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Through space hyperconjugation in half-cage alcohols

Authors
Journal
Journal of Molecular Structure THEOCHEM
0166-1280
Publisher
Elsevier
Publication Date
Volume
677
Identifiers
DOI: 10.1016/j.theochem.2004.01.044
Keywords
  • Hyperconjugation
  • Alcohols
  • Bond Length
Disciplines
  • Chemistry
  • Mathematics

Abstract

Abstract Previous theoretical studies indicate that hyperconjugation and through-space interactions of a steric or electronic nature are mainly responsible for conformational effects in half-cage systems, influencing bond lengths, bond angles and dihedral angles as well as charge distributions. As chemical shieldings are likewise affected, it is of much interest to evaluate their contributions in order to use them for structure elucidation purposes (calculating chemical shifts for individual rotamers has the advantage of detecting effects that would otherwise appear as averages in observed spectra). Half-cage systems provide interesting models due to their rigid molecular framework. On rotation of the single C–O bond of half-cage alcohol derivatives, different groups are forced into close proximity. In our investigation of conformational effects on chemical shifts of “inside” and “outside” half-cage alcohols we found that certain positions on the inside (2) isomer did not reproduce effects observed for the outside (1) isomer. To probe interactions between lone pairs on oxygen and the C9–H9s bond, we used DFT B3LYP/6-31g(d) methodology to optimize geometries and to calculate the second order interaction energy and chemical shifts at 30° intervals. We found that not only is strong hyperconjugation through space present but, besides the oxygen p orbital, there is also a considerable contribution from the oxygen sp orbital. Although this result is not unexpected given the spatial proximity of the orbitals that are involved, to our knowledge the literature on hyperconjugation has, so far, been limited to participation by p orbitals on oxygen.

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