Abstract
The transferability of the one and two centre parameter values used in the Pariser-Parr-Pople method (P-method) has been investigated both for a series of conjugated carbonyl compounds and for the correlation of calculation with different types of experimental data. Electronic excitation energies and ionization potentials involving the pi orbital and lone pair orbital electrons of these compounds have been calculated by the P-method. The same parameter values have been used in spin density calculations of the anion radicals, the spin densities being correlated with the experimental electron spin resonance coupling constants, The method of calculating spin densities involved an unrestricted Hartree-Foch ground state function with annihilation of the major contaminating spin multiplet. The electronic transition energies have been calculated for some of these carbonyl compounds by the molecules-in-molecules method (MIM method). A feature of the P-method and MIM method calculations has been the inclusion of a one centre exchange repulsion term between the lone pair and the 2p orbitals on the oxygen atom of the carbonyl group. This term gives a correction to the pi molecular orbital and the lone pair orbital energies and also gives rise to the small n →pi singlet-triplet splitting observed experimentally. Hydrogen-bonding in hydroxy-quinones has been investigated by correlation between the following. (1) The energy of the lowest unoccupied molecular orbital calculated by the Huckel method. (2) The polarographic half-wave reduction potential. (3) The energy of the charge transfer band using a common donor. Both the experimental observables (3) and (2) were measured in aprotic solvents. An electron spin resonance study of the anion radicals of quinones and hydroxy-quinones in an aprotic solvent was attempted, the formation of the radicals being tried by both chemical and electrochemical means.