Möbius and Hückel Cyclacenes with Dewar and Ladenburg Defects.

Cyclacene nanobelts have not been synthesized in over 60 years and remain one of the last unsynthesized building blocks of carbon nanotubes. Recent work has predicted that Hückel-cyclacenes containing Dewar benzenoid ring isomers are the most stable isomeric forms for several of the smaller sizes of cyclacene belt. Here we give a more complete picture of the isomers that are possible within these nanobelt systems by simulating embedded Ladenburg (prismane) benzenoids in Hückel-[n]cyclacenes (n = 5-14), and embedded Dewar benzenoids in twisted Möbius-[n]cyclacenes (n = 9-14). The Möbius-[9]cyclacene isomer containing one Dewar benzenoid and the Hückel-[5]cyclacene isomer containing two maximally spaced Ladenburg benzenoids are found to be more stable than their conventional Kekulé benzenoid counterparts. The isomers that contain Dewar and Ladenburg benzenoids have larger electronic singlet-triplet energy gaps and lower polyradical character when compared with the conventional isomers.