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Increasing Radical Character of Large [n]cyclacenes Unveiled by Wave Function Theory

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journal contribution
posted on 19.04.2017, 00:00 by Stefano Battaglia, Noelia Faginas-Lago, Dirk Andrae, Stefano Evangelisti, Thierry Leininger
We have investigated the radicality and the vertical singlet–triplet energy gap of [n]­cyclacenes (cyclic polyacenes) as a function of the system size for n even, from 6 to 22. The calculations are performed using the complete active space self-consistent field method and second-order n-electron valence perturbation theory. We present a systematic way for the selection of the active space in order to have a balanced description of the wave function as the size of the system increases. Moreover, we provide didactic insight into the failure of an approach based on a minimal active space. We find that the ground state is an open-shell singlet and its multireference character increases progressively with n. The singlet–triplet gap decreases as a function of the system size and approaches a finite positive value for the limit n → ∞. Finally, an analysis based on the one-particle reduced density matrix suggests a polyradical character for the largest cyclacenes.