Computational Studies\nof Dimerization of [<i>n</i>]‑Cyclacenes

Cyclacenes, C<sub>4<i>n</i></sub>H<sub>2<i>n</i></sub>, consist of <i>n</i> linearly fused benzene\nrings\nthat are arranged to result in a closed nanohoop structure. Cyclacenes\nare thus the cyclic versions of acenes and have so far escaped synthesis.\nIn order to estimate the tendency of [<i>n</i>]-cyclacenes\n(6 ≤ <i>n</i> ≤ 20) to undergo dimerization,\nwhich is assumed to be a major pathway of degradation under oxygen-free\nconditions, we here report the energy of dimerization as computed\nby density functional theory using spin-restricted, spin-unrestricted,\nand thermally assisted-occupation (TAO) formalisms. It is found that\nthe energy of dimerization increases with increasing size of <i>n</i> but that this increase is not monotonic for the smaller\nmembers of the series. This is due to the combination of the cryptoannulenic\neffect and the inherent strain of the cyclacenes. The energy of dimerization\nof the largest member inspected, [20]-cyclacene, is −59.3 kcal/mol,\nwhile we expect it to converge to −46 kcal/mol for <i>n</i> → ∞ based on comparison with data obtained\nfor acenes.