A study of the transferability of one and two-centre parameter values in the application of Pariser-Parr-Pople type methods to conjugated carbonyl compounds

The energies and intensities of the n→π* and π→π* transitions of eight conjugated carbonyl compounds, ranging from formaldehyde to 9:10-anthraquinone, have been calculated by the Pariser-Parr-Pople method using a set of parameter values which has been used to calculate the n and π-electron ionization potentials of the same molecules. The results for the π→π* transitions generally agree well with experiment although the agreement for the larger molecules is better than that for the smaller. Reasons for this are suggested. Calculated singlet-triplet splittings for the n→π* transitions agree well with experiment but the calculated absolute energies of the n→1 π* transitions are uniformly 1–2 ev too small. This difference can be removed by giving the lone-pair orbital a new energy basis, and the origin of the discrepancy appears to lie in the approximations of Koopmans's theorem.