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Results of a calculational study at the B3LYP/6-3/[G.sup.*] 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 ah 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. Key words: delocalized radical dimerization, phenanthroimidazole dimer, Gomberg's dimer, X-ray structure, C--N bond dissociation energy. Resume : On a effectue des calculs au niveau B3LYP/6-3/[G.sup.*] sur la dimerisation du radical phenanthroimidazole. On a obtenu les energies minimales optimisees de six structures dimeres correspondant aux liaisons C--C, C--N et N--N. On a trouve que le dimere de plus faible energie resulte de la liaison N--N (12). Toutefois, ce resultat de calcul ne correspond pas a l'etude de structure par diffraction des rayons X qui a ete faite sur le dimere 13 isole experimentalement et dans laquelle on observe une liaison C--N, tel qu'on peut aussi le deduire d'une etude par spectroscopie RMN. Pour essayer d'expliquer les causes de la variance entre l'energie minimale deduite sur la base de calculs theoriques et la structure observee experimentalement, on a fait appel aux interactions steriques ainsi qu'au caractere planaire maintenu et, en consequence, a la conjugaison complete des deux moities de phenanthroimidazole dans 12 par opposition a la deformation et au manque de caractere coplanaire dans les autres structures dimeres. II est aussi important de noter que les calculs de densite de spin revelent la presence d'une densite de spin negligeable sur les azotes qui rend la formation du dimere 12 par une liaison N--N tres improbable. On a evalue a 16,5 kcal/mol (1 cal = 4,184 J) l'energie de dissociation de liaison pour le dimere experimental 13 et cette valeur est de loin l'energie de liaison C--N enregistree a ce jour dans la litterature. De plus, cette valeur est du meme ordre de grandeur que celle calculee pour la liaison BDE C--C de l'hexaphenylethane qui, selon Gomberg en 1900, serait la molecule qui se formerait lors de la dimerisation du radical triphenylmethyle mais qui n'a jamais ete observee experimentalement. Mots cles : dimerisation d'un radical delocalise, dimere phenanthroimidazole, dimere de Gomberg, structure determinee par rayons X, energie de dissociation d'une liaison C--N. [Traduit par la Redaction], Introduction The dimerization of delocalized radicals has been of interest since 1900 when Gomberg concluded that dimerization of the triphenylmethyl radical 1 produced, in a straightforward manner, the anticipated hexaphenylethane [...]