed successive steps to produce ammonia and propylene molecules. / In the last part, two distinct paths are proposed to investigate the carbon nanotube growth mechanism using benzene as the growth seed and propylene as carbon resource. There is an incremental aromatization process, involving both alkylation and dehydrogenation, that leads to linked aromatic rings to form either (3,3) tube or (4,2) tube. Two paths are identified, one via toluene and the other via cumene. The cumene path is more favorable as the barrier is significant lower. / In the second part, A T5 cluster model is used to investigate mechanisms of propylene aromatization to benzene, which involves chemisorption, dimerization, cyclization and dehydrogenation. Propylene can be chemisorbed to form two distinct products, n-propoxide and i-propoxide, which can further be dimerizated to form longer chain olefins 1-hexene and 2-hexene (from n-propoxide), and 4-methyl-1-pentene and 4-methyl-2-penetene (from i-propoxide). Initiated by H2 elimination, these dimerization products can further go through cyclization process to generated either 6-member ring cyclohexene or 5-member ring methyl-cyclopentene. Catalyzed by zeolite, cyclohexene can directly dehydrogenate to form benzene whereas methyl-cyclopentene can dehydrogenate to form fulven, an isomer to benzene. Under acidic zeolite environment, a fulvene can readily be transformed to the thermodynamically more stable benzene. / The growth mechanisms of mono-sized and parallel-aligned single wall carbon nanotube (CNT) in the microporous channels of AlPO4-5 are investigated by density functional theory calculations. Detailed mechanisms are proposed for the decomposition of TPA, the formation of aromatic ring, and the growth of carbon nanotubes. / Liu, Jianwen. / Adviser: Zhifeng Liu. / Source: Dissertation Abstracts International, Volume: 70-09, Section: B, page: . / Thesis submitted in: December 2008. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 98-99). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Mixed-valence compounds are of interest as model systems for the study of electron transfer reactions. The intramolecular electron transfer processes and patterns of charge delocalization in such compounds depend on the interplay between the electronic (V) and the vibronic (L) coupling. One can obtain both parameters from a Hush analysis of the intervalence band that arises upon optical intramolecular electron transfer if the band is intense and well-separated from other bands. This is quite often the case for mixed-valence triarylamines. As such, both Hush analysis and simulation of the intervalence band are widely used to classify these compounds as charge localized (class-II) or delocalized (class-III). Yet one must estimate the diabatic electron transfer distance (R) to calculate V in the Hush formalism. For mixed-valence triarylamines, R is commonly taken as the N-N distance; we show this to be a poor approximation in many cases. The activation barrier to thermal intramolecular electron transfer in a class-II mixed-valence compound is also related to the parameters V and L. Thus, if one can capture the rate of thermal electron transfer at multiple temperatures, then two experimental methods exist by which to extract the microscopic parameters. One technique that is widely used for organic mixed-valence compounds is variable-temperature electron spin resonance (ESR) spectroscopy. But this method is only rarely used to determine thermal electron transfer rates in mixed-valence triarylamines, as the electron transfer in most of the class-II compounds with distinct intervalence bands is too fast to observe on the ESR timescale. We show, for the first time, that one can use ESR spectroscopy to measure thermal electron transfer rates in such compounds. Simulation of ESR spectra based on density functional theory calculation and comparison with optical data also uncover the nature (i.e., adiabatic or nonadiabatic) of the electron transfer process.