Global and Local Aromaticity of Linear and Angular Polyacenes

Ab initio (B3LYP/6-311G∗∗) optimisation of seven linear and five angular polyacenes gave us an opportunity to study aromaticity in terms of HOMA index and its components EN and GEO, the energy of the carbon skeleton of these hydrocarbons was calculated from CC bond lengths. Independently, the Cohen–Benson group additivity values were used to estimate the thermochemical aromatic stabilisation energy (ASE). For individual rings the NICS values were computed and compared with HOMA. An increase of size of both linear and angular polyacenes is associated with a substantial decrease in their aromaticity, with a greater decrease for the linear polyacenes. This is well documented by both HOMA and Cohen–Benson ASE.