PROTONATION OF BENZO[A]PYRENE, DIBENZO[A,E]PYRENE AND BENZO[E] PYRENEIN SUPERACIDS - NMR-STUDIES OF CHARGE-DISTRIBUTION IN PERSISTENT ARENIUM IONS AND AM1 CALCULATIONS

Benzo[a]pyrene 1 and dibenzo[a,e]pyrene 2 are monoprotonated with FSO3 H-SO2ClF to give persistent arenium ions of protonation at C-6 (1H(+)) and C-8 (2H(+)), respectively. The low temperature protonation of ben zo[e]pyrene 3 under a variety of conditions produced a mixture of aren ium ions of attack at C-1 (3aH(+)) and C-3 (3bH(+)). The charge distri bution pattern in the resulting arenium ions (as probed by low tempera ture C-13 and 1H NMR spectroscopy) remains very much analogous to thos e of alkyl(cycloalkyl)pyrenes, showing significant phenalenium ion cha racter (predominant charge alternation at the periphery of the pyrene moiety). The arenium ion energies and charges were examined by semiemp irical theory (AM1). For 1, in agreement with experiment, protonation at C-6 is most favoured and the changes in charge distribution Delta Q [q(c)(ion) - q(c)(neutral)] indicated a very distinct phenalenium unit . Attack at other sites is less favoured, with that at C-2 being least likely. The extent of positive charge delocalisation becomes more lim ited as the arenium ion energies increase. In accord with experiment, AM1 calculations indicate that for 2, the arenium ion of protonation a t C-8 is the most stable followed by those of C-1 and C-3 which are 4. 5 and 5.1 kcal mol(-1) less stable. Charge delocalisation away from th e protonation site in these cations is predicted to be more extensive and involve a phenalenium moiety. For 3, in line with experiment, the arenium ion energies for attack at C-l and C-3 are almost identical an d most favoured. The results indicate that despite the pronounced effe ct that benzannellation has on carcinogenicity, the preferred site(s) of electrophilic attack are those which lead to the most highly deloca lised carbocations. Among these, formation of a robust phenalenium ion moiety is always favoured. Possible implications of these findings in relation to carcinogenesis models mediated by arenium ions are discus sed. The oxidation dications of 1-2 were also probed by the AM1 method .