Local aromaticity of [n]acenes, [n]phenacenes, and [n]helicenes (n = 1-9)

The local aromaticity of the six-membered rings in three series of benzenoid compounds, namely, the n]acenes, n]phenacenes, and n]helicenes for n = 1-9, has been assessed by means of three probes of local aromaticity based on structural, magnetic, and electron delocalization properties. For n]acenes our analysis shows that the more reactive inner rings are more aromatic than the outer rings. For n]phenacenes, all indicators of aromaticity show that the external rings are the most aromatic. From the external to the central ring, the local aromaticity varies in a damped alternate way. The trends for the n]helicene series are the same as those found for n]phenacenes. Despite the departure from planarity in n]helicenes, only a very slight loss of aromaticity is detected in n]helicenes as compared to the corresponding n]phenacenes. Finally, because of magnetic couplings between superimposed six-membered rings in the higher members of the n]helicenes series, we have demonstrated that the NICS indicator of aromaticity artificially increases the local aromaticity of their most external rings.