The ionization energies of bridged [14]annulenes and of dicyclohepta[cd, gh] pentalene

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Abstract
The ionization energies JJ, of 1,6;8,13‐alkanediylidene‐[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 = −εJ) on the basis of correlation diagrams and with the help of simple molecular orbital models.If the bridge is an ethane‐, propane‐ or butane‐diylidene group, the π‐orbital 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 basis‐AOs 2pμ, 2pν of the peripheral π‐system should be smaller than the twist angles θμν determined by X‐ray analysis, i.e. that the pi;‐ribbon adjusts elastically and is no longer locally orthogonal to the σ‐frame.In the non‐alternant 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 (π(B1u))and antibonding (π*(B3g)) orbital of the central double bond. In 2 the replacement of the two latter orbitals by the Walsh‐orbitals of the cyclopropane moiety leads to the sequence b1, b2, a1, a2.From the data observed for 1 to 5 and for 1,6‐methano‐[10]annulene [11], a crude estimate for the orbital energies of the hypothetical all‐cis D10h‐[10]‐ and D14h‐[14]annulenes can be derived.