Carbocations (M + H)(+) and oxidation dications (M(2+)) from benzo[a]pyrene and its nonalternant isomers azulenophenalenes: a theoretical (DFT, GIAO, NICS) study.

The arenium ions of protonation and the two-electron oxidation dications derived from benzo[a]pyrene (BaP) 1 and three of its nonalternant isomers namely azuleno[5,6,7-cd]phenalene 2 (a strong carcinogen reported to be as potent as BaP) azuleno[1,2,3-cd]phenalene 3 (a strong mutagen/weak carcinogen), and azuleno[4,5,6-cd]phenalene 4 (a weak mutagen) were studied by DFT at the B3LYP/6-31G(d) level. The most favored sites for electrophilic attack were identified on the basis of relative protonation energies in the arenium ions. Computed NMR chemical shifts (GIAO NMR), the NPA-derived charges (and changes in charges), as well as NICS (and DeltaNICS) were employed to derive charge delocalization maps and to gauge relative aromaticity/antiaromaticity in the resulting carbocations and oxidation dications. Quantitative correlations between the experimental (superacid) (13)C data and GIAO chemical shifts, and between computed changes in charges and GIAO Deltadelta (13)C values were explored for benzo[a]pyrenium ion (1cH(+)) and its singlet oxidation dication (1(2+)) as representative cases. For the studied PAHs (1-4), formation of singlet dications were computed to be strongly favored except in 4 for which the triplet lies 5 kcal/mol lower than singlet. Relative carbocation stability data and the derived charge delocalization patterns are assessed in light of the available chemical and toxicological data on these compounds. The present study is the first of its kind to examine the carbocations and oxidation dications derived from biologically active nonalternant analogues of BaP for which no stable ion data are available. It also validates and extends the experimental data for BaP carbocation and oxidation dication and provides a means to gauge the success of GIAO NMR in predicting NMR data for PAH-arenium ions.