Helvetica Chimica Acta p. 23 - 38 (1985)
Update date:2022-08-11
Topics:
Honegger, Evi
Heilbronner, Edgar
Urbanek, Thomas
Martin, Hans-Dieter
The ?-orbital manifold of cubane 1, as suggested by its PE spectrum, is divided into sets separated by a 3eV gap extending from ca. -10.5 eV to ca. -13.5 eV.Halogen substituents with np AO basis energies falling into this gap (e.g.Cl or Br) will, therefore, hyperconjugate appreciable with both sets.Interaction with the lower-lying set will lead to the usual destablization ('normal'hyperconjugation), whereas interaction with the set above will necessarily lead to a 'stabilization' ('inverted' hyperconjugation).As a result the lone-pair ionization energies of Cl or Br substituted cubanes (derived from PE spectra) are much larger than naively expected for an alkyl halide containing as much as 8 C-atoms.In particular no significant shift of the eg-1 lone-pair bands in the PE spectra of 1,4-dichloro- and 1,4-dibromocubane can be detected with respect to the first ionization energies of the free atoms Cl and Br, or of HCl and HBr.
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