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Eine helicale Konformation und damit eine sterisch eher ungünstige Struktur kennzeichnet die Pentaketone 1 und 2. Aus einem einfachen Modell folgt, daß der strukturbestimmende Parameter die Minimierung des Gesamtdipolmoments ist.

The He@,) photoelectron (PE) spectra and electronic spectra of the 2,8,9-trifunctional [3.3.3]propellanes 3, 6, 8, and 9 together with the corresponding 2-mono-and &8-difunctional species have been recorded. The PE investigations reveal a small split between the x bands in the PE spectra of 2 and 3 and the n-lone pair bands in those of 7 and 9. This is rationalized by assuming a small through-space interaction among the functional groups. A considerable It-n splitting is found in the case of 5, 6, and 8. This splitting is due to a fairly strong interaction of n and n orbitals with the Q frame of the corresponding propellane. The first band of the electronic spectra of 6, 8, and 9 is assigned to n* t n transitions on the basis of CNDO/s-CI calculations. These investigations suggest a small split among the lowest unoccupied n* orbitals. Propellanes are intercbting model compounds for studying structure-reactivity relationships'). Of special interest with respect to investigations of the interaction of proximate TI and/or n-orbitals are 2,8,9-trifunctional C3.3.3lpropellanes like 3,6,8, and 9, which provide a sterically fixed framework for three proximate C = C or C = 0 double bonds. To study the interaction among the valence orbitals of these compounds we used He&) photoelectron (PE) and UV/VIS spectroscopy. Together with 3,6, 8, and 9, whose syntheses have been reported'), we also studied the 2-mono-and 2,8-difunctional propellanes 1, 2, 4, 5, and 7, which have been prepared in recent years 334). PE SpectraThe P E spectra of the 2,8,9-trifunctional C3.3.31propel-lanes 3, 6, 8, and 9 are shown in Figure 1. While the spectrum of 3 shows only one broad feature in the low energy region, two peaks are found for 9 at 9.3 and 9.8 eV. 6 und 8 fmdet man eine betriichtliche n-nAufspaltung. Diese Aufspaltung ist durch eine starke Wechselwirkung zwischen n-und n-Orbitalen mit dem a-(?eriist des entsprechenden PropeIlans bedingt. Die ersten Banden der Elektronenspektren von 6,s und 9 wurden aufgrund von CNDO/s-CI Rechnungen n*+-n-Obergtingen zugeordnet. Diese Untersuchungen sprechen f k eine kleine Aufspaltung der tiefsten unbesetzten Orbitale.ratio of the areas below the envelopes amountb to 2 : I . I n the P E spectra of the less symmetrical species 6 and 8 (Czu) three peaks at low energy can be distinguished. The recorded vertical ionization energies, I",,, of 1-9 are collected in Table 1.To interpret these data we proceed empirically by comparing the spectra of the trifunctional species with the monoand difunctional compounds. This has been carried out in Figure 2.A comparison of 1-3 shows that the interaction among the TI bonds is minute. A somewhat larger split (B = 0.15-0.2 eV) is found by comparing the mono-, bis-, and tris-carbonyl compounds 4,7, and 9. The small split in both cases is rather unexpected in view of the apparent favourable arrangement of the double bonds in 2 and 3 for homoconjugation or for lone-pair interaction in 7 and 9. As in 3 a similar small interaction has been observed in the related triviny...

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