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Stretch Effects Induced by Molecular Strain on Weakening σ‑Bonds: Molecular Design of Long-Lived Diradicals (Biradicals)

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journal contribution
posted on 07.09.2012 by Manabu Abe, Haruka Furunaga, Dongxia Ma, Laura Gagliardi, Graham J. Bodwell
Stretch effects induced by two types of molecular strain were examined by quantum chemical calculations at the B3LYP/6-31G­(d), B3LYP/cc-PVDZ, CCSD/6-31G­(d), and CASPT2/ANO-RCC-VTZP level of theory, to design persistent multiradicals such as localized diradicals and oxyallyls. The cooperative molecular strain (Type-1) induced by the spiro[5.5]­undecane and bicyclo[2.1.0]­cyclopentane structures was found to significantly destabilize in energy the ring-closed compounds of the diradicals, leading to small energy differences between the diradicals and the σ-bonded compounds. Another stretch effect (Type-2) induced by macrocyclic systems was also found to energetically destabilize the corresponding ring-closed structures of the 1,3-diradicals. The computational studies predict that the two types of stretch effects are quite effective in lowering the energy barriers of the bond-breaking reaction of the ring-closed compounds and in generating long-lived localized diradicals and oxyallyl derivatives.