ABSTRACT AXIAL LIGAND SUBSTITUTION REACTION KINETICS OF PYRIMIDINE-2-THIONATO BRIDGED BINUCLEAR PLATINUM(III) COMPLEXES G&ouml / y, Aytun&ccedil / M. S. Department of Chemistry Supervisor: Prof. Dr. H&uuml / seyin iS&ccedil / i Co-supervisor: Assoc. Prof.Dr. Seniz &Ouml / zalp Yaman September 2007, 89 pages The kinetics of the ligand substitution reactions, which is represented by the equation, [Pt2(C4H3N2S)4X2] + 2Y- Pt2(C4H3N2S)4Y2 + 2X- where X- = Cl-, Br-, I- and Y- = Cl-, Br-, I- are studied in acetonitrile in the presence of excess Y- ion concentrations, under constant ionic strength. All reactions are reversible. The rate of the above reaction is dependent on binuclear complex and entering ligand concentrations. Thus general rate equation can be written as Rate = k [Y-]a[Pt2(C4H3N2S)4X2]b The reaction rates are first order with respect to the substrate complex (b=1). The experimentally determined values of the order of the reaction with repect to entering ligand, &ldquo / a&rdquo / , are 0.96&plusmn / 0.057 (X=I-, Y=Cl-), -0.49&plusmn / 0.037 (X=Cl-, Y=I-), 0.28&plusmn / 0.023 (X=I-, Y=Br-), 0.48&plusmn / 0.044 (X=Br-, Y=I-), 0.53&plusmn / 0.042 (X=Br-, Y=Cl-), and -0.21&plusmn / 0.014 (X=Cl-, Y=Br-). The rate constants are 12.1&plusmn / 2.05 M-1s-1 (X=I-, Y=Cl-), (5.7&plusmn / 1.6)x10-3 M1/2s-1 (X=Cl-, Y=I-), 0.3&plusmn / 0.27 M-0.3s-1 (X=I-, Y=Br-), 0.53&plusmn / 0.11 M-1/2s-1 (X=Br-, Y=I-), 1.74&plusmn / 0.16 M-1/2s-1 (X=Br-, Y=Cl-), and 1.71&plusmn / 0.37x10-2 M0.2s-1 (X=Cl-, Y=Br-). To obtain information about the energetics of the reactions, the temperature dependence of the rate constants is determined and the activation parameters &amp / #916 / H* and &amp / #916 / S* are calculated. The values &amp / #916 / S* are negative and, in the range of -81 and -236 J K-1 mol-1. These results support an associative-interchange, Ia, mechanism. All data obtained in this work are used to propose a mechanism which will be consistent with the experimentally determined rate law.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 225-227). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 225-227). Also available on the Internet.

Thesis (Ph. D.)--Institute of Paper Chemistry, 1972. / Includes bibliographical references (p. 145-149).

CO hydrogenation and CO oxidation are two important processes addressing the energy and environmental issues of great interest. Both processes are carried out using metallic catalysts. The objective of this dissertation is to study the catalytic processes that govern these two reactions from a molecular perspective using quantum mechanical calculations. Density Functional Theory (DFT) has proven to be a valuable tool to study adsorption, dissociation, chain growth, reaction pathways etc., on well-defined surfaces. DFT was used to study the CO reduction reactions on promoted cobalt catalyst surfaces and CO oxidation mechanisms on cobalt surfaces. CO hydrogenation via Fischer-Tropsch Synthesis (FTS) is a process used to produce liquid fuels from synthesis gas. The economics of the Fischer-Tropsch process strongly depends on the performance of the catalyst used. The desired properties of a catalyst include selectivity towards middle distillate products such as diesel and jet fuel, higher activity and longer catalyst life. Catalysts are often modified by adding promoters to obtain these desirable properties. Promoters can influence the reaction pathways, reducibility, dispersion, activity and selectivity. In FTS, understanding the effect of promoters in the molecular scale would help in tailoring catalysts with higher activity and desired selectivity. Preventing deactivation of catalyst is important in FTS to increase the catalyst life. Deactivation of Co catalyst can occur by reoxidation, C deposition, sintering, formation of cobalt-support compounds etc. Designing catalyst with resistance to deactivation by the use of promoters is explored in this dissertation. The influence of promoters on the initiation pathways of CO hydrogenation is also explored as a first step towards determining the selectivity of promoted catalyst. The influence of Pt promoter on O removal from the surface of Co catalyst showed that Pt promoter reduced the activation barrier for the removal of O on both flat and stepped Co surfaces. An approximate kinetic model was developed and a volcano plot was established. The turn-over frequency (TOF) calculated based on the activation barriers showed that Pt promoted Co surface had a higher rate than unpromoted Co surface. The effect of Pt and Ru promoters on various pathways of C deposition on Co catalyst was studied to gain a mechanistic understanding. The promoters did not affect the subsurface C formation but they increased the barriers for C-C and C-C-C formation and also decreased the barriers for C-H formation. The promoters also influence the stabilities of C compounds on the Co surface suggesting that Pt and Ru promoters would decrease C deposition on Co catalysts. The effect of Pt promoter on unassisted and H-assisted CO activation pathways on Co catalyst was studied. Pt promoted Co surface followed H-assisted CO activation. Pt promoter decreased the activation barriers for CO activation pathways on Co catalyst thereby increasing the activity of Co catalyst. CO oxidation is a process used to prevent poisoning of fuel cell catalysts and reduce pollution of the atmosphere through exhaust gases containing CO. Expensive catalysts like Pt are widely used for CO oxidation which significantly increases the cost of the process and hence it is necessary to search for alternative lower cost catalysts. Understanding the mechanism of a reaction is the first step towards designing better and efficient catalyst. DFT is helpful in determining the basic mechanism and intermediates of reactions. The mechanism of CO oxidation on CoO catalyst was explored. Four possible mechanisms for CO oxidation on CoO catalyst were studied to determine the most likely mechanism. The mechanism was found to be a two-step process with activation barrier for formation of CO2 larger than the barrier for formation of the intermediate species.

This thesis describes research efforts dedicated to the development of palladium(II)-catalyzed oxidative Heck and Heck/Suzuki domino reactions, and the applications of a new microwave heating technology, purpose-built for continuous flow in organic synthesis. Paper I describes the development of a ligand-modulated approach for attaching aryl groups to a chelating vinyl ether. By switching the ligand being used, selectivity for the arylation could be shifted to obtain three different outcomes: internal α- or terminal β-arylation, as well as a serendipitously discovered domino α,β-diarylation process. The latter was proposed to be an effect of para-benzoquinone, effectively acting as a stabilizing π-acidic ligand with the ability to suppress β-hydride elimination. Paper II explores the performance of a new microwave heating technology in combination with continuous flow. The novel nonresonant microwave applicator allowed rapid heating of common laboratory solvents and reaction mixtures above their boiling points with stable and reproducible temperature profiles. The technology was successfully applied to small-scale method development and subsequent scale-out of palladium-catalyzed reactions, heterocycle synthesis and classical organic transformations such as the Fischer indole synthesis. Paper III focuses on developing regioselective oxidative decarboxylative Heck reactions with electron-rich olefins. Successful internal α-arylations were achieved using various olefins and ortho-substituted aromatic acids. The mechanism was also studied by ESI-MS analysis. Key cationic organopalladium intermediates were identified, as well as an unexpected palladium(II)-complex which was isolated and characterized. Its experimentally deduced structure was in accordance with the lowest energy minimum found by DFT calculations. Preliminary findings suggested that the complex acts as a catalyst trap. Paper IV studies the mechanism of the reaction in Paper III by means of DFT calculations. Reductive elimination was identified as the rate-determining step when using a linear enamide as the olefin, due to its propensity to form low energy chelates. Its chelating properties also played a key role in the stability of the isolated palladium(II)-complex. The complex, which can act as a catalyst trap, was characterized by X-ray crystallography.

The biosynthesis of the steroid precursor molecule lanosterol is a remarkable process in which the enzyme-bound substrate 2,3-S-oxidosqualene forms four new carbocyclic rings by a cascade of cation-alkene addition reactions, followed by a series of 1,2-methyl and hydride shifts. The work presented in this thesis is a computational study of the reactions of compounds designed to model the oxidosqualene-lanosterol cyclisation in order to establish details of the mechanism of this amazing cyclisation. The initiation of oxidosqualene cyclisation has been modelled by the intermolecular reaction of protonated oxirane and methylpropene. The SN2-like ring opening of the protonated epoxide is strongly exothermic with a low barrier to reaction; the geometry of the gas phase reaction has been found to be significantly affected by hyperconjugative stabilisations and low energy steric interactions. The energy profile and geometry of this reaction can now be compared to analogous intramolecular reactions such as the formation of the lanosterol A-ring. The competing five- and six-membered cyclisations of a series of substituted A-ring model compounds was investigated. It has been found that the facile cleavage of the protonated epoxide causes the reaction to behave more as an electrophilic addition than as a nucleophilic ring-opening substitution. This behaviour accounts for the general preference of protonated epoxides to react at the more substituted carbon atom, while epoxides in neutral or basic media react at the least sterically hindered carbon. With consideration for Baldwin's rules for ring closure, it is seen that the series of model compounds generally favours six-membered ring formation endo at the epoxide. The formation of the lanosterol B-ring was studied using a bicyclic model system. Previous computational studies had predicted the B-ring to close with readily with an activation energy of less than 1 kcal mol-1, however the present study has found a significant barrier to cyclisation of ca. 5-7 kcal mol-1 in this gas-phase model at the HF/6-31G(d) level of theory. This barrier is thought to arise from the closure of the B-ring in a sterically hindered twist-boat conformation.

[Truncated abstract] This study was carried out to investigate the teaching and learning processes occurring in the topic of reaction mechanisms in three tertiary level organic chemistry courses and focussed on investigating perceptions about the importance of teaching and learning about reaction mechanisms and about the difficult aspects of the topic .... In the organic chemistry courses under investigation, students achieved many of the explicitly stated aims that their lecturers identified. The students rarely achieved implicit outcomes anticipated by the lecturer. Lecturers demonstrate a tendency to use particular structural representations when discussing certain types of reaction process. The study identified that students commonly use these same types when working through particular reaction processes. In addition, it was found that the use of a particular structure could cue students into thinking about only one type of reaction process taking place in a given reaction. The use of language that is consistent with a consideration of only single reaction particles was also commonly observed in lectures. While this can be adequate in some circumstances, other aspects of reaction processes are better considered in terms of multiple reaction particles ... The project proposes an integrated model, which takes into account the many levels (macroscopic, single particle molecular, multiple particle molecular and intramolecular) involved when describing reaction processes. It is felt that a consideration of the levels discussed in this model is useful when teaching and learning about reaction mechanisms.

Thesis (M.S.)--State University of New York at Binghamton, Department of Chemistry, 2006. / Includes bibliographical references.

Thesis (Ph.D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Huang, Ching-Hua; Committee Member: Kim, Jaehong; Committee Member: Pavlostathis, Spyros; Committee Member: Stack, Andrew; Committee Member: Yiacoumi, Sotira