Semi-empirical molecular orbital methods and ab initio calculations in dye chemistry: Computational studies towards the design and synthesis of organic pigments

The main goal of this study was to determine the scope and limitations of some state-of-the-art methods in computational chemistry, including both molecular mechanics and semi-empirical and ab initio quantum mechanics, in the prediction of key properties of selected colorants and their intermediates. Specifically, prediction of equilibrium molecular geometry, wavelength of maximum absorption, mutagenicity via quantitative structure activity relationships (QSAR), photostability, and the photodegradation mechanism of some nonmutagenic organic pigments prepared from nonmutagenic benzidine analogs was studied. It is known that molecular geometry influences significantly electronic and thermodynamic properties of all compounds. Hence, the accurate prediction of geometries was assessed by the development of protocols using semi-empirical and ab initio methods, for use in comparisons with X-ray crystallographic data. In this regard, single crystals of ten compounds were grown and the associated X-ray structures solved.; The effectiveness of each model and protocol was tested by comparison of predicted bond angles, bond lengths, intramolecular hydrogen bond distances, and torsion angles to X-ray data. Electronic properties such as wavelength of maximum absorption were calculated using PPP, ab initio and ZINDO and compared with experimental lambdamax data.; In the case of semi-empirical calculations, different methods (AM1, PM3, CNDO, INDO, ZINDO, PPP) were employed. With regard to geometry optimization a combination of manual adjustments followed by MM2/PM3 calculations was superior to MM2/AM1 in the prediction of the equilibrium geometry of compounds 149--152. In addition, better results were obtained using PM3 versus AM1 when an optimized energy map was employed for these compounds. While PM3 was effective in predicting the equilibrium geometry of compounds 149--152, AM1 was superior to PM3 in predicting the equilibrium geometry of pigments 153--155. As to the prediction of electronic excitation energy, ZINDO was effective in predicting the wavelength of maximum absorption of pigments 153--155 and the equilibrium geometry and spin state of compounds 174a-b.*; In the case of ab initio calculations, local and non-local DFT methods were used to predict the equilibrium geometry of compounds 149--152, pigments 153--155 and C.I. Pigment Yellow 12. (Abstract shortened by UMI.); *Please refer to dissertation for diagrams.