Diradical character of some fluoranthenes

It is shown that some Kekulean fluoranthenes are diradicals and that their ground state is a triplet. In the energetically less favorable singlet state, these hydrocarbons also exhibit pronounced diradical character. The diradical character y of the compounds under investigation was estimated using the unrestricted symmetry-broken (yPUEIF) and complete active space (yNOON) methods. It was found that the yPUHF values better reproduce the diradical character of the investigated hydrocarbons. It was shown that singly occupied molecular orbital (SOMO) and SOMO-1 of a diradical structure occupy different parts of space with a small shared region, resulting in a spin density distribution over the entire molecule. The spatial diradical distribution in the singlet diradical structures was examined by inspecting the HOMOs and LUMOs for alpha and beta spin electrons. It was shown that the alpha-HOMO and the beta-LUMO (as well as the beta-HOMO and the alpha-LUMO) occupy practically the same part of space. In this way, there are no unpaired electrons in a singlet diradical structure, yet two of them occupy different parts of space, thus allowing the pi-electrons to delocalize.