Electronic levels in simple conjugated systems II. Butadiene

Abstract
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 2pπ 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.

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