Thesis (Ph.D.) - Cairo University - Faculty of Science - Department of Physical Chemistry

The optimized structures of the thione, thiol1, thiol2, thionenol and thiol1enol tautomers and the corresponding thione-thiol and thione-enol transition states of the isomer of the parent molecule (1H) had been calculated. It was found that the calculated geometry of the thione form is in good agreement with the crystal structure data suggesting the predominance of the thione tautomer. The substituent and solvent effects on the relative stabilities of the tautomeric forms of TUDHIPP (1H-9H); (R = H, Cl, F, Br, NO₂, CH₃O, NH₂ and N(CH₃)₂) investigated. The relative stabilities were found to be in the order thione {uF03E} enol{uF03E} thiol in gas phase as well as in solutions. UV-Visible absorption spectra of two series (R = H, Cl, F, NO₂, CH₃O and N(CH₃)₂) were measured in different solvents. A benchmark evaluation against experimental results on the accuracy of different DFT functionals in predicting both reliable ground state geometries and electronic absorption spectra is carried out.The benchmark of the geometric structure indicated that hybrid functionals are found to be the most satisfactory functionals and the best agreement with x-ray data is achieved by using the B3LYP functional. It was found that the expansion of the sconjugation system at position C14 of pyridine ring inside the proposed molecules, leads to small significant shift in wavelength. It was concluded that the slightly blue shifting of the maximum absorption is mainly regulated by the solvent polarizability and much less by solvent polarity

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