Electronic levels in simple conjugated systems II. Butadiene

The method of molecular orbitals, with configuration interaction, as described in part I, is applied to butadiene. The total energy, ultra-violet absorption frequency, oscillator strength and charge distribution are calculated for several singlet and triplet states in general agreement with similar more empirical work using the valence-bond approximation. It appears that without configuration interaction, no satisfactory calculation of the absolute intensities of the N → V transitions can be made. Best-possible molecular orbitals, compounded linearly of 2 pπ atomic orbitals, are obtained. Their use reduces the magnitude of configuration interaction from 2·8 to 1·7 eV in the ground state. It also gives a satisfactory measure of the energy of the first allowed transition, but not of the oscillator strength.