Charge delocalization mode and change in aromaticity of protonated 7‐phenylbenzo[k]fluoranthenes studied by experimental observation and DFT calculations

Abstract

Abstract7‐Phenylbenzo[k]fluoranthene (4), which is one of non‐alternant polycyclic aromatic hydrocarbons, was protonated at the C(12) position in neat CF3SO3H to give a persistent carbocation (4aH+). The cation was successfully observed by NMR measurements at rt. The most deshielded protons were observed as doublet signals at 8.92 and 8.71 ppm for H(1) and H(3). The most deshielded 13C signals appeared at 184.4 and 176.6 ppm for C(12a) and C(7). The positive charge was found to be delocalized to four carbons in a 1‐naphthalenium unit and to two other carbons according to Δδ13C from 4 to 4aH+. DFT calculations indicated that 4aH+ was the most stable among possible protonation carbocations. The NICS(1)zz values of 4aH+ suggested that the five‐membered π‐ring is nonaromatic and that the other benzenoid rings are aromatic. The influence of the 7‐phenyl group on aromaticity and on positive charge delocalization was limited. The next most stable cations were computed to be cations (4bH+ and 4cH+) that were generated by the protonation at the C(3) and C(4) positions, respectively. Their Gibbs energies were similar within 0.7 kcal/mol to that of 4aH+. Their five‐membered π‐rings were proved to be anti‐aromatic according to the NICS(1)zz criteria. 4bH+ and 4cH+ can be depicted by canonical structures with a cyclopentadienyl cation unit, which is suggested to be responsible for their anti‐aromaticity.