1,1-vinylidene and 1,2-vinylene as ferromagnetic coupling units

The ground state multiplicities of the ethylene fragment connected with two methylene radicals in 1,1 position (trimethylenemethane, TMM) and in 1,2 p ositions (butadiene) in the planar structure and a conformation with 90 deg rees twisted methylene groups were calculated by ab initio methods (CASSCF CISD). It is shown that at sufficiently large twist angles (Theta greater t han or equal to 65 degrees) the multiplicity of the ground state changes fr om a triplet to a singlet state in the case of TMM. At 1,2 connection of th e methylene radicals, the ground state of the planar structure is a singlet state (butadiene) and for the 90 degrees twisted conformation a triplet st ate.