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Solid and liquid states

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A number of new methods are presented to determine the reaction path both for chemical systems where the transition state(TS) is known and for the more complicated case when only the endpoints are available. To determine the minimum energy path(MEP) two algorithms were developed.The first MEP method is a quadratic string method (QSM) which is based on a multiobjective optimization framework. In the method, each point on the MEP is integrated in the descent direction perpendicular to path. Each local integration is done on an approximate quadratic surface with an updated Hessian allowing the algorithm to take many steps between energy and gradient calls. The integration is performed with an adaptive step size solver, which is restricted in length to the trust radius of the approximate Hessian. The full algorithm is shown to be capable of practical superlinear convergence, in contrast to the linear convergence of other methods. The method also eliminates the need for predetermining such parameters as step size and spring constants, and is applicable to reactions with multiple barriers. The method is demonstrated for the Muller Brown potential, a 7-atom Lennard-Jones cluster and the enolation of acetaldehyde to vinyl alcohol.The second MEP method is referred to as the Sequential Quadratic Programming Method (SQPM). This method is based on minimizing the points representing the path in the subspace perpendicular to the tangent of the path while using a penalty term to prevent kinks from forming. Rather than taking one full step, the minimization is divided into a number of sequential steps on an approximate quadratic surface. The resulting method is shown to be capable of super-linear convergence. However, the emphasis of the algorithm is on its robustness and its ability to determine the reaction mechanism efficiently, from which transition state can be easily identified and refined with other methods. To improve the resolution of the path close to the transition state, points are clustered close to this region with a reparametrization scheme. The usefulness of the algorithm is demonstrated for the Mu$ller Brown potential, amide hydrolysis and an 89 atom cluster taken from the active site of 4-Oxalocrotonate tautomerase (4-OT) for the reaction which catalyzes 2-oxo-4-hexenedioate to the intermediate 2-hydroxy-2,4-hexadienedioate.When the TS is known we present two methods for integrating the steepest descent path (SDP). Also the concepts of stability and stiffness are elaborated upon. The first SDP method is an optimally combined explicit-implicit method for following the reaction path to high accuracy. Although the SDP is generally considered to be a stiff ODE, it is shown that the reaction path is not uniformly stiff and instead is only stiff near stationary points. The optimal algorithm is developed by combining the explicit and implicit methods with a simple criterion, based on the stiffness, to switch between the two. Using two different methods an algorithm is developed to efficiently integrate the SDP. This method is tested on a number of small molecules.The final method given is based on the diagonally implicit Runge-Kutta framework, which is shown to be a general form for constructing stable, efficient steepest descent reaction path integrators, of any order. With this framework tolerance driven, adaptive step-size methods can be constructed by embedding methods to obtain error estimates of each step without additional computational cost. There are many embedded and non-embedded, diagonally implicit Runge-Kutta methods available from the numerical analysis literature and these are reviewed for orders 2,3 and 4. New embedded methods are also developed which are tailored to the application of reaction path following. All integrators are summarized and compared for three systems. / Dissertation

Chemical vapor deposition (CVD) is a versatile method for material preparation. Materials from films to nanostructures can be obtained by tuning the parameters of the CVD method. In the field of one-dimensional nanostructures, CVD is the most extensively employed method. This dissertation is focused on the synthesis of carbon nanotubes (CNTs) and tungsten oxide nanowires by the CVD method as well as the growth mechanism of these two kinds of one-dimensional materials. The first part of this dissertation focuses on CNTs. After an introduction to CNTs, the synthesis of bulk CNTs with high purity is explored. In chapter 2, the synthesis of few-walled carbon nanotubes (FWNTs) with ethanol as carbon source is explored. It is proved that addition of methanol to ethanol increases the purity of raw FWNT materials. Methanol acts as "carbonaceous impurities remover" to remove impurities deposited on the MgO support and also hinders the formation of such impurities during FWNTs growth. In Chapter 3, the synthesis of double-walled carbon nanotubes (DWNTs) with high purity and small size distribution using FeSi2 as catalyst is described. In FeSi2, iron is dispersed at atomic scale, and thus it is possible to provide uniform iron particles for CNT growth. In chapter 4, a two-stage model for the growth of SWNTs is proposed, which may explain the low yield of SWNT growth in some early results. In this model, growth of SWNTs is divided into two stages, nucleation and growth. These two stages need different carbon feeding rates. To obtain a high yield of SWNTs, different carbon feeding gases should be applied to different stages. In chapter 5, a small amount of Zn is reported to increase the yield of SWNTs grown on SiO2/Si wafers. This phenomenon is abnormal and can not be explained with general accepted CNT growth mechanism. More effort is needed to elucidate the mechanism of high SWNT yield obtained under assist of Zn.The second part of this dissertation is focused on tungsten oxide nanowires. Potassium doped tungsten oxide and a tungsten bronze nanowires were synthesized with a simple CVD system on tungsten plate. A vapor-liquid-solid (VLS) mechanism is employed to explain the growth of these two kinds of nanowires. / Dissertation

<p>Chiroptical spectroscopic techniques such as optical rotation (OR), electronic circular dichroism (ECD), and Raman optical activity (ROA) provide the means to probe molecular dissymmetry, which is the arrangement of atoms that is not superimposable on its mirror image. However, the application of chiroptical methods for conformational and configurational analysis of molecules (or molecular stereochemistry) requires the accurate and efficient computation of chiroptical response properties. Access to a battery of accurate methods, especially for OR, have just emerged over the last decade. A combination of quantum chemical electronic structure methods and chiroptical spectroscopy can now be used to establish a comprehensive understanding of molecular stereochemistry. A key challenge in stereochemical analysis is to compute the chiroptical response of molecules in solution, which is the focus of our research in optical activity.</p><p>Although the common perception is that chiroptical responses are solely determined by a chiral solute's electronic structure in its environment, we demonstrate that chiral imprinting effects on media, and molecular assembly effects can dominate chiroptical signatures. Both these effects are strongly influenced by intermolecular interactions that are essential to accurately describe chiroptical signatures of molecules. Chiroptical response of molecules spans a range of large positive and negative values, and hence, the measured chiroptical response represents an ensemble average of orientations. Thus, accurate prediction of chiroptical properties requires adequate conformational averaging. Here we show that OR and ROA of molecules in solution can be modeled using a combination of structure sampling and electronic structure methods. Method for computing chiroptical response in the condensed phase using continuum solvation models and point charge solvent model, although more efficient than explicit solvent treatments, are often inadequate for describing induced chirality effects. The lack of a priori knowledge of solvent-solute interactions and their influence on the chiroptical signature demands exploration of thermally averaged solute-solvent clusters and comparison with simpler solvent studies.</p> / Dissertation

<p>Few-walled carbon nanotubes (FWNTs) have 2 to 3 layers of sidewalls with diameters ranging from 3 to 8 nm and length around tens of micrometers; they are unique MWNTs with perfect graphitization structure as in single walled carbon nanotubes (SWNTs). Double-walled carbon nanotubes (DWNTs) are special type of FWNTs which are considered as the intermediate between SWNTs and MWNTs, respectively. Many of the applications proposed for DWNTs require the precise control of their diameters. </p><p>In this dissertation we describe a simple, scalable approach for the selective synthesis of high-quality DWNTs by carbon monoxide chemical vapor deposition (CO-CVD) method. The inner-tube diameter distribution measured by HRTEM is about 0.7~1.1nm. The structural correlation and optical properties of high quality DWNTs were also exploited. </p><p>To further exploit the excellent properties of FWNTs, it is necessary to functionalize them and make well dispersed solution. FWNTs have been chemically functionalized via efficient ultrasound-mediated dissolving metal method and ultrasound-mediated oxidation method. Such methods can be generalized towards other types of CNTs and the chemically functionalized CNTs are highly soluble in various organic solvents and aqueous solution respectively.</p><p>The reported fluorescence from inner shells of double-walled carbon nanotubes (DWCNTs) is an intriguing and potentially useful property. A combination of bulk and single-molecule methods was used to study the spectroscopy, chemical quenching, mechanical rigidity, density, and TEM structures of the near-IR emitters in DWCNT samples. It is found that DWCNT inner shell fluorescence is weaker than SWCNT fluorescence by a factor of at least 10,000. The near-IR emission from DWCNT samples actually arises from SWCNT impurities.</p><p>Compared to SWNTs and MWNTs, the thin FWNTs are believed to have extraordinary mechanical properties. The mechanical properties of composite films (CNTs/PVA) have been investigated. It is observed that the Young's modulus of composite films with only 0.2wt % functionalized FWNTs shows a remarkable reinforcement. In addition, the Young's modulus increased steadily with the increased concentration of FWNTs. </p><p>In order to tackle the problem of the poor conductivity of the metal oxides, we have designed a ternary nanocomposite film composed of metal oxide (MnO2), functionalized FWNTs and conducting polymer (CP). Each component in the MnO2/CNT/CP film provides unique and critical function to achieve optimized electrochemical properties. It turned out that the electrodes exhibited excellent electrochemical properties.</p> / Dissertation

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Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 113-122). Available also in a digital version from Dissertation Abstracts.