What to do about unpaired electrons? A hydrocarbon hexaradical with three Closs diradicals linked by 1,3,5-trimethylbenzene as ferromagnetic coupler

摘要

The relative energies of a series of high-spin polyradical organic compounds were investigated using density functional methods. The system under study involved the linking of Closs's triplet diradical units (1,3-cyclopentanediyl) 1,3,5 to benzene. By successively closing the diradical units to form bi-cyclo [2.1.0] pentane substituents, one obtains in addition to the septet hexaradical, a quintet tetraradical, a triplet diradical, and the completely ring-closed singlet. The DZP/BP86 method predicts that the lowest-energy structure is the ring-closed singlet, with the diradical 21.5 kcal/mol, the tetraradical 41.9 kcal/mol, and hexaradical 61.8 kcal/mol higher in energy. Of particular note is the failure of the UHF method to predict even the qualitative ordering of these four species, placing the septet as the lowest-energy state and ring-closed singlet as the highest. For this reason, the BP86 results are taken as somewhat more reliable, given the absence of Hartree-Fock exchange in the hybrid B3LYP functional. Furthermore, a structural analysis of the optimized geometries, in addition to nuclear-independent chemical shift values, suggests that the unusual stability of these high-spin compounds as explained in terms of the delocalization of the adjacent radical(s) into the benzene pi system.