Crystal structures and photoelectron spectra of some trimethanoanthracenes, tetramethanonaphthacenes, and pentamethanopentacenes. Experimental evidence for laticyclic hyperconjugation
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Photoelectron (p.e.) spectra of the series of dienes (), (), ()-(), and crystal structures for the dodecachlorodienes()-() are reported. The spectra revealed large [small pi]-splitting energies of 0.32 and 0.52 eV for () and () respectively. The value of () is attributed to the presence of orbital interactions through six [sigma] bonds (OIT-6-B), and this, taken with the p.e. data for the dienes ()-(), provides convincing evidence for the long-range nature of OIT--B. The larger [small pi]-splitting energy of 0.52 eV observed for (), compared with (), has been explained in terms of the presence of two reinforcing orbital interactions: OIT-6-B and a new variant of hyperconjugation, called laticyclic hyperconjugation, in which the [small pi] MOs of the double bonds mix with the [small pi] MO of the central CH bridge of (). The surprisingly large [small pi]-splitting energy of 0.29 eV observed for () is also due largely to laticyclic hyperconjugation involving both pairs of symmetry adapted [small pi] MOs of the CH bridges. No [small pi]-splitting energy could be detected in the p.e. spectrum of (). From the crystal structures of ()-(), it was found that the distance between neighbouring CH groups, and between a double bond and its closest CH neighbour is about 3 A in all three compounds. HF/STO-3G calculations on model ethene (CH) ethene complexes suggest that laticyclic interactions, like OIT--B, are very long-range processes, the predicted [small pi]-splitting energies amounting to 10 eV for two double bonds separated by 33 A. The relevance of these orbital interactions to biological electron-transfer processes is briefly discussed.
|Journal||J. Chem. Soc., Perkin Trans. 2|
|Number of pages||16|
|Publication status||Published - 1987|