An investigation has been undertaken to study the large closed ladderanes C24H24 and their analogs. Thirteen isoelectronic species have been identified as local minima on the MP2(FU)/6-31G(d) potential energy surface, including three C24H24, four Si24H24, two N-24. and four C24-xH24-xNx (x = 6, 8, 12) isomers. Of these 13 species, 11 are reported for the first time. Their structures, stabilities, HOMO-LUMO gaps, and G3(MP2) heats of formations are computationally obtained and discussed. These results are also compared with those of the related species already available in the literature. Our results show that molecules with 12-membered rings are highly energetic and much less stable than their corresponding isomers that do not have such rings. Isomer II of Si24H24 with anticonformations has a very small HOMO-LUMO gap of 2.56 eV, approaching the semiconductor range. Therefore, it is a candidate of potential semiconductor materials. Meanwhile, the N-24 and other nitrogen-containing species are candidates for high-energy density materials. Our results also indicate that C18H18N6 and C16H16N8 may be good hexa- and octadentate ligands for metal cations.