Inverted Hyperconjugation in Symmetrical 1,4-Dihalocubanes

Article abstract of DOI:10.1002/hlca.19850680104

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 e_g^-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.