2,2′-Bipyridine (BPy), one of the most widely used ligands in coordination chemistry, exists naturally in the s-trans conformation but must preorganize to the s-cis conformer in order to form chelating complexes. Lower stability of the s-cis conformer was mainly attributed to steric 3,3′-hydrogen clashes and nitrogen lone pair-lone pair interactions, but recent trends in the literature suggest that these clashes might be bonding interactions in similar molecules. These close contacts are also present in metal complexes with BPy and are often used as “steric repulsions” in order to explain trends in formation constants. In the present work we investigate the CH•••HC interaction in the free ligand as well as in ZnII(BPy)n(OH2)6-2n and NiII(BPy)n(OH2)6-2n complexes. We use multiple distinct advances in theoretical chemistry in order to arrive at a consistent and coherent model describing these interactions. The Quantum Theory of Atoms in Molecules (QTAIM) reveals the presence of an atomic interaction line (a bond path) for the CH•••HC interaction. Using the Interacting Quantum Atoms (IQA) energy decomposition scheme we show that the CH•••HC interaction is attractive and quantum mechanical in nature. The Extended Transition State coupled with Natural Orbitals for Chemical Valence (ETS-NOCV) energy decomposition scheme show favorable orbital mixing, and Non-Covalent Interaction (NCI) analysis reveals that no steric (Pauli) strain exists in the valence (overlap) regions of the interaction - electron density is concentrated rather than depleted in the bonding region. We also studied various other interactions, ranging from purely repulsive (N--N interaction in the s-cis conformer of BPy), purely electrostatic (CH•••N interaction in s-trans conformer of BPy), H-bonding (CH–N and CH–O bonds in complexes) to coordination bonds and covalent bonds. Using a comparative approach, we show the similarities and differences among the interactions, and conclude that the CH•••HC interaction cannot be classified as a “steric repulsion” - the interaction is similar in properties to every studied known bonding interaction and opposite in nature to the studied known repulsions. Finally, we suggest novel interpretations and understanding of the nature of intramolecular interactions and the field of theoretical chemistry, as well as representing the first work to combine and corroborate QTAIM, IQA, NCI and ETS-NOCV findings. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Chemistry / unrestricted

Целью выпускной квалификационной работы является разработка потенциометрического подхода для определения антиоксидантной емкости с использованием комплексов железа с 2,2’-бипиридином и его производными. В работе были проведены исследования электрохимического поведения комплексов железа с 2,2’-бипиридином и его производными методом циклической вольтамперометрии. Проведен расчет условных констант устойчивости выбранных комплексных соединений. Была исследована антиоксидантная емкость ряда фенольных антиоксидантов, обладающих различной растворимостью, новым разработанным потенциометрическим подходом с использованием в качестве модели окислителя бипиридиновых комплексов. Проведены исследования на многокомпонентных объектах – экстрактов травянистых растений. Модифицированным спектрофотометрическим методом было доказано стехиометрическое протекание реакций комплексов хлорида железа (III) c 2,2’-бипиридином и его синтетическими производными с модельными антиоксидантами. Были выявлены перспективные комплексные соединения с производными бипиридина для дальнейшего исследования антиоксидантных свойств потенциометрическим способом. / The aim of the final qualifying work is to develop a potentiometric approach for determining the antioxidant capacity using iron complexes with 2,2’-bipyridine and its derivatives. The electrochemical behavior of iron complexes with 2,2’-bipyridine and its derivatives was studied by cyclic voltammetry. The conditional stability constants of the selected complex compounds are calculated. The antioxidant capacity of a number of phenolic antioxidants with different solubility was investigated by a newly developed potentiometric approach using bipyridine complexes as an oxidizer model. Studies have been carried out on multicomponent objects – extracts of herbaceous plants. The modified spectrophotometric method proved the stoichiometric course of reactions of iron (III) chloride complexes with 2,2’-bipyridine and its synthetic derivatives with model antioxidants. Promising complex compounds with bipyridine derivatives have been identified for further investigation of antioxidant properties by a potentiometric method.

Electron transfer, ET, and proton-coupled electron transfer, PCET, reactions are central to biological reactions involving catalysis, energy conversion and energy storage. The movement of electrons and protons in either a sequential or concerted manner are coupled in a series of elementary reaction steps in respiration and photosynthesis to harvest and convert energy consumed in foodstuffs or by absorption of light into high energy chemi-cal bonds in the form of ATP. These electron transfer processes may be modulated by conformational dynamics within the protein matrix or at the protein-protein interface, the energetics of which are still not well understood. Photoacoustic calorimetry is an estab-lished method of obtaining time-resolved reaction enthalpy and volume changes on the nanosecond to microsecond timescale. Photoacoustic calorimetry is used here to probe 1) the energetics and volume changes for ET between the self-assembled anionic uroporphy-rin:cytochrome c complex and the role of the observed volume changes in modulating ET within the complex, 2) the enthalpy and volume change for the excited state PCET reac-tion of a tyramine functionalized ruthenium(II) bis-(2,2'-bipyridine)(4-carboxy-4'-methyl-2,2'-bipyrine) meant to be a model for the tyrosine PCET chemistry carried out by cyto-chrome c oxidase and photosystem II, 3) the enthalpy and volume changes related to car-bon monoxide and tryptophan migration in heme tryptophan catabolic enzyme indoleam-ine 2,3-dioxygenase.