Abstract
The unrestricted Hartree-Fock method has been applied to the calculation of the π-electron spin densities in a number of conjugated carbonyl radicals and anion radicals, the parameters being those previously used in the interpretation of the ionization potentials and electronic spectra of the corresponding closed shell molecules. The results have been compared with measured E.S.R. proton hyperfine coupling constants and, in general, there is a disappointing agreement between theory and experiment. Some reasons for this are suggested. An assessment is made of the transferability of parameter values in Pariser-Parr-Pople type calculations of different physical properties.