Propellanes hybridization

The bond between the bridgehead atoms is inverted in the [l.l.l]propellane system 59. The hybrid orbitals are directed away from each other, hi the geminal 0-0 interaction, the greater lobes of the geminal hybrid orbitals at the bridgehead atom can be in phase with each other, while the front lobes of the hybrid orbitals of the normal and inverted bonds at the 1,3-positions are in phase with each other... 

Topological analysis of the total density has a considerable advantage over the use of the deformation densities in that it is reference-density independent. There is no need to define hybridized atoms to analyze the nature of covalent bonding, and the ambiguity when using the standard deformation density, noted above in the discussion on propellanes, does not occur. 

Answer. The orbitals of a normal C—C bond and of the central C—C bond of [l.l.l]propellane are shown in Figures B4.1 a and B4Ab, respectively. For both systems, the interacting orbitals are spn hybrids at about the same energy. The direction of polarization is dictated by the rest of the framework. In the case of the pro-pellane, the hybrid orbitals are pointed away from each other and therefore interact much more weakly, resulting in a much lower a as LUMO. This accounts for the ease of capture of an electron. The anion would have an electron in this orbital, thereby reducing the bond order and weakening the bond further. 

There are two ways in which the state of hybridization at the carbon atoms of [1.1.1]-propellane has been deduced. The results disagree dramatically (Figure 1), and at least one is clearly wrong. The hybridization at the carbon atoms can be derived from (i) the empirical correlations between the s orbital character of a carbon hybrid orbital and the, 3C- H and 13C-13C NMR coupling constants across the bonds it forms, (ii) the analysis of computed wavefunctions, using one of several possible schemes. 

Although it is not surprising that Wiberg extended to tricyclic systems (propellanes) his studies on hybridization of the bridgehead carbons in small-ring bicyclic compounds and the chemistry resulting therefrom, we shall perhaps extend somewhat the realm of the propellane substrates. This is done pragmatically (every definition is, by definition, arbitrary) in order to permit inclusion of certain piquant results. 

The balance between the controlling influence of steric effects and secondary orbital interactions in the chemistry of heterosubstituted propellanes has been analysed. In a discussion of intramolecular interactions in a variety of systems, Leonard probes the unusual hybridization of l-azabicyclo[3.3.3]undecane (1)... 

So far, considerable efforts have been made to examine thermal and acid catalysed rearrangements of propellanes. However, PET reactions and direct photolyses with high-energy photons (e.g. 185-nm excitation) have so far been neglected. It would be a challenging task for future research to fill this gap, because new information can be expected on the nature of the cleavage of the central propellane bond with its inverted carbon hybridization. 

The SET oxidation reactions of a range of tricyclic compounds and [3.3.n]propellanes with a range of oxidizing electrophiles (e.g. N02 BF4 ) have been investigated computationally at the BLYP/6-311- -G //BLYP/6-31G non-hybrid density functional level. The low ionization potentials and high proton affinities of the strained propellanes are used to rationalize the SET reactions. 

CHAiUNCt The unusual molecule [2.2.2]propellane is pictured below. On the basis of the given structural parameters, what hybridization scheme best describes the carbons marked by asterisks (Make a model to help you visualize its shape.) What types of orbitals are used in the bond between them Would you expect this bond to be stronger or weaker than an ordinary carbon-carbon single bond (which is usually 1.54 A long) ... 

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