The first application of computational chemistry presented in this thesis is the
prediction of electronic wavelength maxima, with particular emphasis on the
cytological dyes used in the Papanicolaou cervical smear test. Initial studies of the staining mechanism of the al chemic Papanicolaou ingredients are also presented.
Wavelength maxima were initially calculated using semi-empirical HOMO-LUMO
gaps for a training set of several hundred unsaturated organic compounds. Semiempirical electronic wavelength maxima were substantially underestimated, with increasing deviations at longer wavelengths. The overestimation of the singlet excitation energy can be explained by the formation of a hole in the HOMO upon electronic excitation, causing the singlet excitation energy to be smaller than the HOMO-LUMO gap by an amount which approximates to JH,L - 2KH,L- Additionally, correlations were found between the calculated and experimental wavelength maxima.
These correlations were interpreted in terms of Dahne's triad theory, allowing the electronic properties of any unsaturated organic compound to be characterised by varying degrees of polymethinic, aromatic and polyenic character. Wavelength maxima could be predicted to within 25 nm upon application of specific, character dependent correction factors.
More realistic excitation energies allowing the prediction of J and K, could be
obtained by from post Hartree-Fock methods such as CIS configuration interaction
approaches, and by explicit evaluation of fl.Mfr ( excited state vs ground state) values.
Triad theory could again be used to predict wavelength maxima. Qualitative trends in absorption intensities could only be obtained from oscillator strength for certain unsaturated compounds.
First-principles density functional theory calculations employing the local density
approximation were also performed for a training set of several hundred compounds. However, in complete contrast to the above semi-empirical results, singlet excitation energies obtained from DFT-LDA HOMO-LUMO gaps were underestimated, leading to larger values of wavelength maxima. Polar molecules, such as benzenesulphonic acid, greatly deviate from the correlations, especially for poor basis set quality, due to the failure of the LDA approximation.
The second application concerns the calculation of semi-empirical transition
structures and activation energies for various pericyclic reactions. The studies focus on the diimide reductions, ene-dimerisations and Diels-Alder reactions of
cyclopropenes. Due to a lack of experimental data, quantitative agreement with
experiment cannot yet be determined for most compounds.