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

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.

On Excited States of Alkyl Bridged [14] Annulenes

“…First, we measured the linear dichroic absorption spectrum of the 1,3,6,8-tetramethyl derivative of 1 in stretched polyethylene at 77 K. Second, we performed CNDO-CI calculations on 1-5 using a modified version of the computer program published by Baumann [ 111 with inclusion of singly and doubly excited configurations in the CI procedure. It will be shown that this model performs reasonably satisfactorily for these twisted rc-systems, somewhat in contrast to expectations expressed previously [6] PI. The results are in general agreement with the identification of L and B bands suggested [2] [7], but are at variance with the tentative assignment of the additional transitions [7].…”

“…In the calculations on the bridged [ 14lannul-enes we used geometries derived from experimental X-ray data for 1 [26], 4 [27], and 5 [28], while the results for 2 and 3 were based on geometries calculated by Lindner's combined force field and n-SCF-LCAO-MO method [23] which yields results in close agreement with experiment [29] [30]. The results displayed in Figure 3 indicate that the calculated orbital energies are in excellent agreement with the shifts and relative spacing of the lowest few ionization energies as measured by photoelectron spectroscopy [5] [6]. The only discrepancy concerns the third level of 1 (a, , ) which is predicted at relatively high Estimated by the relation log c = 4.74+ log f [ 111 Observed band origins from [20].…”

“…The only discrepancy concerns the third level of 1 (a, , ) which is predicted at relatively high Table 1. Observed and calculated transition energies E (ev), polarization directions, and log E values for pleiudrrie 6…”

ChemInform Abstract: EXTENDED HUECKEL MOLECULAR ORBITAL CALCULATIONS FOR THE BRIDGED ANNULENES

“…Photoelectron spectroscopy studies [40] of the orbital sequences of some annulenic compounds were consistent with the following order of planarity: 16 > 181) > 12 > 13. If 17 and 18 are considered Lu be similar, and representative of the properties of 15, the sequences are in agreement with the one we have given above.…”

Excited states of bridged [14]annulenes with anthracene perimeter: absorption, polarized emission, linear dichroism, and magnetic circular dichroism

“…This is understandable in view of the above remarks. The similarity of the MCD spectra of hydrocarbons I-IV (Figures [7][8][9][10] reflects the homogeneity of this series of [14]annulenes. On the other hand, the signs of many of the…”