Browsing by Author "Van Niekerk, Daniel Malan Emmanuel"
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- ItemExperimental and computational approaches to investigate the high oxidation state oxo- and hydroxido- redox chemistry of osmium(Stellenbosch : Stellenbosch University, 2016-03) Van Niekerk, Daniel Malan Emmanuel; Gerber, Wilhelmus Jacobus; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The OsVIII oxo/hydroxido complexes that are abundant in mild to relatively concentrated basic aqueous solutions are OsVIIIO4, [OsVIIIO4(OH)]- and two cis-[OsVIIIO4(OH)2]2- species. OsVIII complexes that contain water ligands are thermodynamically unfavoured w.r.t. the abovementioned complexes. OsVIIIO4 reacts with hydroxide in two, consecutive, elementary coordination sphere expansion steps to form the [OsVIIIO4(OH)]- complex and then the cis-[OsVIIIO4(OH)2]2- species. The thermodynamic driving force of the first reaction is the bonding energy of the OsVIII-OH metal-hydroxido ligand, while of the second reaction it is the larger hydration energy of the doubly-charged cis-[OsVIIIO4(OH)2]2- in comparison with that of the two, singly-charged reactants. The DFT-calculated ΔG°rxn in the simulated aqueous phase (COSMO) is -2.42 kcal.mol-1 for the first reaction and -0.61 kcal.mol-1 for the second reaction (PBE-D3 functional) and agree to within 1 kcal.mol-1 with reported values, at -2.69 and 0.33 kcal.mol-1, respectively. The thermodynamically most stable paramagnetic OsVII product species in a 2.0 M NaOH aqueous matrix are the five-coordinate trans-[OsVIIO3(OH)2]- and octahedral mer-[OsVIIO3(OH)3]2- stereoisomers. The thermodynamic driving force of the coordination sphere expansion reaction of trans-[OsVIIO3(OH)2]- with a hydroxide ion to form mer-[OsVIIO3(OH)3]2- is the larger hydration energy of the doubly-charged mer-[OsVIIO3(OH)3]2- complex in comparison with that of the two, singly-charged reactants. The thermodynamically most stable OsVI species obtained via DFT is the diamagnetic trans-[OsVIO2(OH)4]2- species and the spin state of this species was confirmed with Evans method, 1H NMR spectroscopy experiments. DFT-calculated metal-ligand bond lengths of the global energy minimum geometry of trans-[OsVIO2(OH)4]2- correlates well with reported X-ray crystal structure data. QTAIM and EDA analyses indicate that the OsVIII=O, OsVI=O and OsVII=O bonding interactions are ionic (closed-shell) and that OsVIII-OH, OsVI-OH and OsVII-OH bonding interactions are polar covalent (dative) while NCI analysis indicates that relatively weak intramolecular hydrogen bonding interactions occur between neighbouring oxo and hydroxido ligands in the [OsVIIIO4(OH)]- and cis-[OsVIIIO4(OH)2]2- species. Diamagnetic OsVIII oxo/hydroxido, [OsVIIIO4(OH)n]n- (n = 1, 2), (of d0 electron configuration) and trans-[OsVIO2(OH)4]2- species (d2) react in a 1:1 mole ratio, in a 2.0 M NaOH aqueous matrix, to form paramagnetic OsVII product species (d1), equation 1. [Os ] + [Os ] 2 [Os ] 1 The forward (comproportionation) reaction, equation 1, is first order w.r.t. the OsVIII and OsVI concentrations while the reverse (disproportionation) reaction is second order w.r.t. the OsVII concentration. From non-linear least squares fits of stopped-flow UV-Vis spectroscopy kinetic traces, rate constants, thermodynamic activation energies and standard reaction energies were determined. The forward reaction is endothermic whilst the activation enthalpy of the reverse reaction is negative. A kinetic isotope effect (KIE) for the forward and reverse reactions, equation 1, are both approximately 1.1 providing evidence that the ratedetermining step of the reactions coincide with the transfer of a proton. DFT-calculated standard reaction energies for the OsVI & OsVIII comproportionation reaction at 298.15 K (e.g. PBE, ΔH°rxn, ΔS°rxn and ΔG°rxn are 21.13 kcal.mol-1, 71.06 cal.mol-1.K-1 and -0.06 kcal.mol-1, respectively) compare well with experimentally determined observed values (ΔH°rxn(obs), ΔS°rxn(obs) and ΔG°rxn(obs) are 17.1 1.2 kcal.mol-1, 61.0 4.3 cal.mol-1.K-1 and -1.1 2.5 kcal.mol-1, respectively). The DFT-calculated, most energetically favoured comproportionation reaction mechanism consists of (i) the formation of a (singlet state) adduct, [OsVIII=O···HO-OsVI]3-, (ii) ‘spin-forbidden’, concerted electron-proton transfer (EPT) from the OsVIII monomer to the trans-[OsVIO2(OH)4]2- monomer to form a (triplet state) dimer, [OsVII-OH···O=OsVII]3-, (iii) separation of the OsVII monomers and ultimately (iv) interconversion of the separated species to form a combination of the trans-[OsVIIO3(OH)2]- and mer-[OsVIIO3(OH)3]2- stereoisomers. EPT from the OsVIII monomer to the OsVI monomer is the rate-determining step of the reversible comproportionation reaction, corroborating the experimental evidence (KIE). Good agreement between experimental and DFT-calculated activation energies were obtained for the stepwise EPT reaction pathway. Sequential electron proton transfer (ET-PT or PT-ET) mechanisms for equation 1 are shown, via DFT calculations, not to occur in a 2.0 M NaOH matrix at 298.15 K. From non-linear least squares fits of UV-Vis spectroscopy kinetic traces, rate constants, thermodynamic activation energies and standard reaction energies for the reduction of OsVIII with methanol (CH3OH or CD3OH) in a 2.0 M NaOH aqueous matrix were determined. Cleavage of an α-C-H bond in CH3OH and the cleavage of an α-C-D bond in CD3OH are the rate-determining step for the two reactions. The relatively large KIE value, kH/kD, of approximately 11.82 (298.15 K) obtained is indicative of a normal, primary isotope effect. The complete, five-step DFT-calculated reaction pathway consists of (i) non-covalent dimer formation of the reactants, (ii) hydrogen atom transfer (HAT) of an α-C-H bond of CH3OH to cis-[OsVIIIO4(OH)2]2-, (iii) PT of the O-H hydrogen of ˙CH2OH to mer-[OsVIIO3(OH)3]2-, (iv) ET from ˙CH2O- to trans-[OsVIIO4(OH)2]-, and (v) separation of the products. The organic and inorganic monomers involved in the elucidated reaction mechanism do not separate from one another between steps (ii) to (iv), but stay bound to one another via intermolecular hydrogen bonding interactions in order to firstly, decrease the PT/ET transfer distance between them, and secondly, to avoid the formation of highly energetically unfavourable cationic, anionic or radical monomer species. The mechanistic study agreed with our experimentally determined result that the rate-determining step of the reaction involves the cleavage of an α-C-H bond in CH3OH. Fair agreement between activation energies of DFT-calculated (PBE-D3 functional), 19.1 kcal.mol-1 (Δ‡H°), -38.1 cal.mol-1.K-1 (Δ‡S°) and 30.5 kcal.mol-1 (Δ‡G°), and experimentally determined, 14.4 1.2 kcal.mol-1 (Δ‡H°), -12.5 4.1 cal.mol-1.K-1 (Δ‡S°) and 18.1 2.4 kcal.mol-1 (Δ‡G°), parameters were obtained.
- ItemDie konstruksie van makrostrukture : die ontwikkeling van graad 10-leerders se vermoe om makrostrukture te konstrueer en proposisies te herroep deur die verbetering van hul struktureringsvaardighede en vraagstellingsvaardighede(Stellenbosch : Stellenbosch University, 2001-03) Van Niekerk, Daniel Malan Emmanuel; Meyer, J. C.; Stellenbosch University. Faculty of Arts and Social Sciences. Dept. of Psychology .ENGLISH ABSTRACT: The construction of macro-structures. Four reading-improvement courses were offered to grade ten pupils in an attempt to improve their general reading skills and more specifically their ability to abstract the gist (macrostructure - Van Dijk and Kintsch, 1983) of texts as well as their ability to recall detail about texts. The macro-structures that learners constructed and the number of propositions recalled from a specific section of the text during a pre-test were compared to the macrostructures they formulated and the number of propositions that they recalled during a post-test. The courses were presented in Afrikaans to four of the five grade 10 classes in a Western Cape school. (The course was also presented in English to the fifth class but that course was not taken into consideration.) The courses were conducted over eight periods of approximately 40 minutes. The content of the first three periods were the same for all four courses. During the first period the learners did a reading exercise in order to determine their reading speed and reading index. During the second and third periods they were taught skimming and study skills respectively. The first course focused on questioning skills in the last five sessions. The second course was presented in order to improve learners' structuring skills. The third course was a combination of the first two courses. A conventional speed reading course was presented to learners who did the fourth course. Multiple regression analyses were conducted to determine whether (1) the construction of macro-structures and (2) the recall of propositions from the texts could be ascribed to questioning skills, structuring skills or the combination of those skills. This was done after variance in the dependant variables brought about by differences in intelligence was controlled statistically. The results of this study indicate a statistically significant improvement to construct macro-structures for the learners who completed the courses aimed at improving structuring and questioning/structuring abilities. This improvement can be ascribed to the skills that they acquired during the courses. Traditional schema theories regard schemas or schemata as pre-fabricated structures that need only to be activated by readers. In these courses, however the emphasis was placed on the formation of a structure by the reader; on the activity performed by the reader. Hence it was referred to as a structuring course. During the courses the learners were encouraged not only to "chunk" the content of the texts, but to generalize and to construct the structure of the texts. They were encouraged to use their own words or categories. Several studies indicated that an improvement in the ability to structure texts lead to an improvement in text comprehension. There are five other reasons why structuring the content of texts will facilitate the construction of macro-structures. The first is that when subjects are given the task to remember unorganized material they structure the material to be remembered spontaneously. The second is that the semantic organization of material facilitate recall and the third that normal perception is highly structured. The fourth argument is that experts make effective use of schemes. The fifth argument is that the activity of structuring forces the reader to process the text at a deeper level of processmg. Since it was assumed that top achievers are good readers who have mastered the ability to construct macro-structures efficiently, it was argued that they would benefit more from a course that aimed at improving their question-generating skills. Thus, the fact that there was not a statistically significant improvement in their ability to construct macrostructures can be attributed to the fact that they have already mastered that ability sufficiently. It is also possible to attribute the fact that there was not a statistically significant improvement to the fact that they did not master the question-generating activities. The results of this study also indicated that learners who did the structuring, questioning and questioning/structuring courses improved their ability to recall propositions from the texts in a pre-test to a post-test. Their improved ability to recall propositions, can be ascribed to the fact that detail can be inferred from a hierarchical structure since detail facts are subsumed under thematic propositions. Put differently, micro-propositions can be inferred from macro-propositions. The improvement in the ability of subjects who did the questioning course can be ascribed to the fact that questioning helps to focus attention and that the activity of questioning leads to deeper levels of processing. Three assumptions concerning macro-structures were made in this study. It was assumed that the construction of macro-structures is an automatic and integral part of the (normal) reading process. Readers do not construct macro-structures only when they are required to do so. Good readers construct better macro-structures than poor readers.