Using density functional theory we have studied the structural properties of eleven C54 isomers that appear in the C60 fragmentation. We have evaluated the relative stability of the different isomers with respect the most stable one, which corresponds to the structure with the minimum number (four) of adjacent pentagons. On average, the length of a bond shared by pentagons and/or hexagons increases in the order hexagon-hexagon, hexagon-pentagon and pentagon-pentagon. However, we have found that the central bond in the confluence of four hexagons, i.e. a pyrene substructure, is anomalously large, becoming in some cases the largest one. We have also evaluated the nucleus-independent chemical shifts (NICS) at the center of every individual ring in the most stable isomers. For the chlorine derivatives, our calculations show that the favorite position for chlorine addition are the bonds shared by pentagons.