Ab initio valence bond calculations. IX. Ionization potentials of ethylene, allyl radicall, trans-, and cis-butadiene

π-electron ab initio valence bond calculations on the ground and ionized states of ethylene, allyl radical, trans-, and cis-butadiene have been performed using the 6-31G basis set after contraction. Vertical ionization potentials have been computed both using the π core of the molecule and that of the cation: the VB results and those obtained by means of the Koopmans' theorem and the SF method have been compared with the experimental data. The wave functions have been interpreted in terms of VB structures by means of a population analysis.