Dissociation of the phenanthroimidazole dimer - A highly delocalized radical - Gomberg revisited

Eric Kiepek, Yuhui Zhou, Shmaryahu Hoz, Ester Rozental, Peter M. Kazmaier, Erwin Buncel

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Results of a calculational study at the B3LYP/6-3/G* level concerning the dimerization of the phenanthroimidazole radical are reported herein. The optimized minimum energies of six dimer structures corresponding to C-C, C-N, and N-N bonding have been obtained. The lowest energy dimer is found to be that resulting from N-N bonding (12). However, this does not correspond to an X-ray structure study of the experimentally isolated dimer (13), which reveals C-N bonding, as is also deduced from an NMR investigation. The variance between the experimental structure and the calculational lowest energy one is examined on the basis of steric interactions: virtually retained planarity, and hence, full conjugation in the two phenanthroimidazole halves in 12, vs. twisting and lack of coplanarity in the other dimer structures. Importantly, spin density calculations reveal a negligible spin density on the nitrogens, rendering dimer 12 formation through N-N bonding highly improbable. A bond dissociation energy of 16.5 kcal/mol (1 cal = 4.184 J) is calculated for the experimental dimer 13, which is by far the lowest C-N bond energy recorded so far in the literature. As well, this value is of the same order of magnitude as the calculated C-C BDE in hexaphenylethane, which had been postulated by Gomberg in 1900 as the molecule formed on dimerization of the triphenymethyl radical, but so far not observed experimentally.

Original languageEnglish
Pages (from-to)1448-1459
Number of pages12
JournalCanadian Journal of Chemistry
Issue number9
StatePublished - Sep 2005


  • C-N bond dissociation energy
  • Delocalized radical dimerization
  • Gomberg's dimer
  • Phenanthroimidazole dimer
  • X-ray structure


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