PI-ASTERISK NEGATIVE-ION RESONANCE STATES OF (E)-1,3,5-HEXATRIENE AND (Z)-1,3,5-HEXATRIENE - PREDICTION OF ELECTRON-AFFINITIES NEAR THRESHOLD

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