Masters Degrees (Chemistry)

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Browsing Masters Degrees (Chemistry) by Author "Botha, Ebrahiem"

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    Quantum computational, structural and electrochemical properties of substituted dithizones and photochromic dithizonato phenylmercury complexes
    (University of the Free State, 2010) Botha, Ebrahiem; Von Eschwege, Karel Grobler; Conradie, Jeanet
    English: For the purpose of investigating the influence on the photochromic reaction of its phenylmercury complexes, a series of dithizone derivatives were at first synthesized. This includes symmetrical dithizone derivatives with electron-donating ability; ortho, meta & para-methyl and methoxy, as well as the 3,4-dimethyl derivative, including some of its disulphide and dehydrodithizone oxidation products. The synthesis of extended aromatic ring dithizone derivatives was attempted, i.e. instead of a phenyl ring the dithizone was extended to have napthyl, fluorenyl, anthracenyl and pyrenyl functional groups. Synthetic procedures were optimized, while conveniently utilizing ammoniumsulphide instead of NH3(g) and H2S(g) as reagent. Phenylmercury(II) complexes were prepared from the successfully synthesized ortho and meta-methoxy, and ortho, meta and para-methyl, and napthyldithizone derivatives. All except the ortho-methyl and naphthyl dithizonates were observed to be photochromic. The ortho-methoxy dithizonate, for the first time, exhibited an alternative color reaction; instead of the usual orange to blue, this compound changed from pink to purple. From the corresponding substituted dithizone the disulphide of ortho-fluorodithizone was synthesized through auto-oxidation, chemical oxidation and electrochemical oxidation. Successful growth of a sizeable crystal of the latter compound ended in solving, for the first time, the X-ray crystal structure of a dithizone disulphide, giving conclusive evidence in support of the long standing proposal of an unstable disulphide intermediate in the oxidation reaction of dithizone. Solving also the X-ray crystal structure of ortho-methoxydehydrodithizone gave indirect structural evidence in support of the foregoing synthesis product. A comprehensive ADF DFT quantum computational study included the successfully and unsuccessfully prepared dithizones and dithizonatophenylmercury complexes. Results unambiguously showed the symmetric form of dithizone and its analogues to be the most stable. TDDFT calculations simulate the trends seen in experimental UV/visible spectra. Graphical representations show molecular orbitals (HOMO & LUMO) extending over the entire length of the ligand, confirming the molecule’s structural importance in the photochromic reaction. Extended dithizonates pose a steric hindrance for photochromism to take place. Electrochemical studies were performed on the above successfully synthesized ligands and complexes, as well as some fluoro-substituted dithizones, exploring the electronic borders within which photochromism ultimately takes place. Complete redox schemes are proposed for the observable redox waves in the cyclic voltammograms of all species involved. An approximate linear relation between experimental oxidation data of the dithizonates and the calculated HOMO energies was found (R2 = 0.89).