Computational Studies of Dimerization of n-Cyclacenes

Cyclacenes, C4nH2n, consist of n linearly fused benzene rings that are arranged to result in a closed nanohoop structure. Cyclacenes are thus the cyclic versions of acenes and have so far escaped synthesis. In order to estimate the tendency of n-cyclacenes (6 ≤ n ≤ 20) to undergo dimerization, which is assumed to be a major pathway of degradation under oxygen-free conditions, we here report the energy of dimerization as computed by density functional theory using spin-restricted, spin-unrestricted, and thermally assisted-occupation (TAO) formalisms. It is found that the energy of dimerization increases with increasing size of n but that this increase is not monotonic for the smaller members of the series. This is due to the combination of the cryptoannulenic effect and the inherent strain of the cyclacenes. The energy of dimerization of the largest member inspected, 20-cyclacene, is −59.3 kcal/mol, while we expect it to converge to −46 kcal/mol for n → ∞ based on comparison with data obtained for acenes.