DFT study of cycloparaphenylenes and heteroatom-substituted nanohoops.

Nanohoops are macrocycles formed of aromatic rings linked in a 1,4' fashion. Cycloparaphenylenes 1 and nitrogen analogues formed from the building blocks pyridinyl (2), pyrazinyl (3), pyridazinyl (4), and pyrimidinyl (5) are examined at B3LYP/6-31G(d). The nanohoops contain 3-24 repeat units. The strain energy of the nanohoops exponentially decreases with the number of building blocks n, and this strain strongly correlates with the bend angle at the ipso carbons. Nitrogen substitution reduces the o,o' steric interactions between neighboring rings. Nanohoops 3 and 5 have ribbon-like structure with dihedral angles between neighboring rings near zero. Nanohoops 5 are the least strained and, with their ribbon structure, are suggested as synthetic targets for possible interesting bulk properties and structures.