The ground state spin multiplicity of Schlenk-type biradicals and the influence of additional linkage to ladder type structures Original Research Article

The semi-empirical AM1 molecular orbital plus configuration interaction model is applied to a series of Schlenk-type biradicals with the aim to assess the ordering of the ground state spin multiplicities and their relative stabilities. The model biradicals are composed by variation of the radical function and also by incorporation of secondary inter-ring bridges (CH2, C=O, O, S, NH, S=O, SO2) that fix a ladder-type structure. The singlet-triplet ordering is found to vary both upon changes of the radical center and of the additional inter-ring bridge. The CI results are interpreted by reference to the distribution of the spin-active electrons, the near-degeneracy of the corresponding MOs, the equilibrium molecular geometry, the density distribution of partially occupied localized orbitals (RHF) and the spin-density waves for the triplet ground state (UHF). In the absence of a secondary bridge, the stability of the triplet versus the singlet decreases with the variation of radical center in the order .N, .CCH3, .CH, .NO, .CPh, .+NH, .+NPh, where Ph is phenyl. A saturated secondary bridge that forces the benzene rings into planarity does not reverse the order of the lowest spin states. In contrast, conjugated inter-ring bridges that perturb stronger the spin-carrying π-system may give rise to singlet ground state.