π * negative ion resonance states of (E)- and (Z)-1,3,5-hexatriene. Prediction of electron affinities near threshold

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Deviations from the pairing theorem previously noted for the linear polyenes are explained on the basis of 1,3-π,π interactions in the negative ion states. Such interactions also contribute significantly to the stabilization of the π * −2 negative ion state of (Z)-relative to (E)-1,3,5-hexatriene. Koopmans' theorem (KT) does not explain the equal π * −3 electron affinities of (E)- and (Z)-hexatriene but CI-singles calculations closely approximate the relative experimental values. Scaled KT and Δε SCF calculations lead to the predictions that (a) the crossover between unbound and bound first π * negative ion states in the linear polyenes occurs between hexatriene and octatetraene, (b) the first π * negative ion states of fulvene and bicyclo[1.1.0]but-1(3)-ene are bound and (c) that of cyclobutadiene is unbound.