Models 1 and 2 and the corresponding extended models are isomers, permitting the calculation of their relative energies. The small models are isoenergetic: their electronic energies in the intermediate spin ground states differ by less than 1 kcal mol -1 . In contrast, the IS ground state and the less than 3 kcal mol -1 destabilized HS state of model 1 Large are stabilised by at least 15 kcal mol -1 compared to the respective states of 2 Large (see also Table for all energy data). This indicates that the arrangement of seven-membered rings as in 1 is more likely to occur in larger graphene-like sheets than 2, for which the deformation of the saddle structure would incur an energetic cost.