Thesis (doctoral)--Norges tekniske høgskole, 1983. / Cover title. "Oktober 1983." Includes bibliographical references.

This study is concerned with the nature of the relatively unusual bonding which occurs in organolithium compounds as a direct result of the oligomerization and possible explanations for that bonding.

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No description available.

The research reported in this thesis comprised a cross-age inquiry of learners' mental models for chemical bonding. Learners were chosen purposefully from three academic levels-senior secondary school (Year-13, age range 17-18 years old), undergraduate (age range 19-21 years), and postgraduate (comprising MSc and PhD; age range 22- 27 years). The principal research goal was to establish learners' preferred mental models for the concept of chemical bonding. Other research goals were to establish if and how learners made use of analogy to understand chemical bonding and to establish the prevalence of learners' alternative conceptions for chemical bonding. The research inquiry was conducted from within a constructivist paradigm; specifically the researcher ascribed to a social and contextual constructivist belief system.Based on a review of the science education literature a decision was made to classify mental models into four classes according to the typology of Norman (1983), namely, the target system, a conceptual model, the users' or learners' mental model and the scientists' conceptualisation. A conceptual theme for the inquiry was developed based on this typology resulting in the identification of target systems-metallic, ionic and covalent bonding. Subsequently, target models for each of the three target systems were identified, namely, the sea of electrons model and the band theory for metallic bonding; the electrostatic model, and the theoretical electrostatic model for ionic bonding; and the octet rule, the valence bond approach, the molecular orbital theory and the ligand field theory for covalent bonding. A conceptual model, consisting of a summary of the salient points of the target models, was developed by the researcher. Once validated by four of the instructors involved in the inquiry, this formed the scientists' conceptualisation for the target ++ / models.Learners' mental models were elicited by the use of a three phase semi-structured interview protocol for each of the three target systems based on the translation interface developed by Johnson and Gott (1996). The protocol consisted of showing participants samples of common substances and asking them to describe the bonding in these materials. In addition, participants were shown Interviews About Events (IAE), focus cards which depicted events involving chemical bonding or contained depicted models of bonding for the three target systems. Transcriptions of audio-tapes combined with diagrams produced by the participants formed the data corpus for the inquiry. Learners' mental models were compiled into inventories for each of the target systems. Examination of inventories enabled identification of commonality of views which were validated by four instructors-two instructors from the teaching institutions involved in the inquiry, and two instructors independent of the inquiry.The research reported in this thesis revealed that learners across all three academic levels preferred simple or realist mental models for chemical bonding, such as the sea of electrons model and the octet rule. Learners frequently used concepts from other more sophisticated models to aid their explanations when their preferred mental models were found to be inadequate. Senior level learners were more critical of mental models, particularly depicted models provided on IAE focus cards. Furthermore, senior level learners were able to describe their mental models in greater detail than their younger counterparts. However, the inquiry found considerable commonality across all three levels of learner, suggesting mental models are relatively stable.Learners' use of analogy was classified according to Dagher's (1995a) typology, namely, simple, narrative, peripheral and compound. Learners' use of ++ / analogy for the understanding of chemical bonding was found to be idiosyncratic. When they struggled to explain aspects of their mental models for chemical bonding, learners made extensive use of simple analogy, that typically involved the mapping of a single attribute between the target and source domains. There did not appear to be any correlation between academic ability or academic level and use of analogy. However, learners made greater use of compound analogy for the target systems of metallic and ionic bonding, mostly as a result of the use of analogical models during instruction.This inquiry revealed prevalent alternative conceptions for chemical bonding across all three academic levels of learner. This is a somewhat surprising result considering that the mental models preferred by learners were typically simple, realist models they had encountered during instruction. Learners' alternative conceptions often concerned simple conceptions such as ionic size, the presence of charged species in non- polar molecular compounds, and misunderstandings about the strength of bonding in metals and ionic substances. The inquiry also revealed widespread confusion about intermolecular and intramolecular bonding, and the nature of lattices structures for ionic and metallic substances.The inquiry resulted in a number of recommendations. It is proposed that it may be more beneficial to teach less content at the introductory level, that is, delivering a curriculum that is more appropriate for non-specialist chemistry majors. Hence, one recommendation is for instructors to examine the intended curriculum carefully and be more critical regarding the value of inclusion of some course content. A second recommendation is that sophisticated models of chemical bonding are better taught only at advanced stages of the degree program, and that teaching from a contructivist view of ++ / learning may be beneficial. The third recommendation relates to the fact that learners spontaneously generated analogies to aid their explanations and conceptual understanding, consequently, learners may benefit from greater use of analogy during instruction.

The reactions of tert-butyl-, sec-butyl-, and n-butyllithium with dimethylfluorovinylsilane include addition to the double bond to give both silene and silenoid intermediates, fluorine substitution, and a novel vinyl substitution. For the tert-butyllithium reaction, product stereochemistry and trapping experiments using both cyclopentadiene and methoxytrimethylsilane show that silenes are not formed in THF. In hexane about 67% of the 1,3-disilacyclobutanes obtained arise from silene dimerization while 33% are formed by silenoid coupling. In hexane the order of reactivity for addition, t-Bu > sec-Bu > n-Bu, is opposite that for fluorine substitution. The vinyl substitution is most significant with secondary alkyllithium reagents including the tert-butyllithium adduct to dimethylfluorovinylsilane and with sec-butyllithium itself. Evidence for the formation of vinyllithium or ethylene in the process could not be obtained.

Although various organometallic carbene complexes have found use in industry or research, they still lack some fundamental footing in theory. These complexes have found significant use in catalysis. This is especially true for Schrock carbene complexes in olefination reactions. A few such titanium-based olefination catalyst examples are the Tebbe reagent, Petasis reagent and the Ziegler-Natta catalyst. The nature of organometallic carbene bonding is still not well defined theoretically. Schrock carbenes are expected to have a covalent bonding nature, but multiconfigurational studies have shown this to not be the best description of the bonding. Furthermore, expected Schrock carbenes have been shown to be Fischer carbenes due to the electronic structure. This work investigates the nature of the carbene bond in titanium Schrock carbene complexes by utilising DFT and further application of MO, NBO, QTAIM and FALDI methods. This allows for a modernised description of the nature of this bond as well as the identification of an important long-range ligand-ligand interaction that has not been reported on previously. The research aims to define the nature of titanium Schrock carbene bonding on a theoretical basis by the use of integrated cross-sections on the electron and orbital densities to determine the σ- and π-character of the interaction. These cross-sections provided the means to determine the major components of the bonding interaction. This is further investigated by defining FALDI fragment-based delocalisation indices which revealed the presence of long-range ligand-ligand interactions. The FALDI fragment approach also provided the means to quantify the inter-fragment delocalisation along with intra-fragment localisation and delocalisation which would prove useful for further investigation into the characteristics or various chemical interactions. The fragment-based description should prove to be more intuitive to the chemist than diatomic interactions between atoms where a chemical bond or interaction is not classically expected. This study was followed by a decomposition of the molecular orbitals into localised and delocalised components from atomic contributions which provides a novel approach to determining the bond order in compounds. This provided a quantitative means to describe which atoms contribute to the formation of each molecular orbital as well as providing a measure of the degree to which these atoms are contributing localised as well as delocalised electrons to the molecular orbital. / Dissertation (MSc)--University of Pretoria, 2020. / DST-CSIR Interbursary Support Program / Chemistry / MSc / Unrestricted

Les polyoxovanadates fonctionalisés (POVs) présentent des structures de type cœur – cluster super-octahédral à l’échelle nanométrique, des propriétés électroniques et magnétiques facinantes, de nombreux isomers possédant des propriétés thermodynamiques de type réduction – oxydation et des applications catalytiques prometteuses. Parmi ces différentes propriétés, nous nous sommes intéressées aux propriétés de transfert de charge et de fluorescence. Cependant, comprendre des comportements de transfert de charge, et des mécanismes de fluorescence de ces héxavanadates fonctionnalisées est un encore véritable challenge.La diffraction des rayons X haute résolution permet l’analyse des propriétés électroniques et topologiques, et représente une méthode performante pour étudier les liaisons chimiques et la réactivité chimique, basées sur la détermination des propriétés électroniques et électrostatiques. Par conséquent ces propriétés ont été déterminées pour des polyoxovanadates et sont discutées au niveau atomique.Dans ce manuscrit, nous présentons les résultats de : i) la densité électronique expérimentale, la topologie et les propriétés électrostatiques de deux hexavanadates fonctionnalisées (V6), [(C4H9)4N]2[V6O13{(OCH2)3CCH2OCCH2CH3}2] (V6-C3) and Na2[V6O13{(OCH2)3CCH2OH}2]·3.5H2O (V6OH) ; ii) les calculs théoriques ab initio, d’une série de composés fonctionnalisées (V6) et du décavanadate (V10). Les informations chimiques déduites de l’analyse de la densité électronique sont utilisées pour une meilleure compréhension de la distribution de la densité de charge, des propriétés de fluorescences, du comportement en terme de fonctionnalisation, et des propriétés biologiques. / The functionalized polyoxovanadates (POVs) exhibit nanoscale superoctahedral cluster-core structures, fascinating electronic and magnetic properties, various thermodynamically stable redox isomers, and potential catalytic capabilities. Among of the various properties, we are interested in the charge transfer and fluorescent properties. However, understanding such a charge transfer behavior and fluorescence mechanism of these functionalized hexavanadates is still a formidable challenge.High resolution X-ray crystallography allows the analysis of the electronic and topological properties, and provides a method to study the chemical bonding and chemical reactivity based on charge density and the electrostatic properties determination. Experimental and theoretical charge density analysis of functionalized polyoxovanadates has been carried out and the related properties have been discussed at the atomic level.In this manuscript, we present the results of: i) experimental charge density and related electronic and topological properties of two functionalized hexavanadates (V6), [(C4H9)4N]2[V6O13{(OCH2)3CCH2OCCH2CH3}2] (V6-C3) and Na2[V6O13{(OCH2)3CCH2OH}2]·3.5H2O (V6OH); ii) theoretical calculations on a series of functionalized V6 compounds, and decavanadate (V10). The chemical information from charge density analysis is used for a better understanding of the charge density distribution, charge transfer, fluorescent properties, functionalization behavior, and biological activities.

Models of chemical bonding are essential for contemporary chemistry. Even the explosive development of the computational resources including, both hardware and software, cannot eliminate necessity of compact, intuitive, and efficient methods of representing chemically relevant information. The Lewis model of chemical bonding, which was proposed eleven years before the formulation of quantum theory and preserves its pivotal role in chemical education and research for more than ninety years, is a vivid example of such a tool. As chemistry shifts to the nanoscale, it is becoming obvious that a certain shift of the paradigms of chemical bonding is inescapable. For example, none of the currently available models of chemical bonding can correctly predict structures and properties of sub-nano and nanoclusters. Clusters of main-group elements and transition metals are of major interest for nanotechnology with potential applications including catalysis, hydrogen storage, molecular conductors, drug development, nanodevices, etc. Thus, the goals of this dissertation were three-fold. Firstly, the dissertation introduces a novel approach to the description of chemical bonding and the algorithm of the software performing analysis of chemical bonding, which is called Adaptive Natural Density Partitioning. Secondly, the dissertation presents a series of studies of main-group element and transition-metal clusters in molecular beams, including obtaining their photoelectron spectra, establishing their structures, analyzing chemical bonding, and developing generalized model of chemical bonding. Thirdly, the dissertation clarifies and develops certain methodological aspects of the quantum chemical computations dealing with clusters. This includes appraisal of the performance of several computational methods based on the Density Functional Theory and the development of global optimization software based on the Particle Swarm Optimization algorithm.

This thesis focuses on how school textbooks and teachers present models of chemical bonding in upper secondary schools in Sweden. In science, as well as in science education, models play a central role, but research has shown that they often are difficult for students to understand. In science education, models are presented to students mainly through textbooks and teachers, and textbooks influence teachers’ teaching. The aim of this thesis was to investigate how textbooks and teachers present models of chemical bonding with respect to students’ difficulties in understanding. To analyze representations of models, an analytical framework based on research reports of students’ difficulties in understanding related to models in general and chemical bonding in particular was developed. The chapters of chemical bonding in five chemistry textbooks were analyzed. Further, ten Chemistry teachers’ lesson plans about chemical bonding and semi-structured interviews with the teachers concerning their teaching were analyzed. This analysis concerned teachers pedagogical content knowledge (PCK) of teaching chemical bonding, with focus on knowledge of students’ difficulties in understanding and teaching strategies that take these difficulties into account. The results show that the teachers could specify examples of students’ learning difficulties, but the teaching strategies to promote the students’ understanding were limited. This indicates a deficient interaction between knowledge of difficulties in understanding and teaching strategies, two essential components of teachers’ PCK. Further, the models of chemical bonding represented in the textbooks and by the teachers might cause students’ difficulties in understanding. This indicates a gap between research of students’ difficulties in understanding and teaching practices as well as textbooks’ development. Further, the teachers’ representations of models were strongly influenced by the textbooks. Implications for textbooks’ authors, pre-service as well as in-service teachers are addressed. / <p>Papper 2 "Upper secondary teachers knowledge..." ingick i lic.uppsatsen som manuskript med titeln: "Swedish upper secondary teachers’ pedagogical content knowledge to teach chemical bonding". Nu publicerad. </p>