Synthesis and PE Spectra of 1‐Ethynyl‐ and 1,4‐Diethynylbicyclo[2.2.2]octane, and of Related 1,4‐Dihalobicyclo[2.2.2]octanes

Abstract

The assignment of the PE spectra of 1-substituted and of 1,Cdisubstituted bicyclo[2.2.2]octanes or related cage-molecules can present serious difficulties, which have to be kept in mind when attempting a rationalization of such an assignment in terms of qualitative concepts. As long as the molecules consist only of first and second row atoms, the usual ab initio or semiempirical models provide a rather safe basis as we shall see by discussing in detail the PE spectra of l-ethynylbicyclo[2.2.2]octane (1) and of 1,4-diethynylbicyclo[2.2.2]octane (2). However, the problem becomes more involved when the substituents are heavy atoms, e.g., halogen atoms. Thus a recent investigation of the He(Ia) PE spectra of symmetrical 1 ,4-dihalonorbonanes2) has shown that the assignment of such spectra is anything but straightforward. Extensive mixing between the substituent and parent-molecule orbitals does no longer allow a unique classification of the observed PE bands as e.g. dominantly n; ' or D-' bands.Schrtne I

Nonconjugated Hydrocarbons as Rigid‐Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

“…The corresponding splitting of the ionization energies with the assigned MOs (DI n )r epresents the degree of interaction. Initial studies on cubane, [95] cubane derivatives, [96] and BCO derivatives [97] showedo nly very small splitting, characteristics for only av ery feeble interaction of the linked systems. Ad ifferent picture was observed for BCP derivatives.…”

Coupling between Substituents as a Function of Cage Structure: Synthesis and Valence Ionized States of Bridgehead Disubstituted Parent and Hexafluorinated Bicyclo[1.1.1]pentane Derivatives C5X6Y2

“…Next, the effects of the axially symmetric bridgehead substituents Y are considered in Figures 13 and 14, patterned after the work of Heilbronner and co‐workers16, 17 (experimental data have been used except for Y=F). Three types of lines are used in Figure 13 to indicate the full details of the orbital correlations for Y=I: dashed lines for σ‐symmetry and full lines for π‐symmetry orbitals at the bridgeheads, and dotted lines for orbitals of other types.…”

“…Nevertheless, there is a one‐to‐one correspondence between the Koopmans’ and the more accurate SAC‐CI description, such that the ionized states predicted by the latter can be well described by stating from which MO an electron is being removed. This permits the use of MO correlation diagrams of the type used so successfully for similar compounds by Heilbronner and co‐workers16, 17 instead of state correlation diagrams, thus greatly simplifying the following discussion. Figures 13 and 14 show the correlation of the important MOs associated with the ionized states through the Koopmans’ theorem.…”

“…The splitting of the π‐symmetry orbitals in 1,3‐Y 2 ‐ 1 is large: 0.9 (Cl), 0.8 (Br), and 0.6 eV (I); it is even larger in 1,3‐Y 2 ‐ 2 : 1.8 (Cl), 1.2 (Br), and 0.7 eV (I) (calculated values). For comparison, in bicyclo[2.2.2]octane substituted with bridgehead halogens,16 in which the lowest 1E′′ and 1E′ states are also due to ionizations from the halogen lone‐pair orbitals, the splitting is only 0.14 eV for Br substituents. For Cl substituents, the splitting cannot be much larger, but its exact value is unclear due to unresolved bands.…”

“…The latter is at −10.8 eV, and the amplitude is 0.43. The other, bicyclo[2.2.2]octane,16 is diagonally dominated. At the optimized staggered geometry, the symmetric e MO of the cage lies at −13.34 eV, and its amplitude at the bridgehead is 0.35, whereas the antisymmetric e orbital is located at −10.94 eV, and its amplitude at the bridgehead is nearly the same at 0.34 (HF/D95).…”