AbstractThe ionization energies JJ, of 1,6;8,13āalkanediylideneā[14]annulenes (2 to 5) and of dicyclohepta[cd,gh]pentalene (1) have been determined by photoelectron spectroscopy, using HeI radiation. The data are interpreted in terms of Koopmans' theorem (JJ = āĪµJ) on the basis of correlation diagrams and with the help of simple molecular orbital models.If the bridge is an ethaneā, propaneā or butaneādiylidene group, the Ļāorbital sequence, in descending order of orbital energies, is (in C2v): b1, b2, a2, a1. The sequence is due to a complicated and not uniquely definable interplay of inductive, conjugative and homoconjugative effects. A detailed analysis of these effects suggests that the effective angle of twist between two consecutive basisāAOs 2pĪ¼, 2pĪ½ of the peripheral Ļāsystem should be smaller than the twist angles ĪøĪ¼Ī½ determined by Xāray analysis, i.e. that the pi;āribbon adjusts elastically and is no longer locally orthogonal to the Ļāframe.In the nonāalternant hydrocarbon 1 of symmetry D2h, the sequence is 2b2g, 3b1u, 2b3g, 1au, 2b1u. The sequence 3b1u above 2b3g, i.e. the reverse of b2 above a1 in the bridged [14]annulenes, is explained as being due to the interaction of the semilocalized perimeter orbitals b1u and b3g with the bonding (Ļ(B1u))and antibonding (Ļ*(B3g)) orbital of the central double bond. In 2 the replacement of the two latter orbitals by the Walshāorbitals of the cyclopropane moiety leads to the sequence b1, b2, a1, a2.From the data observed for 1 to 5 and for 1,6āmethanoā[10]annulene [11], a crude estimate for the orbital energies of the hypothetical allācis D10hā[10]ā and D14hā[14]annulenes can be derived.