NMR and DFT Study on Onium Ions Derived from Substituted Fluoranthenes and Benzo[k]fluoranthenes

Abstract Fluoranthene and benzo[k]fluoranthene (3) are nonalternant polyaromatic hydrocarbons. Their derivatives, 3-acetyl, 8-acetyl, 3-nitro, and 3-aminofluoranthenes (4, 5, 7, and 8) were reacted in FSO3H/SO2ClF and the solutions were observed by NMR measurements at low temperatures, which showed the formation of PAH-substituted onium ions. The most deshielded 13C peaks in fluoranthene frames were observed at 155.7 and 148.7 ppm for 4H+, 154.4 ppm for 5H+, 159.1 and 139.6 ppm for 7H+, and 139.7 ppm for 8H+. Distribution of the positive charges were estimated on the basis of changes in 13C NMR chemical shifts between onium ions and their corresponding parent compounds. Only limited delocalization of positive charges into the aromatic rings was found to occur. GIAO-derived NMR chemical shifts calculated by the DFT method were generally consistent with the experimental chemical shifts. DFT calculations suggested that benzo[k]fluoranthene (3) is favored to be protonated at C-3/C-7 positions. GIAO-derived NICS(1)zz were computed to elucidate aromaticity/antiaromaticity, and the results suggested that the five-membered rings are antiaromatic for cations 4H+, 7H+, 8H+, and 3aH+ (3-benzo[k]fluoranthenium ion).