resulting in better conjugation with the core π system. Unlike in 2, co-planarity is possible in 9 due to the absence of substituents on the other half of the pentalenide, showing electronic preference for full conjugation where sterically possible. Experimentally this change in conjugation due to different dihedral angles between the pentalenide core and the aryl substituents could also be seen in the NMR shifts of the quaternary C 1 and C 3 atoms: whereas in 2 they resonated at 109.5 ppm (30.2° twist angle), in 9 they shifted upfield to 103.5 ppm (Table ). The ACID plot of 12 (Figure ) showed a relatively uniform aromatic ring current around the pentalenide perimeter similar to that of 2 (cf. Figure ). The plot of the NICS Z-scans of 12 also showed little difference in aromaticity between the two subunits, while the X scan indicated a slightly stronger aromatic circuit in the Me,Ph-substituted ring (-12.8 ppm) as shown by lower NICS shifts than in the Ph2-substituted ring (-10.9 ppm; Figure and Table ). All three phenyl angles in 12 fell in the range of 30-34°, showing a methyl group to cause the same level of partially interrupted conjugation as in the tetra-aryl pentalenides 2 and 3. In order to understand the degree of charge localisation and substituent effects in these pentalenides, NBO (natural bond orbital) calculations of 1, 2, 3, 9, 10, and 12 were carried out. For this analysis, we separated the pentalenide core into two five-membered subunits with individual substituents (Cp